Health authorities in the United States and Europe reported an increasing number of travel-associated episodes of sexual transmission of Zika virus (ZIKV) following the 2015–2017 ZIKV outbreak. This, and other scientific evidence, suggests that ZIKV is sexually transmissible in addition to having its primary mosquito-borne route. The objective of this systematic review and evidence synthesis was to clarify the epidemiology of sexually transmitted ZIKV.
Methods and findings
We performed a living (i.e., continually updated) systematic review of evidence published up to 15 April 2018 about sexual transmission of ZIKV and other arthropod-borne flaviviruses in humans and other animals. We defined 7 key elements of ZIKV sexual transmission for which we extracted data: (1) rectal and vaginal susceptibility to infection, (2) incubation period following sexual transmission, (3) serial interval between the onset of symptoms in a primary and secondary infected individuals, (4) duration of infectiousness, (5) reproduction number, (6) probability of transmission per sex act, and (7) transmission rate. We identified 1,227 unique publications and included 128, of which 77 presented data on humans and 51 presented data on animals. Laboratory experiments confirm that rectal and vaginal mucosae are susceptible to infection with ZIKV and that the testis serves as a reservoir for the virus in animal models. Sexual transmission was reported in 36 human couples: 34/36 of these involved male-to-female sexual transmission. The median serial symptom onset interval in 15 couples was 12 days (interquartile range: 10–14.5); the maximum was 44 days. We found evidence from 2 prospective cohorts that ZIKV RNA is present in human semen with a median duration of 34 days (95% CI: 28–41 days) and 35 days (no CI given) (low certainty of evidence, according to GRADE). Aggregated data about detection of ZIKV RNA from 37 case reports and case series indicate a median duration of detection of ZIKV of 40 days (95% CI: 30–49 days) and maximum duration of 370 days in semen. In human vaginal fluid, median duration was 14 days (95% CI: 7–20 days) and maximum duration was 37 days (very low certainty). Infectious virus in human semen was detected for a median duration of 12 days (95% CI: 1–21 days) and maximum of 69 days. Modelling studies indicate that the reproduction number is below 1 (very low certainty). Evidence was lacking to estimate the incubation period or the transmission rate. Evidence on sexual transmission of other flaviviruses was scarce. The certainty of the evidence is limited because of uncontrolled residual bias.
The living systematic review and sexual transmission framework allowed us to assess evidence about the risk of sexual transmission of ZIKV. ZIKV is more likely transmitted from men to women than from women to men. For other flaviviruses, evidence of sexual transmissibility is still absent. Taking into account all available data about the duration of detection of ZIKV in culture and from the serial interval, our findings suggest that the infectious period for sexual transmission of ZIKV is shorter than estimates from the earliest post-outbreak studies, which were based on reverse transcription PCR alone.
Why was this study done?
- Sexual transmission of Zika virus (ZIKV) is now documented, but the risks of transmission are not well understood.
- It is not known whether other flaviviruses can be transmitted through sexual intercourse.
What did the researchers do and find?
- We developed a sexual transmission framework for ZIKV infection that identified 7 key elements related to ZIKV sexual transmission, and we conducted a living systematic review through 15 April 2018 of available evidence about each element.
- We found that, where documented, sexual transmission of ZIKV is much more common from men to women than from women to men. For sexual transmission of ZIKV, the median serial interval—the time between onset of symptoms in 2 sexual partners—is 12 days.
- The median duration of ZIKV RNA persistence in semen is longer (34 days) than in the female genital tract (12 days). ZIKV can be detected for longer periods using reverse transcription polymerase chain reaction compared to viral culture.
- We found no evidence of sexual transmission for any other arthropod-borne flaviviruses.
Citation: Counotte MJ, Kim CR, Wang J, Bernstein K, Deal CD, Broutet NJN, et al. (2018) Sexual transmission of Zika virus and other flaviviruses: A living systematic review. PLoS Med 15(7): e1002611. https://doi.org/10.1371/journal.pmed.1002611
Academic Editor: Lorenz von Seidlein, Mahidol-Oxford Tropical Medicine Research Unit, THAILAND
Received: February 16, 2018; Accepted: June 14, 2018; Published: July 24, 2018
Copyright: © 2018 Counotte et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the manuscript and its supporting Information files.
Funding: MJC and JW received salary support from Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (CH) http://www.snf.ch/ Award Number: 320030_170069 & 320030_176233. MJC received salary support from World Health Organization contracts 2017/725385-0 and 2017/733494-0. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: I have read the journal's policy and the authors of this manuscript have the following competing interests: Nicola Low receives a stipend as a Specialty Consulting Editor for PLOS Medicine, and serves on the journal’s editorial board.
Abbreviations: DENV, dengue virus; GRADE, Grading of Recommendations Assessment, Development and Evaluation; JEV, Japanese encephalitis virus; NHP, nonhuman primate; RT-PCR, reverse transcription polymerase chain reaction; SYRCLE, SYstematic Review Centre for Laboratory animal Experimentation; WNV, West Nile virus; YFV, yellow fever virus; ZIKV, Zika virus
Zika virus (ZIKV) can be transmitted between humans through sexual contact, although it is most commonly transmitted by infected Aedes spp. mosquitoes [1,2]. Sexual transmission of ZIKV has important implications for public health, for people living in endemic regions, and for sexual partners of travellers returning to non-endemic regions from endemic regions because ZIKV infection during pregnancy can cause congenital infection of the foetus and because ZIKV infection can trigger the immune-mediated neurological condition Guillain-Barré syndrome [3,4]. ZIKV is an RNA flavivirus. Flaviviruses are a genus of viruses from the Flaviviridae family, of which the majority are transmitted to vertebrates by infected mosquito or tick vectors .
Scientists working in Senegal in 2008 were the first to report presumed sexual transmission of ZIKV in a case report that documented their own symptoms and serological findings . One scientist developed symptoms after returning to the US, and his wife, who had not travelled outside the US, became unwell 4 days later. The large ZIKV outbreak (2015–2017) in the Americas resulted in additional reports of travel-associated ZIKV sexual transmission in the US and Europe, which Moreira and colleagues synthesised descriptively in a systematic review of the literature up to December 2016 . In vivo and in vitro experimental studies have provided evidence of the biological plausibility of this route of infection .
While possible sexual transmission has been established, there are many unanswered questions about the transmissibility of ZIKV through sexual intercourse. For mosquito-borne ZIKV infection, the incubation period and duration of viral shedding in serum have been estimated, allowing implications for blood donation to be assessed . Additional information about parameters related to person-to-person transmission of ZIKV has not been systematically collated or quantified, although several narrative reviews have been published [10,11]. Evidence about sexual transmission of other arthropod-borne flaviviruses in humans, including West Nile virus (WNV), yellow fever virus (YFV), Japanese encephalitis virus (JEV), and dengue virus (DENV) , has not been synthesised, but WNV and YFV have been detected in human semen [13,14]. The primary objective of this review was to systematically review evidence about defined aspects of the sexual transmission of ZIKV. Secondary objectives were to systematically review evidence about the sexual transmissibility of other arthropod-borne flaviviruses and to establish these reviews using a living systematic review approach .
Sexual transmission framework
In March 2017, we developed a sexual transmission framework for ZIKV [16,17], based on standard concepts about person-to-person transmission of infection . The framework includes key elements in the course of an infection in an individual and transmission to a sexual partner, some of which can be measured and others that can only be determined indirectly or through modelling. Fig 1 shows these elements and the relationships between them: (1) susceptibility to infection, (2) incubation period after sexual transmission, (3) serial interval, (4) duration of infectiousness, (5) reproduction number, (6) probability of transmission per sex act, and (7) transmission rate. The framework does not include transmission from and to mosquitoes, which would be needed to estimate the proportion of all ZIKV infections due to sexual transmission. The sexual transmission framework defined the outcomes and informed the structure of the review.
Numbered circles show the 7 key elements. Dark blue circles are elements for which evidence is based on empirical research. Light blue circles denote elements derived from mathematical modelling studies and in vivo studies. (A) Transmission between 2 individuals. The horizontal arrows show the time course of the disease for the primary infected individual (I), who is infected, and the secondary individual (S), who starts as susceptible (element 1). The vertical red arrow represents a Zika virus transmission event, after which there is an incubation period (element 2) before symptoms develop. Element 3 is the serial interval, i.e., the period between the start of symptoms in the primary and the secondary individual. Element 4 is the duration of infectiousness. After the infection, individuals can become immune. (B) Relation between different elements at population level. The reproduction number (element 5) is the result of the contact rate, the probability of transmission per act (element 6), and the duration of infectiousness (4). The transmission rate (element 7) can be estimated using the reproduction number (5) and the serial interval (3). Adapted with permission from World Health Organization .
Living systematic review
We performed this review as a living systematic review  because research into many aspects of ZIKV is a new and fast-moving field. Several studies are ongoing  and have published interim results , and updated results could affect public health decisions. The protocol for this review was registered on 19 May 2017 in the database PROSPERO (CRD42017060338) . We summarise the details that make the review a living systematic review in S1 Text. Future updates will be reported quarterly online (http://zika.ispm.unibe.ch/stf/) and in the online comments section of this publication. Reporting is in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (S1 PRISMA Checklist).
