Zika virus (ZIKV) originated and continues to circulate in a sylvatic transmission cycle between non-human primate hosts and arboreal mosquitoes in tropical Africa. Recently ZIKV invaded the Americas, where it poses a threat to human health, especially to pregnant women and their infants. Here we examine the risk that ZIKV will establish a sylvatic cycle in the Americas, focusing on Brazil. We review the natural history of sylvatic ZIKV and present a mathematical dynamic transmission model to assess the probability of establishment of a sylvatic ZIKV transmission cycle in non-human primates and/or other mammals and arboreal mosquito vectors in Brazil. Brazil is home to multiple species of primates and mosquitoes potentially capable of ZIKV transmission, though direct assessment of host competence (ability to mount viremia sufficient to infect a feeding mosquito) and vector competence (ability to become infected with ZIKV and disseminate and transmit upon subsequent feedings) of New World species is lacking. Modeling reveals a high probability of establishment of sylvatic ZIKV across a large range of biologically plausible parameters. Probability of establishment is dependent on host and vector population sizes, host birthrates, and ZIKV force of infection. Research on the host competence of New World monkeys or other small mammals to ZIKV, on vector competence of New World Aedes, Sabethes, and Haemagogus mosquitoes for ZIKV, and on the geographic range of potential New World hosts and vectors is urgently needed. A sylvatic cycle of ZIKV would make future elimination efforts in the Americas practically impossible, and paints a dire picture for the epidemiology of ZIKV and our ability to end the ongoing outbreak of congenital Zika syndrome.
Citation: Althouse BM, Vasilakis N, Sall AA, Diallo M, Weaver SC, Hanley KA (2016) Potential for Zika Virus to Establish a Sylvatic Transmission Cycle in the Americas. PLoS Negl Trop Dis 10(12): e0005055. https://doi.org/10.1371/journal.pntd.0005055
Published: December 15, 2016
Copyright: © 2016 Althouse 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.
Funding: We gratefully acknowledge the impetus for this work provided by the NSF-RCN on Infectious Disease Evolution Across Scales (RCN-IDEAS) program. This work was supported by grants 1R15AI113628-01 (KAH), RO1AI069145 (BMA, SCW, KAH), R24AI120942 (NV, SCW), 1U01AI115577-01 (NV, SCW, KAH) from the U.S. National Institutes of Health. BMA thanks Bill and Melinda Gates for their active support of this work and their sponsorship through the Global Good Fund. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
The invasion of Brazil by Zika virus (ZIKV) is the latest upheaval in a decade-long emergence of viruses transmitted by the mosquito Aedes aegypti in the Americas [1, 2]. Dengue virus (DENV) moved into Florida in 2009 , Arizona in 2014 and Hawaii in 2015 ; chikungunya virus (CHIKV) was introduced into the Caribbean in 2013 and spread extensively and rapidly across both Central and South America , reaching dozens of countries within a year of introduction ; and, in 2015, ZIKV was first detected in Brazil. With ZIKV came a spike in cases of congenital microcephaly and Guillain-Barre syndrome [2, 7]. The introduction of ZIKV to the Americas had been predicted well in advance of the event [8, 9], however, the association of ZIKV infection with neuropathology and teratogenicity were only revealed during the spread of the virus through the Pacific and into Brazil . Hayes  did warn in 2009 that the spread of ZIKV warranted concern despite lack of contemporary evidence for severe ZIKV disease. He reminded the scientific community that West Nile virus was also considered a “relatively innocuous pathogen” until it ushered outbreaks of neuroinvasive disease into Europe and the Americas. In response to a growing body of evidence linking ZIKV infection with teratogenic effects [11–14], the World Health Organization declared the ZIKV outbreak a public health emergency of international concern in February of 2016 [1, 15].