The search includes the electronic databases PubMed, Embase, bioRxiv, arXiv, PeerJ, and LILACS and online repositories from the US Centers for Disease Control and Prevention, the European Centre for Disease Prevention and Control, the Pan American Health Organization, and the World Health Organization from the earliest date of each database and without language restrictions. The searches include Medical Subject Headings (MeSH) terms and keywords for ZIKV and flaviviruses together with terms and keywords for viral persistence and sexual transmission (S1 Text). An automated search is run every day, with results deduplicated and imported into REDCap (Research Electronic Data Capture). We checked reference lists of included studies to identify additional relevant studies. For this report, we identified studies published before and up to 15 April 2018.
We included observational studies, in vitro and in vivo studies, and mathematical modelling studies that directly addressed any of the elements of the sexual transmission framework in either humans or animals for ZIKV or another arthropod-borne flavivirus. We included observational studies that reported 1 or more cases of sexual transmission, 1 or more measurements of presence of virus in bodily fluids, or both. As bodily fluids we included semen, cervical and vaginal secretions, and saliva; diagnostic methods included reverse transcription polymerase chain reaction (RT-PCR) and viral culture. We did not include reviews, editorials, or commentaries that did not report original data. Table 1 provides an overview of the eligibility criteria for each outcome. Primary outcomes can be directly estimated from observational studies, and secondary outcomes are calculated or inferred from indirect evidence.
Study selection and data extraction
One reviewer screened titles and abstracts of retrieved papers. If retained in the first step, the same reviewer screened the full text of the paper. One reviewer extracted data into piloted extraction forms in REDCap . A second reviewer verified exclusion decisions and data entry.
Synthesis of the evidence
We provide descriptive summaries of findings about the elements of the ZIKV sexual transmission framework for basic research studies (element 1), observational epidemiological studies (elements 2–4), and mathematical modelling studies (elements 5–7). In addition, we used data from included studies to calculate estimates for the serial interval (i.e., the period between the start of symptoms in the primary and the secondary individual) and the duration of the detection of ZIKV. We report the median serial interval and its interquartile range. To estimate the duration of detection of ZIKV positivity, we conducted interval-censored survival analysis and fitted Weibull distributions using the “straweib” package [23,24] in R (version 3.4.1), based on previous studies [20,23,24]. We assumed that all infected patients were RT-PCR or viral culture positive at symptom onset. We report median estimated durations and corresponding 95% confidence intervals. Additional information about the methods is provided in S2 Text. For other flaviviruses, we summarise findings from all study types descriptively.
Certainty assessment of the evidence
We assessed the methodology of included studies using specific checklists for each study type. For observational studies, we used the National Institutes of Health (NIH) Quality Assessment Tool for Case Series Studies  and UK National Institute for Health and Care Excellence (NICE) checklists for case–control studies and cohort studies . For in vivo studies we used the SYstematic Review Centre for Laboratory animal Experimentation (SYRCLE) risk of bias tool for animal studies , and for mathematical modelling studies, the International Society for Pharmacoeconomics and Outcomes Research (ISPOR) Questionnaire to Assess Relevance and Credibility of Modelling Studies . We performed the assessment by a consensus-driven approach among multiple reviewers. We appraised the certainty of the key elements according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) tool [29–31] (S3 Table). In accordance with GRADE, assessments of the overall certainty of evidence from observational studies started at low certainty. We downgraded the level of certainty for small sample size and evidence from case reports or case series. We assessed outcomes of mathematical modelling studies as high, medium, low, or very low certainty.
We identified 1,227 unique citations and excluded 901 by title and abstract screening (Fig 2). Of the remaining 326 potentially eligible citations with relevant abstracts, 128 publications were eligible for inclusion. Table 2 summarises characteristics of the included studies.
Numbers of studies screened, assessed for eligibility, and included in the review, with reasons for exclusions at each stage.
Basic research studies
We included 41 in vivo and in vitro studies of ZIKV [32–69] (Table 2). Of these 41 studies, 6 were in vitro studies and 35 were studies in in vivo animal models: 12 in nonhuman primates (NHPs) such as cynomolgus macaques (Macaca fascicularis), rhesus macaques (M. mulatta), and common marmosets (Callithrix jacchus) and 23 in mice. In 1 study, both guinea pigs and NHPs were used . These studies provide insight into the underlying biological mechanisms of susceptibility to ZIKV infection through sexual transmission and substantiate the biological plausibility of this transmission route.
Susceptibility (element 1)
In mouse and NHP models, the vaginal and rectal mucosae were shown to be susceptible to infection with ZIKV [36,40,47,49,51,59]. When ZIKV-infected male mice were mated with uninfected female mice, the female mice became infected [47,53,69]. Female-to-male transmission of ZIKV in mice was unsuccessful . In rhesus macaques, systemic infection through oropharyngeal mucosal inoculation with ZIKV was only successful after inoculation with a very high dose of virus, suggesting a very low risk of oral mucosal transmission . Four rhesus macaques became viraemic after intranasal or intragastric inoculation with ZIKV . In guinea pigs, direct transmission between animals infected subcutaneously with ZIKV and co-housed uninfected animals was seen .
Human prostate cells, testicular cells, and mature spermatozoa are susceptible in vitro to ZIKV infection [35,60–62]. Human Sertoli cells can support high levels of ZIKV replication and persistence . In multiple mouse models, using different strains of ZIKV, the testes seem to be a preferred site for viral replication, able to sustain high viral loads for a longer duration than other organs [32,34,35,43,47,49,51,58,63]. In some of these models, ZIKV caused inflammation of the testes [35,39,41,47,49], reduced testicular size, and decreased levels of testosterone [49,56,68,71]. The testes of experimentally infected NHPs harboured high levels of ZIKV [37,38]. High titres of ZIKV RNA were detectable in semen until day 28 in rhesus and cynomolgus macaques . However, 1 group of rhesus macaques had only low levels of viral RNA in the testes and no detectable virus in the prostate or epididymis . In common marmosets, ZIKV RNA in semen was sporadically detected .
The female genital tract of macaques was able to sustain ZIKV replication for shorter durations and with lower viral loads than the male genital tract [33,38,46]. Although cervical and endometrial cells were susceptible in vitro , virus was not detected in the female genital tract in mice or NHPs for longer than 7 days after infection [33,46]; in 1 study, the ovary sustained higher titres up to 14 days post-infection . Intravaginal infection of mice led to systemic infection [36,40,42,47,48,50,67,68,69] and to adverse congenital outcomes . In pregnant female mice, sexual transmission led to more ZIKV dissemination to the female reproductive tract, compared with subcutaneous or intravaginal inoculation . In NHPs the incubation time following infection was longer for intravaginal infection compared with subcutaneous infection .
In mice, viral titres were lower in the salivary glands than the testes and ovaries . In NHPs, viral RNA was detected in saliva up to 28–42 days [33,37,38,52,57], and ZIKV could be cultured at day 7 and day 14 [38,65].
Risk of bias in in vivo studies
Most studies did not describe in detail the methods used to avoid bias. Detailed certainty assessment of the in vivo studies is provided in S3 Table.
ZIKV transmission between sexual partners.
As of 15 April 2018, the US Centers for Disease Control and Prevention reported that, of 5,672 cases of ZIKV infection in the US, 52 were acquired through sexual transmission . The European Centre for Disease Prevention and Control reported, as of 13 March 2017, 20 cases of sexual transmission out of 1,737 cases for which the route of transmission was known .
We included 67 reports about ZIKV sexual transmission; measurement of ZIKV infection status using RT-PCR or viral culture in samples of semen, vaginal fluid, or saliva; or both [6,20,74–138]. Twenty-four of these studies reported on 36 couples in which a primary partner with ZIKV infection, who returned from a ZIKV endemic area, is suspected to have transmitted ZIKV to a secondary partner [6,78,79,82,84,85,87,88,90,91,96,100,102,105,113,115,119–121,123,124,127,131,137] (Table 3). Thirty-four of 36 episodes of transmission were from man to woman (94%), 1 (3%) was from woman to man , and 1 (3%) was from man to man . Penile–vaginal intercourse was reported as the most likely mode of transmission between men and women, but oral and anal intercourse were mentioned as possible transmission routes in some reports [6,78,79,84,85,88,90,91,96,100,115]. One study reported transmission of ZIKV from a vasectomised man to his female sexual partner . Amongst primary infected individuals, 27/36 (75%) were symptomatic, 2/36 (6%) were asymptomatic, and symptom status was not reported for the remaining 7/36 (19%).
ZIKV was detected by RT-PCR in blood, urine, saliva, or semen in 14/36 (39%) primary partners and in 18/36 (50%) secondary partners. No diagnostic method was stated for 29/72 (40%) individuals. In 5/36 (14%) couples, the secondary partner also had a history of travel to an endemic region [88,91,115,124,137].
Incubation period (element 2) and serial interval (element 3).
We were not able to extract information on the incubation period following sexual exposure to ZIKV, since dates of exposure of the primary partner and dates of sexual intercourse with the secondary partner were rarely reported. Thirteen reports about 15 couples reported on dates of symptom onset for both partners. The median serial interval was 12 days (interquartile range: 10–14.5 days) [6,78,79,84,85,88,90,96,102,115,124,131,137] and the maximum was 44 days .
Duration of infectiousness (element 4).
Duration of infectiousness was not measured directly in any included study. Observational studies measured the duration of detection of ZIKV in bodily fluids in case reports, case series, and prospective cohort studies. S2 Text provides additional information.
Case reports and case series.
We included 48 publications describing 180 individuals who underwent diagnostic testing by RT-PCR or viral culture on semen, vaginal fluid, or saliva at 1 or more time points [75,76,80,81,83–86,88–95,97–100,102–104,107–117,122,124–126,128–138]. In semen (data available from 37 case reports and case series from 119 individuals; Fig 3; S2 Text), the median duration of RT-PCR positivity was 39.6 days (95% CI: 29.9–49.0 days) and the maximum was 370 days . The median duration based on viral culture was 9.5 days (95% CI: 1.2–20.3 days) (data from 22 men in 11 reports), with a maximum of 69 days . The median duration of ZIKV positivity in any fluid from the female genital tract was 13.9 days (95% CI: 7.2–19.6 days) based on RT-PCR (data from 15 women in 7 reports), with a maximum of 37 days . The median duration of ZIKV positivity in saliva was 7.3 days (95% CI: 4.2–10.8 days) based on RT-PCR (data from 76 individuals in 23 reports), with a maximum of 91 days . There were too few data for analysis of viral culture specimens in female genital tract fluids and saliva.
The x-axis indicates time in days from symptom onset. The labels on the y-axis represent the date of publication of the studies, in chronological order, with the last date indicating the date of this analysis. Green lines represent the duration of RT-PCR positivity in individuals from case reports and case series (n = 119), extending to the last positive RT-PCR measurement. Green dots at day 0 represent an assumption of RT-PCR positivity for patients with no sample taken at symptom onset. Blue lines represent the interval between the last positive measurement and the first subsequent negative measure (red dot). The black dotted line represents the publication of the WHO interim guidelines  and the suggested duration of protected sexual intercourse advised in the guidelines (6 months, black triangle). The black dots and whisker bars represent median aggregated values and 95% confidence intervals for [a] a prospective cohort (n = 55 men)  and [b] the aggregation of all available case reports and case series. Maximum values in these datasets are shown with a red diamond or a red greater than symbol (for values outside the range of the graph). Lines for which the date is not provided are from the same date as the line above. RT-PCR, reverse transcription PCR; ZIKV, Zika virus.
Prospective cohort studies.
One cohort study enrolled 150 women and men with symptomatic ZIKV infection in Puerto Rico. ZIKV was detected by RT-PCR in 31/55 men, with a median duration of persistence of 34 days (95% CI: 28–41 days). ZIKV RNA was only detected in saliva or vaginal fluids in a few participants . A second cohort study, amongst people returning from ZIKV endemic areas or infected in the US, detected ZIKA RNA in the semen of 60/184 symptomatic men . The mean time to ZIKV RNA clearance was 54 days (95% CI: 53–55 days). The median duration was not reported, but plotted at approximately 35 days. Only 3 out of 19 of the semen samples provided within 30 days after symptom onset could be cultured; none of the 59 samples provided after 30 days could be cultured.
Risk of bias in observational studies.
Studies varied widely in risk of bias and completeness of reporting (S3 Table). Many studies reporting on transmission events did not use reliable diagnostic methods in both partners, potentially leading to misclassification bias. The median duration of ZIKV persistence was higher in case reports and case series than in the prospective cohort study.
Mathematical modelling studies
Reproduction number (element 5), transmission probability (element 6), and transmission rate (element 7).
We included 2 mathematical modelling studies, both of which used a deterministic structure [139,140]. Gao et al. used surveillance data from Brazil, Colombia, and El Salvador ; Towers et al. used data from Colombia . Both studies derived the reproduction number for ZIKV sexual transmission: 0.136 (95% CI: 0.009–0.521)  and “likely below 1” . The 2 studies calculated the proportion of ZIKV infections resulting from sexual transmission as 3.04% (95% CI: 0.12%–45.73%)  and 23% (95% CI: 1%–47%) . Neither study provides new information about the transmission probability per sex act or the transmission rate for sexual transmission of ZIKV.
Risk of bias of mathematical modelling studies.
For both modelling studies, the data used to populate the model were not suitable to derive the outcome. The surveillance data on which these studies based their results did not distinguish between vector-transmitted ZIKV and sexually transmitted ZIKV. The results of these studies did not provide information about the size of the risk of sexual transmission. External validation for both models is lacking. Detailed certainty assessment is shown in S3 Table.
We included 18 studies reporting on the sexual transmission potential of other arthropod-borne flaviviruses [14,141–157]. Ten of the 18 studies (56%) were in vitro experiments or observations in animals, and 8 studies (44%) were case reports or case series. JEV was demonstrated to be transmissible from male to female pigs via semen [141–144]. Persistence of virus was demonstrated for at least 17 days in boars . JEV can be cultured from the seminal fluids of pigs . In humans, we found 1 case report of male-to-female sexual transmission of WNV, although the secondary partner also lived in a mosquito endemic area . WNV was found postmortem in the prostate and testis of a 43-year-old man on immunosuppressive therapy following a kidney transplant . Intravaginal inoculation of WNV in mice led to local acute inflammation followed by systemic illness in a proportion of the animals . The testes of 6 Japanese macaques (M. fuscata) showed low DENV neutralising antibody titres . Experimentally DENV-infected pigtail macaques (M. nemestrina) showed dissemination of virus in the prostate gland and seminal vesicles . DENV RNA could be detected in experimentally infected mice 3 days after infection . In humans, 4 case reports describe the presence of DENV in saliva, diagnosed by either RT-PCR or viral culture, for up to 7 days [152–155]. DENV RNA was demonstrated in the vaginal secretion of 1 patient up to 18 days after onset of symptoms . Female mice that were mated to male mice infected with tick-borne encephalitis virus had worse reproductive outcomes than ones mated to a group of non-infected males; in 1 female mouse, the virus was detected . YFV was demonstrated in the urine and semen of a patient by RT-PCR 21 days after onset of symptoms .
This systematic review summarises published data related to sexual transmission of ZIKV and other arthropod-borne flaviviruses published on or before 15 April 2018. In animals, vaginal and rectal mucosae are susceptible to ZIKV, with the testis as a preferred site of replication. Male-to-female transmission was more frequent than female-to-male transmission in animal models and in humans. In humans, we estimated the serial interval for sexually transmitted infection to be 12 (interquartile range: 10–14.5) days. ZIKV was detectable in semen for a median of 34 (95% CI: 28–41) days by RT-PCR and 9.5 (95% CI: 1.2–20.3) days by viral culture. In mathematical modelling studies, the reproduction number for sexual transmission of ZIKV was below 1. The overall certainty of the evidence was low. We found no evidence that other arthropod-borne flaviviruses can be sexually transmitted.
What the study adds to existing research
The ZIKV sexual transmission framework allowed us to synthesise evidence from both animal and human studies in a structured way, taking into account the risks of bias in the included studies. Susceptibility of tissues to ZIKV could only be assessed in animal models. There were consistent findings in animal models that help to explain the overrepresentation of reported cases of human male-to-female transmission, even though mice are not a natural host for ZIKV and in vivo studies often use immunocompromised animals . First, vaginal mucosae are more susceptible than urethral mucosae to infection [36,40,47,49,51,59]. Second, high levels of ZIKV replication in the testes in mice and sustained detection of viral RNA and of virus in tissue culture in mice and NHP models are consistent with the longer duration of detection in men than women. Rectal mucosa is also susceptible to ZIKV, so, although only observed once , unprotected anal intercourse is also a likely route of ZIKV transmission. The risk of bias of the included in vivo studies, as assessed with the SYRCLE tool, was high. Most of these studies explored the suitability of animal models or investigated pathophysiological pathways, and potential sources of bias were rarely reported.
Our analysis shows that, when assessed from case reports and case series, the duration of detection of ZIKV in semen by RT-PCR is overestimated; all reports are of people with ZIKV detected, and a small number of outliers influence the estimate. A prospective cohort study that consecutively enrolled people with symptomatic ZIKV infection estimated a shorter duration of persistence . Case reports and case series are early sources of information about a new disease, but, by the nature of these studies, researchers report novel and unusual findings. Parameters and effect sizes estimated from aggregating data from these sources are likely to be overestimates, reflecting the so-called random high: extreme values in a distribution that are observed by chance and are more likely to be reported because they are noteworthy . As evidence has accumulated in well-designed prospective studies, the estimated duration of persistence of ZIKV RNA has decreased. Notably, the prospective cohort study in Puerto Rico found ZIKV in semen in only half of men with symptomatic infection and vaginal fluid in only 1 of 50 women. Similarly, ZIKV was found in semen in only 60/183 (33%) ZIKV-infected men in the US . Persistence of viral RNA in body fluids is often used as a proxy for the duration of ZIKV infectiousness, although it remains unclear whether the presence of viral RNA corresponds with infectious virus. ZIKV RNA positivity persists for longer than detection of ZIKV in viral culture in both mice  and human semen samples. However, viral cultures might underestimate the duration of infectiousness if low pH or other specimen-dependent factors produce false negative results [83,159]. The estimated serial interval was based on observations from only 15 couples, but was consistent with that of several respiratory infectious diseases . The serial interval for sexual transmission was towards the lower end of estimates for mosquito-borne transmission (10–23 days) .
Some elements of the infection process, such as the incubation period, transmissibility of ZIKV per sex act, and transmission rate could not be observed. In the mathematical models published so far [139,140,162], the estimates were based on assumptions about the transmissibility of mosquito-borne infection. Estimates from our review might provide more reliable data for use in future modelling studies. The potential for sustained sexual transmission of ZIKV appears low, based on the reproduction number estimated in the mathematical modelling studies. The estimated reproduction number was higher for mosquito-borne transmission, 1.96 (95% CI: 0.45–6.23) than for sexual transmission , although this number is highly dependent on the geographical location .