ZIKV is unusual among the arthropod-borne viruses (arboviruses) in its capacity for sustained transmission in a human-endemic cycle. This capacity is shared by three other arboviruses that are also, not coincidentally, transmitted in the human cycle by Aedes aegypti: DENV, CHIKV and yellow fever virus (YFV). For all four viruses, human-endemic lineages emerged from ecologically and evolutionarily distinct, sylvatic, enzootic cycles transmitted between mostly arboreal Aedes spp. vectors and non-human animal hosts [8, 16, 17]. While non-human primates (hereafter primates) have generally been considered the major reservoir hosts for the sylvatic transmission cycle of all four viruses, this paradigm is based on scant evidence and researchers in the field have repeatedly cautioned that other animal species may play key roles in the transmission dynamics of these viruses [8, 16, 18, 19]. The ancestral sylvatic cycles of YFV, CHIKV and ZIKV occur in Africa, while the DENV ancestral cycle occurs in Southeast Asia with later transport to West Africa and enzootic establishment . YFV was transported from Africa to the Americas in infected humans and mosquitoes via the slave trade in the 17th and 18th centuries  and spilled back into a sylvatic cycle, maintained in New World monkey species, which persists today. Sylvatic YFV outbreaks are a regular occurrence in Brazil and demand swift reactive vaccination to control [16, 22–24]. Such sylvatic circulation in Brazil generates a persistent risk for a grave outbreak of the virus in Brazilian megacity such as São Paolo; this risk is elevated during epizootics of the virus [25–30]. DENV, in contrast, has not established a sylvatic transmission cycle in the Americas despite widespread circulation of the virus across the Americas in the human-endemic cycle . Whether ZIKV will emulate YFV or DENV is an open and urgent question. If the virus establishes a sylvatic cycle in the Americas, then mosquito control and even herd immunity from vaccination or limitation of transmission from demographic turnover will not suffice to eradicate it from the region.
ZIKV was first isolated in 1947 from a sentinel monkey in the Ziika forest of Uganda. Intriguingly the sentinel species used was the rhesus macaque, demonstrating the susceptibility of Asian primates to ZIKV. The next year ZIKV was isolated from Ae. africanus in the area, suggesting mosquito-borne transmission of the virus. As laid out in the comprehensive review by Hayes, the virus was subsequently detected across a wide swath of tropical Africa via serosurveys of monkeys as well as virus isolation from monkeys and several species of sylvatic Aedes . Notably these mosquitoes were collected in the forest canopy but also on the forest floor. Infection of humans living in proximity to sylvatic cycles was detected via serosurveys and clinical surveillance. Seroprevalence was variable and quite high (up to 40%) in some human populations, but disease was invariably mild, generally manifesting as fever, headache, rash and conjunctivitis. In 2007, an outbreak of ZIKV in Libreville, the capital of Gabon was thought to have been vectored by the peri-urban mosquito species Aedes albopictus . Importantly, experimental studies of the interaction among different African arboviruses have shown evidence for both enhancement [32, 33] and interference .
Over the same time period that the ZIKV transmission cycle was being investigated in Africa, circulation of ZIKV was documented in several countries in Asia. Albert Rudnick, the pioneer of sylvatic DENV research, isolated ZIKV from Ae. aegypti in Malaysia . A serological study in 1977-78 in Central Java revealed that a high percentage of febrile patients had antibodies against ZIKV . Subsequently ZIKV infection was documented in travellers returning from Indonesia , Thailand , and Malaysia  and in residents of Indonesia [39, 40], Cambodia , the Philippines , and Thailand . One of the cases of Zika infection in a traveller was notable because disease onset occurred five days after being bitten by a monkey in Indonesia . Anti-ZIKV antibodies have also been detected semi-captive orangutans in Malaysia . To date there has been no solid evidence of an Asian sylvatic cycle of ZIKV, but such a sylvatic cycle could be widespread and still go undetected due to the lack of surveillance for sylvatic arboviruses in Southeast Asia . Thus it remains uncertain whether all human ZIKV infections in Asia derive from the human-endemic cycle or whether some may occur due to spillover from an as-yet undescribed sylvatic cycle in the region. The lineage of ZIKV that circulates in Asia is distinct from the African lineages of the virus, and it is the Asian lineage that spread across the Pacific and into Brazil .