This review did not find evidence supporting sexual transmission of other arthropod-borne flaviviruses. The continual updating of the literature search identified a finding of YFV in urine and semen . However, it remains to be clarified for many viruses if detection in semen means that there is a risk of sexual transmission .
Strengths and weaknesses of the study
The strengths of our living systematic review are the high coverage of the body of published literature, the structured overview, and the reanalysis of individual patient data on persistence of ZIKV. The automation of search and deduplication processes makes it feasible to keep the review updated as new information becomes available. Updated analyses of the data from case reports show regression to the mean of the median estimate of the duration of RNA detection in semen (https://zika.ispm.unibe.ch/stf/). Future updates of this review will also allow for incorporation of techniques to synthesise mathematical modelling studies, such as multi-model ensembles. This study also has limitations. Screening and data extraction were not done by 2 independent reviewers because of time constraints, but we believe that we reduced errors by having a second reviewer check the decisions and data extracted. The statistical methods used to estimate the duration of persistence of ZIKV in bodily fluids assume that all samples are positive for ZIKV at time 0 , which might not be the case. Additionally, the sexual transmission framework might not include all factors that are required to investigate the risks of sexual transmission of ZIKV. The certainty of this body of evidence was assessed as being low or very low because of bias in the observational study designs and the indirectness of evidence from animal studies. The certainty of the evidence base could increase if the design and reporting of both animal and human studies improve and if their findings are consistent with, and increase the precision of, the evidence presented here.
Implications and next steps for researchers, clinicians, and policymakers
The risk of sexual transmission of ZIKV is particularly relevant for women who are pregnant or planning a pregnancy and for people with high levels of sexual partner change such as some groups of men who have sex with men and women at high risk. An expert group has used the ZIKV sexual transmission framework to stimulate discussion about research priorities . One important limitation to the generalisability of findings from our review is that the data that we analysed about sexual transmission of ZIKV in humans relied largely on information from travellers returning from endemic areas with symptomatic ZIKV infection and their sexual partners. This group probably differs from people in endemic regions in ways that could affect sexual transmission of ZIKV, such as previous exposure to other flaviviruses . Additional studies in ZIKV endemic settings could enrol travellers who work in areas with mosquito-borne ZIKV transmission and who return to families living nearby but in areas, e.g., at high altitude, where the vector does not survive . There are unanswered questions about the potential for asymptomatic ZIKV infection following sexual transmission, since ZIKV transmitted by mosquitoes often causes asymptomatic infection [165–167], about clinical differences between ZIKV infections acquired through sexual and mosquito-borne routes, and about the long-term consequences of ZIKV in the genital tract, such as its effects on the testes and on male infertility. Research about the potential for sexual transmission of other flaviviruses is needed, although these viruses often display different symptomatology or affinity for different species.
Clinicians and policymakers need information that helps to advise both opposite sex and same sex couples on how to reduce the risk of sexual transmission of ZIKV. The relationship between detectable RNA in semen and infectiousness therefore needs to be further investigated in both laboratory and epidemiological studies. Current guidelines for travellers returning from endemic areas advise 6 months of protected intercourse [1,2]. As more information becomes available, a revision of the duration of protection might be indicated.
This living systematic review gives an up-to-date synthesis of information about the sexual transmission of ZIKV with a structured framework. Planned regular updates will allow timely updating of relevant data from a rapidly expanding evidence base. We did not quantify the absolute risk of sexual transmission of ZIKV, but it appears small based on information about the proportion of people with symptomatic ZIKV who have ZIKV detected in genital secretions and the short median duration of detection of ZIKV in semen and vaginal fluid. Taking into account all available data about the duration of detection of ZIKV in culture and from the serial interval, our findings suggest that the infectious period for sexual transmission of ZIKV is shorter than estimates from the earliest post-outbreak studies, which were based on RT-PCR alone.
S1 Table. Detailed description of case reports and case series describing sexual transmission of ZIKV.
S2 Table. Detailed description of in vivo and in vitro studies.
S3 Table. Certainty assessment of outcomes and studies.
The views expressed in this article are those of the authors and do not necessarily represent the official positions of the World Health Organization, the University of Bern, the National Institute of Allergy and Infectious Diseases, or the Centers for Disease Control and Prevention.
- 1. Petersen EE, Meaney-Delman D, Neblett-Fanfair R, Havers F, Oduyebo T, Hills SL, et al. Update: interim guidance for preconception counseling and prevention of sexual transmission of Zika virus for persons with possible Zika virus exposure—United States, September 2016. MMWR Morb Mortal Wkly Rep. 2016;65(39):1077–81. pmid:27711033
- 2. World Health Organization. Prevention of sexual transmission of Zika virus: interim guidance update. Geneva: World Health Organization; 2016 [cited 2017 Nov 1]. Available from: http://apps.who.int/iris/bitstream/10665/204421/1/WHO_ZIKV_MOC_16.1_eng.pdf.
- 3. World Health Organization. WHO statement on the first meeting of the International Health Regulations (2005) (IHR 2005) Emergency Committee on Zika Virus and Observed Increase in Neurological Disorders and Neonatal Malformations. Saudi Med J. 2016;37(3):332–3.
- 4. Krauer F, Riesen M, Reveiz L, Oladapo OT, Martinez-Vega R, Porgo TV, et al. Zika virus infection as a cause of congenital brain abnormalities and Guillain-Barre syndrome: systematic review. PLoS Med. 2017;14(1):e1002203. pmid:28045901
- 5. Chambers TJ, Hahn CS, Galler R, Rice CM. Flavivirus genome organization, expression, and replication. Annu Rev Microbiol. 1990;44:649–88. pmid:2174669
- 6. Foy BD, Kobylinski KC, Chilson Foy JL, Blitvich BJ, Travassos da Rosa A, Haddow AD, et al. Probable non-vector-borne transmission of Zika virus, Colorado, USA. Emerg Infect Dis. 2011;17(5):880–2. pmid:21529401
- 7. Moreira J, Peixoto TM, Siqueira AM, Lamas CC. Sexually acquired Zika virus: a systematic review. Clin Microbiol Infect. 2017;23(5):296–305. pmid:28062314
- 8. Morrison TE, Diamond MS. Animal models of Zika virus infection, pathogenesis, and immunity. J Virol. 2017;91(8):e00009–17. pmid:28148798
- 9. Lessler J, Ott CT, Carcelen AC, Konikoff JM, Williamson J, Bi Q, et al. Times to key events in Zika virus infection and implications for blood donation: a systematic review. Bull World Health Organ. 2016;94(11):841–9. pmid:27821887
- 10. Grischott F, Puhan M, Hatz C, Schlagenhauf P. Non-vector-borne transmission of Zika virus: a systematic review. Travel Med Infect Dis. 2016;14(4):313–30. pmid:27425793
- 11. Hastings AK, Fikrig E. Zika virus and sexual transmission: a new route of transmission for mosquito-borne flaviviruses. Yale J Biol Med. 2017;90(2):325–30. pmid:28656018
- 12. Kuno G, Chang GJ, Tsuchiya KR, Karabatsos N, Cropp CB. Phylogeny of the genus Flavivirus. J Virol. 1998;72(1):73–83. pmid:9420202
- 13. Salam AP, Horby PW. The breadth of viruses in human semen. Emerg Infect Dis. 2017;23(11):1922–4. pmid:29048276
- 14. Barbosa CM, Di Paola N, Cunha MP, Rodrigues-Jesus MJ, Araujo DB, Silveira VB, et al. Yellow fever virus in urine and semen of convalescent patient, Brazil. Emerg Infect Dis. 2018;24(1):176–8. pmid:29058663
- 15. Elliott JH, Turner T, Clavisi O, Thomas J, Higgins JP, Mavergames C, et al. Living systematic reviews: an emerging opportunity to narrow the evidence-practice gap. PLoS Med. 2014;11(2):e1001603. pmid:24558353
- 16. World Health Organization. Sexual transmission of Zika virus: current status, challenges and research priorities. Geneva: World Health Organization; 2017 [cited 2018 Jun 22]. Available from: http://www.who.int/iris/handle/10665/259583.
- 17. Kim CR, Counotte M, Bernstein K, Deal C, Mayaud P, Low N, et al. Investigating the sexual transmission of Zika virus. Lancet Glob Health. 2018;6(1):e24–5. pmid:29241605
- 18. Giesecke J. Modern infectious disease epidemiology. Boca Raton (FL): CRC Press; 2017.