Research on the sylvatic cycle of ZIKV since 2007 has focused primarily on West Africa. Phylogenetic analysis indicates that the virus has been introduced into West Africa at least twice in the twentieth century  and that West Africa contains ZIKV strains that are distinct from those elsewhere in Africa . Analyses of mosquitoes collected annually over the last fifty years in Kedougou, Senegal demonstrated that ZIKV is amplified in mosquito collections at approximately four year intervals, that rainfall is a positive predictor of ZIKV isolations in mosquitoes, and that there was little positive or negative association between amplification of ZIKV and of three other Aedes-borne arboviruses that circulate in the region, YFV, DENV-2 and CHIKV . Moreover our field studies in Kedougou during the 2011 ZIKV amplification showed that the virus was present in all major land cover classes in the region but was detected significantly more often in the forest than in other land cover types . In this study, ZIKV was detected in ten separate species of Aedes, with Ae. hirsutus, Ae. unilineatus, Ae. metallicus, and Ae. africanus having the highest minimum infection rates of collected species. ZIKV was also found in Culex perfuscus. Finally, one pool of male Ae. furcifer was found to be positive, indicating possible vertical transmission of ZIKV. To follow up these field observations, Diagne et al. tested the vector competence of multiple Senegalese Aedes species for ZIKV in the laboratory and found that only Ae. luteocephalus and Ae. vittatus were capable of transmitting the virus . ZIKV has previously been isolated from two of the three monkey species resident in Kedougou: African green monkeys (Chlorocebus sabaeus) and patas monkeys (Erythrocebus patas) (reviewed in ) In combination with previous field studies in Africa, these findings demonstrate that the transmission dynamics of ZIKV are complex and that a diverse network of Aedes vector species and primate host species participate in the maintenance of the sylvatic ZIKV cycle.
Here, we used a mathematical model that we have previously employed to study the sylvatic DENV cycle in Senegal  to identify the conditions of host and vector density and connectivity that would permit the establishment of an American sylvatic cycle of ZIKV.
Establishing a Sylvatic ZIKV Cycle
Our model extends, to our knowledge, the only previous dynamic model of mosquito-borne viruses in non-human primate hosts [54, 55]. While our previous modeling study was focused on sylvatic DENV , the strong similarities between sylvatic DENV and sylvatic ZIKV transmission cycle make the model a good fit for both viruses. Here we use the model to ask whether ZIKV will establish a self-sustaining transmission cycle in a population of susceptible hosts with competent vector mosquito populations after introduction of a single ZIKV-infected host. We assume host and vector species interact as separate populations, and thus populations correspond to separate species. Here we explore primates and mosquitoes as the hosts and vectors.
Briefly, mosquitoes and primates are born susceptible to ZIKV infection, and are infected at a rate proportional to the number of bites given or received per day and a probability of infection. Primate species differ in their life history, particularly birthrate and lifespan. We assume birthrate = 1/lifespan, which is conservative as age of fertility completion is younger than age of mortality for many primates  (see S1 Text and S1 Fig for a discussion of the rate of primate population turnover and [57, 58]). Transmission probabilities vary seasonally due to differences in rainfall and temperature . We explore three per-bite infection probabilities (0.3, 0.6, 0.9) with an average of 0.5 bites per day. This gives forces of infection 0.15, 0.3, 0.45, which is in line with observed sylvatic DENV forces of infection from primate collections in Kedougou, Senegal in 2010-2012 . These forces of infection ranged from 0.09 (95% CI: 0.07, 0.11) for Guinea baboons (Papio papio) in 2012, to 0.41 (95% CI: 0.26, 0.76) for African green monkeys (Chlorocebus sabaeus) in 2012. After infection, primates recover at a fixed rate (4 days ) while mosquitoes are infected for the remainder of their life. We employ the stochastic version of the model simulated using a Gillespie stochastic simulation algorithm with the Binomial Tau leap approximation (BTL) to examine the effects of population size, primate birthrate, and force of infection on the probability of ZIKV establishment. Simulations were run and we calculated the proportion of simulations not becoming extinct after introduction of a ZIKV infected host (ie, establishing a sylvatic cycle). Full model equations and parameters are given in S1 Text and S1 Table.
Model simulations suggest the probability of establishment is highly dependent on the primate birthrate (Fig 1). In low and medium force of infection settings (0.15 and 0.3) primates with lifespans of 15 and 25 years show little probability of sylvatic establishment (panels d, g, h). However, if there exists a rapidly reproducing primate (lifespan of 5 years), establishment of a sylvatic cycle is nearly assured (panels a, b, c). Generally, increasing numbers of primates relative to mosquitoes lowers the probability of establishment, as might be expected as the force of infection is directly proportional to the number of mosquitoes and inversely proportional to the number of primates [50, 61]. A network of as few as 6,000 primates with 10,000 mosquitoes is capable of supporting the establishment of a ZIKV sylvatic cycle.
Figure shows heat maps of the probability of ZIKV establishment in 3 years for 50 simulations per parameter set with colors ranging from blue (no simulations establishing) to red (all simulations establishing). Contour lines show 0.25, 0.5, 0.75, and 0.95 probability of establishment. For each plot, the x-axis shows the total number of mosquitoes (in two populations) and the y-axis shows the total number of non-human primates (in two populations). Left to right the panels indicate increasing in force of infection, and top to bottom decreasing non-human primate birthrate (as 1/lifespan). Other parameters: mean mosquito lifespan = 7 days; mean ZIKV recovery in NHP = 4 days; mosquito vertical transmission of ZIKV = 0; rate of yearly ZIKV introduction = 0.
Box 1 summarizes key priorities for the research needed to better assess the risk and consequences of the establishment of a sylvatic ZIKV cycle in the Americas. The most pressing of these are laboratory studies of New World primate infections with ZIKV. To our knowledge, the susceptibility of New World monkeys to ZIKV has not yet been tested, and it is possible that they are insusceptible to ZIKV infection or generate only low levels of viremia insufficient to infect potential sylvatic vectors. However, as we have pointed out in a previous review, there are also free-living populations of several Old World monkey species in the Americas, some of which, notably African green monkeys (which as noted above had high forces of sylvatic DENV infection in Senegal) are known to be hosts of sylvatic ZIKV in Africa [16, 62]. Our model predicts that the presence of a rapidly reproducing primate or other mammal that is a competent host for ZIKV vastly increases the chances of establishment of a sylvatic cycle. There is some serological evidence that vertebrates other than primates may also serve as enzootic reservoirs of ZIKV [39, 63].
Box 1: Research Priorities to Predict the Likelihood of Establishment of a Sylvatic Zika Virus Cycle in the Americas
Priority 1: Laboratory studies of host competence of New World primate species for ZIKV. These studies should (i) incorporate multiple species of New World primates, (ii) compare virus dynamics resulting from virus delivery by needle versus mosquito vector, (iii) monitor intra-host infection dynamics over a duration that exceeds that currently documented for viremia in humans, (iv) utilize both culture and direct mosquito feeding to detect viremia, and (v) monitor pathogenesis of the virus with a focus on potential impact of the virus on survival .
Priority 2: Laboratory studies of host competence of New World mammal species other than primates for ZIKV. In particular we would recommend that canopy-dwelling species such as squirrels and bats be considered for such studies.
Priority 3: Laboratory studies of vector competence of New World mosquito species, particularly vectors with the capacity to maintain maintenance a sylvatic ZIKV cycle, such as species in the genera Sabethes and Haemogogus, and potential bridge vectors such as Aedes albopictus that could move the virus from the sylvatic cycle to humans.
Priority 4: Serosurveys to detect spillback of ZIKV into feral populations of Old World primate species living in the Americas. An ideal site for such a study would be the island of St. Kitts. As of this writing, ZIKV had not reached St. Kitts. The island hosts a large population of African green monkeys (Chlorocebus sabaeus) that come into close contact with humans . It should be possible to establish a baseline serosurvey in these animals prior to the arrival of ZIKV and then monitor them for spillback and possible sylvatic circulation of ZIKV.
Priority 5: Ecological monitoring to detect spillback and sylvatic circulation of ZIKV in the Americas. Serological surveillance of primate populations living in close proximity to urban centers should be initiated to detect initial spillback of ZIKV into monkeys. In parallel, more isolated primate populations should be sampled to detect unambiguous sylvatic circulation. Additionally, mosquitoes should be collected at sites where serosurveys are occurring and should be screened for ZIKV. Studies of mosquitoes should be as inclusive as possible, to avoid failure to detect ZIKV in species in which it is not, a priori, expected to be found.
Priority 6: Active surveillance of human populations living close to areas known for sylvatic introduction to assess and monitor the threat of bridging transmission from the sylvatic back into the urban cycle.