- 19. Calvet GA, Kara EO, Giozza SP, Botto-Menezes CHA, Gaillard P, de Oliveira Franca RF, et al. Study on the persistence of Zika virus (ZIKV) in body fluids of patients with ZIKV infection in Brazil. BMC Infect Dis. 2018;18(1):49. pmid:29357841
- 20. Paz-Bailey G, Rosenberg ES, Doyle K, Munoz-Jordan J, Santiago GA, Klein L, et al. Persistence of Zika virus in body fluids—preliminary report. N Engl J Med. 2017 Feb 14. pmid:28195756
- 21. Zambrano H, Waggoner JJ, Almeida C, Rivera L, Benjamin JQ, Pinsky BA. Zika virus and chikungunya virus coinfections: a series of three cases from a single center in Ecuador. Am J Trop Med Hyg. 2016;95(4):894–6. pmid:27402518
- 22. Harris PA, Taylor R, Thielke R, Payne J, Gonzalez N, Conde JG. Research electronic data capture (REDCap)—a metadata-driven methodology and workflow process for providing translational research informatics support. J Biomed Inform. 2009;42(2):377–81. pmid:18929686
- 23. Gu X, Shapiro D, Hughes MD, Balasubramanian R. Stratified Weibull regression model for interval-censored data. R J. 2014;6(1):31–40. pmid:26835159
- 24. Gomez G, Calle ML, Oller R, Langohr K. Tutorial on methods for interval-censored data and their implementation in R. Stat Model. 2009;9(4):259–97.
- 25. National Heart, Lung, and Blood Institute. Study quality assessment tools: Quality Assessment Tool for Case Series Studies. Bethesda; National Heart, Lung, and Blood Institute; 2014 [cited 2017 Nov 1]. Available from: https://www.nhlbi.nih.gov/health-pro/guidelines/in-develop/cardiovascular-risk-reduction/tools/case_series.
- 26. National Institute for Health and Care Excellence. The guidelines manual: process and methods [PMG6]. London: National Institute for Health and Care Excellence; 2012 [cited 2017 Nov 1]. Available from: https://www.nice.org.uk/process/pmg6/chapter/introduction.
- 27. Hooijmans CR, Rovers MM, de Vries RB, Leenaars M, Ritskes-Hoitinga M, Langendam MW. SYRCLE’s risk of bias tool for animal studies. BMC Med Res Methodol. 2014;14(1):43. pmid:24667063
- 28. Jaime Caro J, Eddy DM, Kan H, Kaltz C, Patel B, Eldessouki R, et al. Questionnaire to assess relevance and credibility of modeling studies for informing health care decision making: an ISPOR-AMCP-NPC Good Practice Task Force report. Value Health. 2014;17(2):174–82. pmid:24636375
- 29. Balshem H, Helfand M, Schunemann HJ, Oxman AD, Kunz R, Brozek J, et al. GRADE guidelines: 3. Rating the quality of evidence. J Clin Epidemiol. 2011;64(4):401–6. pmid:21208779
- 30. Guyatt GH, Oxman AD, Sultan S, Glasziou P, Akl EA, Alonso-Coello P, et al. GRADE guidelines: 9. Rating up the quality of evidence. J Clin Epidemiol. 2011;64(12):1311–6. pmid:21802902
- 31. Egger M, Johnson L, Althaus C, Schoni A, Salanti G, Low N, et al. Developing WHO guidelines: time to formally include evidence from mathematical modelling studies. F1000Res. 2017;6:1584. pmid:29552335
- 32. Rossi SL, Tesh RB, Azar SR, Muruato AE, Hanley KA, Auguste AJ, et al. Characterization of a novel murine model to study Zika virus. Am J Trop Med Hyg. 2016;94(6):1362–9. pmid:27022155
- 33. Dudley DM, Aliota MT, Mohr EL, Weiler AM, Lehrer-Brey G, Weisgrau KL, et al. Natural history of Asian lineage Zika virus infection in macaques. bioRxiv. 2016 May 10.
- 34. Lazear HM, Govero J, Smith AM, Platt DJ, Fernandez E, Miner JJ, et al. A mouse model of Zika virus pathogenesis. Cell Host Microbe. 2016;19(5):720–30. pmid:27066744
- 35. Chan JF, Yip CC, Tsang JO, Tee KM, Cai JP, Chik KK, et al. Differential cell line susceptibility to the emerging Zika virus: implications for disease pathogenesis, non-vector-borne human transmission and animal reservoirs. Emerg Microbes Infect. 2016;5:e93. pmid:27553173
- 36. Yockey LJ, Varela L, Rakib T, Khoury-Hanold W, Fink SL, Stutz B, et al. Vaginal exposure to Zika virus during pregnancy leads to fetal brain infection. Cell. 2016;166(5):1247–56.e4. pmid:27565347
- 37. Koide F, Goebel S, Snyder B, Walters KB, Gast A, Hagelin K, et al. Development of a Zika virus infection model in cynomolgus macaques. Front Microbiol. 2016;7(6):2028. pmid:28066354
- 38. Osuna CE, Lim SY, Deleage C, Griffin BD, Stein D, Schroeder LT, et al. Zika viral dynamics and shedding in rhesus and cynomolgus macaques. Nat Med. 2016;22(12):1448–55. pmid:27694931
- 39. Ma W, Li S, Ma S, Jia L, Zhang F, Zhang Y, et al. Zika virus causes testis damage and leads to male infertility in mice. Cell. 2016;167(6):1511–24.e10. pmid:27884405
- 40. Khan S, Woodruff EM, Trapecar M, Fontaine KA, Ezaki A, Borbet TC, et al. Dampened antiviral immunity to intravaginal exposure to RNA viral pathogens allows enhanced viral replication. J Exp Med. 2016;213(13):2913–29. pmid:27852793
- 41. Govero J, Esakky P, Scheaffer SM, Fernandez E, Drury A, Platt DJ, et al. Zika virus infection damages the testes in mice. Nature. 2016;540(7633):438–42. pmid:27798603
- 42. Tang WW, Young MP, Mamidi A, Regla-Nava JA, Kim K, Shresta S. A mouse model of Zika virus sexual transmission and vaginal viral replication. Cell Rep. 2016;17(12):3091–8. pmid:28009279
- 43. Julander JG, Siddharthan V, Evans J, Taylor R, Tolbert K, Apuli C, et al. Efficacy of the broad-spectrum antiviral compound BCX4430 against Zika virus in cell culture and in a mouse model. Antiviral Res. 2017;137:14–22. pmid:27838352
- 44. Li XF, Dong HL, Huang XY, Qiu YF, Wang HJ, Deng YQ, et al. Characterization of a 2016 clinical isolate of Zika virus in non-human primates. EBioMedicine. 2016;12:170–7. pmid:27693104
- 45. Panina-Bordignon P, Pagani I, Ghezzi S, Ulisse A, Rubio A, Garavaglia E, et al. Zika virus infects human endometrial stromal cells. Reprod Sci. 2017;24(1):85A.
- 46. Aid M, Abbink P, Larocca RA, Boyd M, Nityanandam R, Nanayakkara O, et al. Zika virus persistence in the central nervous system and lymph nodes of rhesus monkeys. Cell. 2017;169(4):610–20.e14. pmid:28457610
- 47. Duggal NK, Ritter JM, Pestorius SE, Zaki SR, Davis BS, Chang GJ, et al. Frequent Zika virus sexual transmission and prolonged viral rna shedding in an immunodeficient mouse model. Cell Rep. 2017;18(7):1751–60. pmid:28199846
- 48. Hastings AK, Yockey LJ, Jagger BW, Hwang J, Uraki R, Gaitsch HF, et al. tam receptors are not required for Zika virus infection in mice. Cell Rep. 2017;19(3):558–68. pmid:28423319
- 49. Uraki R, Hwang J, Jurado KA, Householder S, Yockey LJ, Hastings AK, et al. Zika virus causes testicular atrophy. Sci Adv. 2017;3(2):e1602899. pmid:28261663
- 50. Uraki R, Jurado KA, Hwang J, Szigeti-Buck K, Horvath TL, Iwasaki A, et al. Fetal growth restriction caused by sexual transmission of Zika virus in mice. J Infect Dis. 2017;215(11):1720–4. pmid:28472297
- 51. Martinez LE, Garcia G, Contreras D, Gong D, Sun R, Arumugaswami V. Pathogenesis of Zika virus infection via rectal route. bioRxiv. 2017 Apr 19.
- 52. Coffey LL, Pesavento PA, Keesler RI, Singapuri A, Watanabe J, Watanabe R, et al. Zika virus tissue and blood compartmentalization in acute infection of rhesus macaques. PLoS ONE. 2017;12(1):e0171148. pmid:28141843
- 53. Winkler CW, Woods TA, Rosenke R, Scott DP, Best SM, Peterson KE. Sexual and vertical transmission of Zika virus in anti-interferon receptor-treated Rag1-deficient mice. Sci Rep. 2017;7(1):7176. pmid:28775298
- 54. Newman CM, Dudley DM, Aliota MT, Weiler AM, Barry GL, Mohns MS, et al. Oropharyngeal mucosal transmission of Zika virus in rhesus macaques. Nat Commun. 2017;8(1):169. pmid:28765581
- 55. Carroll T, Lo M, Lanteri M, Dutra J, Zarbock K, Silveira P, et al. Zika virus preferentially replicates in the female reproductive tract after vaginal inoculation of rhesus macaques. PLoS Pathog. 2017;13(7):e1006537. pmid:28746373
- 56. Sheng ZY, Gao N, Wang ZY, Cui XY, Zhou DS, Fan DY, et al. Sertoli cells are susceptible to ZIKV infection in mouse testis. Front Cell Infect Microbiol. 2017;7:272. pmid:28680856
- 57. Silveira ELV, Rogers KA, Gumber S, Amancha P, Xiao P, Woollard SM, et al. Immune cell dynamics in rhesus macaques infected with a Brazilian strain of Zika virus. J Immunol. 2017;199(3):1003–11. pmid:28667164
- 58. Griffin BD, Muthumani K, Warner BM, Majer A, Hagan M, Audet J, et al. DNA vaccination protects mice against Zika virus-induced damage to the testes. Nat Commun. 2017;8:15743. pmid:28589934
- 59. Haddow AD, Nalca A, Rossi FD, Miller LJ, Wiley MR, Perez-Sautu U, et al. High infection rates for adult macaques after intravaginal or intrarectal inoculation with Zika virus. Emerg Infect Dis. 2017;23(8):1274–81. pmid:28548637
- 60. Spencer JL, Lahon A, Tran LL, Arya RP, Kneubehl AR, Vogt MB, et al. Replication of Zika virus in human prostate cells: a potential source of sexually transmitted virus. J Infect Dis. 2018;217(4):538–47. pmid:28968863
- 61. Bagasra O, Addanki KC, Goodwin GR, Hughes BW, Pandey P, McLean E. Cellular targets and receptor of sexual transmission of Zika virus. Appl Immunohistochem Mol Morphol. 2017;25(10):679–86. pmid:28968270
- 62. Siemann DN, Strange DP, Maharaj PN, Shi PY, Verma S. Zika virus infects human Sertoli cells and modulates the integrity of the in vitro blood-testis barrier model. J Virol. 2017;91(22):e00623–17. pmid:28878076
- 63. McDonald EM, Duggal NK, Brault AC. Pathogenesis and sexual transmission of Spondweni and Zika viruses. PLoS Negl Trop Dis. 2017;11(10):e0005990. pmid:28985234
- 64. Deng YQ, Zhang NN, Li XF, Wang YQ, Tian M, Qiu YF, et al. Intranasal infection and contact transmission of Zika virus in guinea pigs. Nat Commun. 2017;8(1):1648. pmid:29162827
- 65. Chiu CY, Sanchez-San Martin C, Bouquet J, Li T, Yagi S, Tamhankar M, et al. Experimental Zika virus inoculation in a New World monkey model reproduces key features of the human infection. Sci Rep. 2017;7(1):17126. pmid:29215081
- 66. Xu K, Song Y, Dai L, Zhang Y, Lu X, Xie Y, et al. recombinant chimpanzee adenovirus vaccine AdC7-M/E protects against Zika virus infection and testis damage. J Virol. 2018;92(6):e01722–17. pmid:29298885
- 67. Scott JM, Lebratti TJ, Richner JM, Jiang X, Fernandez E, Zhao H, et al. Cellular and humoral immunity protect against vaginal Zika virus infection in mice. J Virol. 2018 Jan 17. pmid:29343577
- 68. Simanjuntak Y, Liang JJ, Chen SY, Li JK, Lee YL, Wu HC, et al. Ebselen alleviates testicular pathology in mice with Zika virus infection and prevents its sexual transmission. PLoS Pathog. 2018;14(2):e1006854. pmid:29447264
- 69. Duggal NK, McDonald EM, Ritter JM, Brault AC. Sexual transmission of Zika virus enhances in utero transmission in a mouse model. Sci Rep. 2018;8(1):4510. pmid:29540804
- 70. Kumar A, Jovel J, Lopez-Orozco J, Limonta D, Airo AM, Hou S, et al. Human Sertoli cells support high levels of Zika virus replication and persistence. Sci Rep. 2018;8(1):5477. pmid:29615760
- 71. Clancy CS, Van Wettere AJ, Siddharthan V, Morrey JD, Julander JG. Comparative histopathologic lesions of the male reproductive tract during acute infection of Zika virus in AG129 and Ifnar(-/-) mice. Am J Pathol. 2018;188(4):904–15. pmid:29378173
- 72. Centers for Disease Control and Prevention. Zika cases in the United States: cumulative Zika virus disease case counts in the United States, 2015–2018. Atlanta: Centers for Disease Control and Prevention; 2018 [cited 2018 Apr 3]. Available from: https://www.cdc.gov/zika/reporting/case-counts.html
- 73. Spiteri G, Sudre B, Septfons A, Beaute J, European Zika Surveillance Network. Surveillance of Zika virus infection in the EU/EEA, June 2015 to January 2017. Euro Surveill. 2017;22(41).
- 74. Mead PS, Duggal NK, Hook SA, Delorey M, Fischer M, Olzenak McGuire D, et al. Zika virus shedding in semen of symptomatic infected men. N Engl J Med. 2018;378(15):1377–85. pmid:29641964
- 75. Fonseca K, Meatherall B, Zarra D, Drebot M, MacDonald J, Pabbaraju K, et al. First case of Zika virus infection in a returning Canadian traveler. Am J Trop Med Hyg. 2014;91(5):1035–8. pmid:25294619
- 76. Hearn PT, Atkinson B, Hewson R, Brooks T. Identification of the first case of imported Zika Fever to the UK: A novel sample type for diagnostic purposes and support for a potential non-vectorborne route of transmission. Am J Trop Med Hyg. 2014;91(5 Suppl 1):62–3.
- 77. Maria AT, Maquart M, Makinson A, Flusin O, Segondy M, Leparc-Goffart I, et al. Zika virus infections in three travellers returning from South America and the Caribbean respectively, to Montpellier, France, December 2015 to January 2016. Euro Surveill. 2016;21(6):30131. pmid:26898198
- 78. Venturi G, Zammarchi L, Fortuna C, Remoli ME, Benedetti E, Fiorentini C, et al. An autochthonous case of Zika due to possible sexual transmission, Florence, Italy, 2014. Euro Surveill. 2016;21(8):30148. pmid:26939607
- 79. Hills SL, Russell K, Hennessey M, Williams C, Oster AM, Fischer M, et al. Transmission of Zika virus through sexual contact with travelers to areas of ongoing transmission—continental United States, 2016. MMWR Morb Mortal Wkly Rep. 2016;65(8):215–6. pmid:26937739
- 80. Mansuy JM, Dutertre M, Mengelle C, Fourcade C, Marchou B, Delobel P, et al. Zika virus: high infectious viral load in semen, a new sexually transmitted pathogen? Lancet Infect Dis. 2016;16(4):405.
- 81. Barzon L, Pacenti M, Berto A, Sinigaglia A, Franchin E, Lavezzo E, et al. Isolation of infectious Zika virus from saliva and prolonged viral RNA shedding in a traveller returning from the Dominican Republic to Italy, January 2016. Euro Surveill. 2016;21(10):30159. pmid:26987769
- 82. Armstrong P, Hennessey M, Adams M, Cherry C, Chiu S, Harrist A, et al. Travel-associated Zika virus disease cases among U.S. residents—United States, January 2015–February 2016. MMWR Morb Mortal Wkly Rep. 2016;65(11):286–9. pmid:27023833
- 83. Bonaldo MC, Ribeiro IP, Lima NS, Dos Santos AA, Menezes LS, da Cruz SO, et al. Isolation of infective Zika virus from urine and saliva of patients in Brazil. PLoS Negl Trop Dis. 2016;10(6):e0004816. pmid:27341420
- 84. D’Ortenzio E, Matheron S, Yazdanpanah Y, de Lamballerie X, Hubert B, Piorkowski G, et al. Evidence of sexual transmission of Zika virus. N Engl J Med. 2016;374(22):2195–8. pmid:27074370
- 85. Deckard DT, Chung WM, Brooks JT, Smith JC, Woldai S, Hennessey M, et al. Male-to-male sexual transmission of Zika virus—Texas, January 2016. MMWR Morb Mortal Wkly Rep. 2016;65(14):372–4. pmid:27078057
- 86. Atkinson B, Hearn P, Afrough B, Lumley S, Carter D, Aarons EJ, et al. Detection of Zika virus in semen. Emerg Infect Dis. 2016;22(5):940. pmid:27088817
- 87. World Health Organization. Zika situation report: Zika virus, microcephaly and Guillain-Barré syndrome. Geneva: World Health Organization; 2016 Apr 21.
- 88. Turmel JM, Abgueguen P, Hubert B, Vandamme YM, Maquart M, Le Guillou-Guillemette H, et al. Late sexual transmission of Zika virus related to persistence in the semen. Lancet. 2016;387(10037):2501. pmid:27287833
- 89. Reusken C, Pas S, GeurtsvanKessel C, Mogling R, van Kampen J, Langerak T, et al. Longitudinal follow-up of Zika virus RNA in semen of a traveller returning from Barbados to the Netherlands with Zika virus disease, March 2016. Euro Surveill. 2016;21(23):30251. pmid:27313200
- 90. Frank C, Cadar D, Schlaphof A, Neddersen N, Gunther S, Schmidt-Chanasit J, et al. Sexual transmission of Zika virus in Germany, April 2016. Euro Surveill. 2016;21(23):30252. pmid:27311329
- 91. Freour T, Mirallie S, Hubert B, Splingart C, Barriere P, Maquart M, et al. Sexual transmission of Zika virus in an entirely asymptomatic couple returning from a Zika epidemic area, France, April 2016. Euro Surveill. 2016;21(23):30254. pmid:27311680
- 92. Jang HC, Park WB, Kim UJ, Chun JY, Choi SJ, Choe PG, et al. First imported case of Zika virus infection into Korea. J Korean Med Sci. 2016;31(7):1173–7. pmid:27366020
- 93. Wu D, Sun J, Zhong H, Guan D, Zhang H, Tan Q, et al. A family cluster of imported ZIKV cases: viremia period may be longer than previously reported. J Infect. 2016;73(3):300–3. pmid:27373766
- 94. Huits RMHG, De Smet B, Ariën KK, Van Esbroeck M, de Jong BC, Bottieau E, et al. Kinetics of Zika virus persistence in semen. Bull World Health Organ. 2016 Jul 6.