Priority 7: Incorporation of species presence records into distribution maps to assess the overlap of host and vector species with potential for sylvatic ZIKV maintenance.
We also do not know the competence of most New World Aedes species to transmit ZIKV. However, it has recently been shown that Ae. albopictus was likely the primary vector of a ZIKV outbreak in humans in Gabon . This mosquito species, which is common in the Americas, has a broad host range and has high potential to serve as a bridge vector to transfer the virus from humans to non-human animals . Ae. albopictus has been spreading through Brazil for decades . Additionally, Sabethes and Haemagogus spp. mosquitoes are tropical New World vectors of sylvatic YFV and thus may be likely vectors of sylvatic ZIKV as well . Additionally, more research is necessary on the extrinsic incubation period of ZIKA in relevant mosquito species [66, 67]. A longer extrinsic incubation period may reduce the potential for transmission for a mosquito of a given lifespan .
The current work is limited by gaps in knowledge, and relies on sylvatic ZIKV transmission dynamics being similar to sylvatic DENV and YFV transmission—a reasonable assumption given the extensive overlap of the two viruses in the hosts and vectors used in West African sylvatic cycles [8, 16, 51]. We note that our model calculates the probability of ZIKV establishment starting from a single infectious introduction without further importation, and does not include vertical transmission of ZIKV within mosquitoes. These features both make our estimates conservative and paints a dire picture for the epidemiology of ZIKV and for prospects of extinguishing the ongoing congenital Zika syndrome outbreak in Brazil.
The International Task Force for Disease Eradication identifies a key factor for considering a disease eradicable as epidemiologic vulnerability, including not having the presence of an animal reservoir . Establishment of a sylvatic cycle of ZIKV would make future elimination efforts in the Americas extremely difficult if not impossible. Taking lessons from sylvatic YFV in Brazil, reactive, and massive vaccination efforts will be necessary if and when a ZIKV vaccine becomes available to control ZIKV transmission , decrease morbidity, and protect unborn infants from teratogenic effects. We use this work to identify and highlight key lines of research that would enable the public health community to understand ZIKV transmission going forward and target surveillance for enzootic ZIKV to those animal populations most likely to sustain virus transmission.
- Sylvatic Zika virus has exhibited periodic amplifications in Senegal for at least the past 50 years.
- Non-human primate species capable of becoming hosts for sylvatic Zika virus are abundant in South America—Brazil in particular.
- Mathematical modeling of Zika virus transmission suggests a high likelihood of establishment of a sylvatic Zika cycle in the forests of South America.
- More research is necessary to ascertain the suitability of wild hosts in the tropics for supporting Zika transmission, as well as the competence of vectors present in South America.
Key papers in the field
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- Diallo D, Chen R, Diagne CT, Ba Y, Dia I, Sall AA, et al. Bloodfeeding patterns of sylvatic arbovirus vectors in southeastern Senegal. Transactions of The Royal Society of Tropical Medicine and Hygiene. 2013;107(3):200?203
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- Hayes EB. Zika virus outside Africa. Emerg Infect Dis. 2009;15(9):1347?50. doi:10.3201/eid1509.090442
S1 Fig. Population turnover of selected non-human primate species.
Figure shows the average number of litters per year and the turnover rate (1/litters per year) against maximum natural lifespan for 156 primate species. Those species found in Brazil are highlighted in green and include: Alouatta caraya, Alouatta seniculus, Aotus azarai, Aotus trivirgatus, Ateles belzebuth, Ateles paniscus, Brachyteles arachnoides, Cacajao calvus, Callicebus cupreus, Callimico goeldii, Callithrix flaviceps, Callithrix jacchus, Callithrix penicillata, Callithrix pygmaea, Cebus apella, Cebus olivaceus, Chiropotes albinasus, Chiropotes satanas, Lagothrix lagotricha, Leontopithecus rosalia, Pithecia monachus, Pithecia pithecia, Saguinus bicolor, Saguinus fuscicollis, Saguinus imperator, Saguinus labiatus, Saguinus midas, Saguinus mystax, Saguinus nigricollis. Data from Ernest et al. Life history characteristics of placental nonvolant mammals: ecological archives E084-093. Ecology. 2003;84(12):3402?3402.
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