- 95. Prisant N, Bujan L, Benichou H, Hayot PH, Pavili L, Lurel S, et al. Zika virus in the female genital tract. Lancet Infect Dis. 2016;16(9):1000–1.
- 96. Davidson A, Slavinski S, Komoto K, Rakeman J, Weiss D. Suspected female-to-male sexual transmission of Zika virus—New York City, 2016. MMWR Morb Mortal Wkly Rep. 2016;65(28):716–7. pmid:27442327
- 97. Mansuy JM, Pasquier C, Daudin M, Chapuy-Regaud S, Moinard N, Chevreau C, et al. Zika virus in semen of a patient returning from a non-epidemic area. Lancet Infect Dis. 2016;16(8):894–5.
- 98. Barzon L, Pacenti M, Franchin E, Lavezzo E, Trevisan M, Sgarabotto D, et al. Infection dynamics in a traveller with persistent shedding of Zika virus RNA in semen for six months after returning from Haiti to Italy, January 2016. Euro Surveill. 2016;21(32):30316. pmid:27542178
- 99. Nicastri E, Castilletti C, Liuzzi G, Iannetta M, Capobianchi MR, Ippolito G. Persistent detection of Zika virus RNA in semen for six months after symptom onset in a traveller returning from Haiti to Italy, February 2016. Euro Surveill. 2016;21(32):30314. pmid:27541989
- 100. Brooks RB, Carlos MP, Myers RA, White MG, Bobo-Lenoci T, Aplan D, et al. Likely sexual transmission of Zika virus from a man with no symptoms of infection—Maryland, 2016. MMWR Morb Mortal Wkly Rep. 2016;65(34):915–6. pmid:27585037
- 101. de Laval F, Matheus S, Maquart M, Yvrard E, Barthes N, Combes C, et al. Prospective Zika virus disease cohort: systematic screening. Lancet. 2016;388(10047):868.
- 102. Harrower J, Kiedrzynski T, Baker S, Upton A, Rahnama F, Sherwood J, et al. Sexual transmission of Zika virus and persistence in semen, New Zealand, 2016. Emerg Infect Dis. 2016;22(10):1855–7. pmid:27454745
- 103. Atkinson B, Thorburn F, Petridou C, Bailey D, Hewson R, Simpson AJ, et al. Presence and persistence of Zika virus RNA in semen, United Kingdom, 2016. Emerg Infect Dis. 2017;23(4):611–5. pmid:27997333
- 104. Nicastri E, Castilletti C, Balestra P, Galgani S, Ippolito G. Zika virus infection in the central nervous system and female genital tract. Emerg Infect Dis. 2016;22(12):2228–30. pmid:27617352
- 105. Gulland A. First case of Zika virus spread through sexual contact is detected in UK. BMJ. 2016;355:i6500. pmid:27908879
- 106. Zheng NC, Liu YH, Chen Q, Chen MT, Zheng CG, Yang ZW, et al. [Clinical manifestations of 10 cases of imported human Zika virus disease in Jiangmen city of China]. Zhonghua Yi Xue Za Zhi. 2016;96(45):3666–8. pmid:27978904
- 107. Oliveira Souto I, Alejo-Cancho I, Gascon Brustenga J, Peiro Mestres A, Munoz Gutierrez J, Martinez Yoldi MJ. Persistence of Zika virus in semen 93 days after the onset of symptoms. Enferm Infecc Microbiol Clin. 2018;36(1):21–3. pmid:28007310
- 108. Suy A, Sulleiro E, Rodo C, Vazquez E, Bocanegra C, Molina I, et al. Prolonged Zika virus viremia during pregnancy. N Engl J Med. 2016;375(26):2611–3. pmid:27959695
- 109. Gaskell KM, Houlihan C, Nastouli E, Checkley AM. Persistent Zika virus detection in semen in a traveler returning to the United Kingdom from Brazil, 2016. Emerg Infect Dis. 2017;23(1):137–9. pmid:27748650
- 110. Murray KO, Gorchakov R, Carlson AR, Berry R, Lai L, Natrajan M, et al. Prolonged detection of Zika virus in vaginal secretions and whole blood. Emerg Infect Dis. 2017;23(1):99–101. pmid:27748649
- 111. Prisant N, Breurec S, Moriniere C, Bujan L, Joguet G. Zika virus genital tract shedding in infected women of childbearing age. Clin Infect Dis. 2017;64(1):107–9. pmid:27682065
- 112. Froeschl G, Huber K, von Sonnenburg F, Nothdurft HD, Bretzel G, Hoelscher M, et al. Long-term kinetics of Zika virus RNA and antibodies in body fluids of a vasectomized traveller returning from Martinique: a case report. BMC Infect Dis. 2017;17(1):55. pmid:28068904
- 113. Russell K, Hills SL, Oster AM, Porse CC, Danyluk G, Cone M, et al. Male-to-female sexual transmission of Zika virus—United States, January–April 2016. Clin Infect Dis. 2017;64(2):211–3. pmid:27986688
- 114. Visseaux B, Mortier E, Houhou-Fidouh N, Brichler S, Collin G, Larrouy L, et al. Zika virus in the female genital tract. Lancet Infect Dis. 2016;16(11):1220.
- 115. Arsuaga M, Bujalance SG, Diaz-Menendez M, Vazquez A, Arribas JR. Probable sexual transmission of Zika virus from a vasectomised man. Lancet Infect Dis. 2016;16(10):1107.
- 116. Mansuy JM, Suberbielle E, Chapuy-Regaud S, Mengelle C, Bujan L, Marchou B, et al. Zika virus in semen and spermatozoa. Lancet Infect Dis. 2016;16(10):1106–7. pmid:27676340
- 117. Torres JR, Martinez N, Moros Z. Microhematospermia in acute Zika virus infection. Int J Infect Dis. 2016;51:127. pmid:27639453
- 118. Matheron S, d’Ortenzio E, Leparc-Goffart I, Hubert B, de Lamballerie X, Yazdanpanah Y. Long-lasting persistence of Zika virus in semen. Clin Infect Dis. 2016;63(9):1264. pmid:27470244
- 119. World Health Organization. Zika virus infection—Chile. Geneva: World Health Organization; 2016 [cited 2018 Jun 19]. Available from: http://www.who.int/csr/don/15-april-2016-zika-chile/en/.
- 120. World Health Organization. Zika virus infection—Peru. Geneva: World Health Organization; 2016 [cited 2018 Jun 19]. Available from: http://www.who.int/csr/don/21-april-2016-zika-peru/en/.
- 121. World Health Organization. Zika virus infection—Argentina and France. Geneva: World Health Organization; 2016 [cited 2018 Jun 19]. Available from: http://www.who.int/csr/don/7-march-2016-zika-argentina-and-france/en/.
- 122. Iovine NM, Lednicky J, Cherabuddi K, Crooke H, White SK, Loeb JC, et al. Coinfection with Zika and dengue-2 viruses in a traveler returning from Haiti, 2016: clinical presentation and genetic analysis. Clin Infect Dis. 2017;64(1):72–5. pmid:27694479
- 123. Boggild AK, Geduld J, Libman M, Yansouni CP, McCarthy AE, Hajek J, et al. Surveillance report of Zika virus among Canadian travellers returning from the Americas. CMAJ. 2017;189(9):E334–40. pmid:28280063
- 124. Duijster JW, Brandwagt DAH, Timen A, van der Eijk AA, Vennema H, Te Wierik MJM. [Zika virus and sexual transmission; when and for how long does one need to use a condom?]. Ned Tijdschr Geneeskd. 2017;161(0):D1622. pmid:28745255
- 125. Sun J, Wu D, Zhong H, Guan D, Zhang H, Tan Q, et al. Returning ex-patriot Chinese to Guangdong, China, increase the risk for local transmission of Zika virus. J Infect. 2017;75(4):356–67. pmid:28712937
- 126. Percivalle E, Zavattoni M, Fausto F, Rovida F. Zika virus isolation from semen. New Microbiol. 2017;40(3):197–8. pmid:28513814
- 127. Borrego BV, Kosanic K, de Ory F, Merino Fernandez FJ, Rodriguez BG. [Zika virus infection acquired through sexual contact: first documented case of local transmission in Spain]. Emergencias. 2017;29(4):290–1.
- 128. de Laval F, Matheus S, Labrousse T, Enfissi A, Rousset D, Briolant S. Kinetics of Zika viral load in semen. N Engl J Med. 2017;377(7):697–9. pmid:28813216
- 129. Joguet G, Mansuy JM, Matusali G, Hamdi S, Walschaerts M, Pavili L, et al. Effect of acute Zika virus infection on sperm and virus clearance in body fluids: a prospective observational study. Lancet Infect Dis. 2017;17(11):1200–8. pmid:28838639
- 130. Garcia-Bujalance S, Gutierrez-Arroyo A, De la Calle F, Diaz-Menendez M, Arribas JR, Garcia-Rodriguez J, et al. Persistence and infectivity of Zika virus in semen after returning from endemic areas: report of 5 cases. J Clin Virol. 2017;96:110–5. pmid:29053990
- 131. Grossi PA, Percivalle E, Campanini G, Sarasini A, Premoli M, Zavattoni M, et al. An autochthonous sexually transmitted Zika virus infection in Italy 2016. New Microbiol. 2018;41(1):80–2. pmid:29112768
- 132. Huits R, De Smet B, Arien KK, Van Esbroeck M, Bottieau E, Cnops L. Zika virus in semen: a prospective cohort study of symptomatic travellers returning to Belgium. Bull World Health Organ. 2017;95(12):802–9. pmid:29200521
- 133. Sanchez-Montalva A, Pou D, Sulleiro E, Salvador F, Bocanegra C, Trevino B, et al. Zika virus dynamics in body fluids and risk of sexual transmission in a non-endemic area. Trop Med Int Health. 2018;23(1):92–100. pmid:29194880
- 134. Barzon L, Percivalle E, Pacenti M, Rovida F, Zavattoni M, Del Bravo P, et al. Virus and antibody dynamics in travelers with acute Zika virus infection. Clin Infect Dis. 2018;66(8):1173–80. pmid:29300893
- 135. Cassuto NG, Marras G, Jacomo V, Bouret D. Persistence of Zika virus in gradient sperm preparation. J Gynecol Obstet Hum Reprod. 2018;47(5):211–12. pmid:29510270
- 136. Biava M, Caglioti C, Castilletti C, Bordi L, Carletti F, Colavita F, et al. Persistence of ZIKV-RNA in the cellular fraction of semen is accompanied by a surrogate-marker of viral replication. Diagnostic implications for sexual transmission. New Microbiol. 2018;41(1):30–3. pmid:29112766
- 137. Desclaux A, de Lamballerie X, Leparc-Goffart I, Vilain-Parce A, Coatleven F, Fleury H, et al. Probable sexually transmitted Zika virus infection in a pregnant woman. N Engl J Med. 2018;378(15):1458–60. pmid:29641959
- 138. Jia H, Zhang M, Chen M, Yang Z, Li J, Huang G, et al. Zika virus infection in travelers returning from countries with local transmission, Guangdong, China, 2016. Travel Med Infect Dis. 2018;21:56–61. pmid:29183824
- 139. Gao D, Lou Y, He D, Porco TC, Kuang Y, Chowell G, et al. Prevention and control of Zika as a mosquito-borne and sexually transmitted disease: a mathematical modeling analysis. Sci Rep. 2016;6:28070. pmid:27312324
- 140. Towers S, Brauer F, Castillo-Chavez C, Falconar AKI, Mubayi A, Romero-Vivas CME. Estimate of the reproduction number of the 2015 Zika virus outbreak in Barranquilla, Colombia, and estimation of the relative role of sexual transmission. Epidemics. 2016;17:50–5. pmid:27846442
- 141. Habu A, Murakami Y, Ogasa A, Fujisaki Y. [Disorder of spermatogenesis and viral discharge into semen in boars infected with Japanese encephalitis virus (author’s transl)]. Uirusu. 1977;27(1):21–6. pmid:203101
- 142. Guerin B, Pozzi N. Viruses in boar semen: detection and clinical as well as epidemiological consequences regarding disease transmission by artificial insemination. Theriogenology. 2005;63(2):556–72. pmid:15626416
- 143. Zhang Y, Jing J, Li X, Wang J, Feng X, Cao R, et al. Integration analysis of miRNA and mRNA expression profiles in swine testis cells infected with Japanese encephalitis virus. Infect Genet Evol. 2015;32:342–7. pmid:25847692
- 144. Althouse GC, Rossow K. The potential risk of infectious disease dissemination via artificial insemination in swine. Reprod Domest Anim. 2011;46(Suppl 2):64–7. pmid:21884281
- 145. Liu WJ, Zhu M, Pei JJ, Dong XY, Liu W, Zhao MQ, et al. Molecular phylogenetic and positive selection analysis of Japanese encephalitis virus strains isolated from pigs in China. Virus Res. 2013;178(2):547–52. pmid:24045128
- 146. Kelley RE, Berger JR, Kelley BP. West Nile virus meningo-encephalitis: possible sexual transmission. J La State Med Soc. 2016;168(1):21–2. pmid:26986863
- 147. Armah HB, Wang G, Omalu BI, Tesh RB, Gyure KA, Chute DJ, et al. Systemic distribution of West Nile virus infection: postmortem immunohistochemical study of six cases. Brain Pathol. 2007;17(4):354–62. pmid:17610522
- 148. Burke SA, Wen L, King NJ. Routes of inoculation and the immune response to a resolving genital flavivirus infection in a novel murine model. Immunol Cell Biol. 2004;82(2):174–83. pmid:15061771
- 149. Fujita N, Izaki S, Oda K, Kimura E, Hotta S. Research on dengue in tissue culture. V. Production of anti-dengue antibodies in tissue cultures from monkeys inoculated with an attenuated type I dengue virus strain. Kobe J Med Sci. 1972;18(3):143–52. pmid:4200059
- 150. Prabandari SA, Iskandriati D, Saepuloh U, Arifin E, Pamungkas J. Tissues lesions associated with primary infection of dengue-3 virus in pigtailed macaques (Macaca nemestrina). Exp Anim. 2015;64:S27.
- 151. Rasinhas ADC, Silva M, Caldas GC, Jacome FC, Leonardo R, Santos FBD, et al. First detection of dengue virus in the saliva of immunocompetent murine model. Mem Inst Oswaldo Cruz. 2018;113(4):e170208. pmid:29412340
- 152. Torres JR, Liprandi F, Goncalvez AP. Acute parotitis due to dengue virus. Clin Infect Dis. 2000;31(5):E28–9. pmid:11073786
- 153. Mizuno Y, Kotaki A, Harada F, Tajima S, Kurane I, Takasaki T. Confirmation of dengue virus infection by detection of dengue virus type 1 genome in urine and saliva but not in plasma. Trans R Soc Trop Med Hyg. 2007;101(7):738–9. pmid:17418320
- 154. Sriprapun M, Laosakul C, Krajiw S, Arunyingmongkol K, Siriyasatien P, Kulwichit W. Prolonged and differential shedding of dengue virus serotype 2 (DEN2) in plasma, PBMCs, saliva and urine of adult patients during acute infection. Clin Microbiol Infect. 2012;18:28.
- 155. Sriprapun M, Laosakul C, Krajiw S, Arunyingmongkol K, Siriyasatien P, Kulwichit W. Time course of concurrent infection with dengue virus serotypes 2 and 4 detected in urine. Asian Biomed. 2015;9(2):197–202.
- 156. Iannetta M, Lalle E, Musso M, Carletti F, Scorzolini L, D’Abramo A, et al. Persistent detection of dengue virus RNA in vaginal secretion of a woman returning from Sri Lanka to Italy, April 2017. Euro Surveill. 2017;22(34):30600. pmid:28857045
- 157. Gerlinskaia LA, Bakhvalova VN, Morozova OV, Tsekhanovskaia NA, Matveeva VA, Moshkin MP. [Sexual transmission of tick-borne encephalitis virus in laboratory mice] [Article in Russian]. Biull Eksp Biol Med. 1997;123(3):327–8. pmid:9162247
- 158. Vandenbroucke JP. Case reports in an evidence-based world. J R Soc Med. 1999;92(4):159–63. pmid:10450190
- 159. Muller JA, Harms M, Schubert A, Jansen S, Michel D, Mertens T, et al. Inactivation and environmental stability of Zika virus. Emerg Infect Dis. 2016;22(9):1685–7. pmid:27367466
- 160. Vink MA, Bootsma MC, Wallinga J. Serial intervals of respiratory infectious diseases: a systematic review and analysis. Am J Epidemiol. 2014;180(9):865–75. pmid:25294601
- 161. Majumder MS, Cohn E, Fish D, Brownstein JS. Estimating a feasible serial interval range for Zika fever. Bull World Health Organ. 2016 Feb 9.
- 162. Coelho FC, Durovni B, Saraceni V, Lemos C, Codeco CT, Camargo S, et al. Higher incidence of Zika in adult women than adult men in Rio de Janeiro suggests a significant contribution of sexual transmission from men to women. Int J Infect Dis. 2016;51:128–32. pmid:27664930
- 163. Funk S, Kucharski AJ, Camacho A, Eggo RM, Yakob L, Murray LM, et al. Comparative analysis of dengue and Zika outbreaks reveals differences by setting and virus. PLoS Negl Trop Dis. 2016;10(12):e0005173. pmid:27926933
- 164. Dejnirattisai W, Supasa P, Wongwiwat W, Rouvinski A, Barba-Spaeth G, Duangchinda T, et al. Dengue virus sero-cross-reactivity drives antibody-dependent enhancement of infection with Zika virus. Nat Immunol. 2016;17(9):1102–8. pmid:27339099
- 165. Duffy MR, Chen TH, Hancock WT, Powers AM, Kool JL, Lanciotti RS, et al. Zika virus outbreak on Yap Island, Federated States of Micronesia. N Engl J Med. 2009;360(24):2536–43. pmid:19516034
- 166. Aubry M, Teissier A, Huart M, Merceron S, Vanhomwegen J, Roche C, et al. Zika virus seroprevalence, French Polynesia, 2014–2015. Emerg Infect Dis. 2017;23(4):669–72. pmid:28084987
- 167. Gallian P, Cabie A, Richard P, Paturel L, Charrel RN, Pastorino B, et al. Zika virus in asymptomatic blood donors in Martinique. Blood. 2017;129(2):263–6. pmid:27827826