Browse Subject Areas

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Survival analysis of confirmed elephant endotheliotropic herpes virus cases in Thailand from 2006 – 2018

  • Khajohnpat Boonprasert,

    Roles Data curation, Formal analysis, Resources, Writing – original draft

    Affiliation Center of Elephant and Wildlife Research, Chiang Mai University, Chiang Mai, Thailand

  • Veerasak Punyapornwithaya,

    Roles Formal analysis, Methodology

    Affiliations Center of Elephant and Wildlife Research, Chiang Mai University, Chiang Mai, Thailand, Department of Food Animal Clinic, Faculty of Veterinary Medicine, Chiang Mai University, Chiang Mai, Thailand

  • Pallop Tankaew,

    Roles Methodology, Resources, Writing – review & editing

    Affiliation Center of Elephant and Wildlife Research, Chiang Mai University, Chiang Mai, Thailand

  • Taweepoke Angkawanish,

    Roles Resources, Writing – review & editing

    Affiliation Elephant Hospital, National Elephant Institute, Forest Industry Organization, Lampang, Thailand

  • Supaphen Sriphiboon,

    Roles Methodology, Resources, Writing – review & editing

    Affiliation Department of Large Animal and Wildlife Clinical Sciences, Faculty of Veterinary Medicine, Kasetsart University, Kamphaeng Saen Campus, Nakornpathom, Thailand

  • Chatchote Titharam,

    Roles Conceptualization, Resources, Supervision, Writing – review & editing

    Affiliations Center of Elephant and Wildlife Research, Chiang Mai University, Chiang Mai, Thailand, Department of Companion Animal and Wildlife Clinics, Faculty of Veterinary Medicine, Chiang Mai University, Chiang Mai, Thailand

  • Janine L. Brown,

    Roles Supervision, Writing – review & editing

    Affiliation Center for Species Survival, Smithsonian Conservation Biology Institute, Front Royal, Virginia, United State of America

  • Chaleamchat Somgird

    Roles Conceptualization, Data curation, Project administration, Supervision, Validation, Writing – review & editing

    Affiliations Center of Elephant and Wildlife Research, Chiang Mai University, Chiang Mai, Thailand, Department of Companion Animal and Wildlife Clinics, Faculty of Veterinary Medicine, Chiang Mai University, Chiang Mai, Thailand

Survival analysis of confirmed elephant endotheliotropic herpes virus cases in Thailand from 2006 – 2018

  • Khajohnpat Boonprasert, 
  • Veerasak Punyapornwithaya, 
  • Pallop Tankaew, 
  • Taweepoke Angkawanish, 
  • Supaphen Sriphiboon, 
  • Chatchote Titharam, 
  • Janine L. Brown, 
  • Chaleamchat Somgird


The elephant endotheliotropic herpesvirus (EEHV) has been a known cause of death of young elephants in Thailand for over a decade. In this study, we report on the demography, disease characteristics and mortality of 58 elephants with confirmed EEHV hemorrhagic disease between January 2006 and August 2018 using retrospective data subjected to survival analysis. Median age of EEHV presentation was 29 months, and the mortality rate was 68.97% with a median survival time of 36 h. Most EEHV cases occurred in the north of Thailand, the region where most of the country’s captive elephants reside. The hazard ratio analysis identified application of medical procedures and antiviral medications as being significant factors correlated to the risk of death. Our results indicate a need to focus EEHV monitoring efforts on young elephants and to follow current protocols that advise starting treatments before clinical signs appear.


The elephant endotheliotropic herpes virus (EEHV) is a cause of rapid death in young Asian elephants (Elephas maximus) that present with signs of acute hemorrhagic disease or “EEHV-HD” [1,2]. The first report of disease associated with a herpes virus in an elephant was in 1971, and involved pulmonary lymphoid nodules and cutaneous papillomas in an African elephant (Loxodonta africana) in Kruger National Park [3]. In 1990, Ossent and others described a lethal case in an Asian elephant that presented with severe hemorrhage in multiple organs, particularly heart, trunk, stomach, and intestine, although the cause was unknown [4]. The discovery of DNA from the causative agent EEHV1 in a 16-month-old female calf in 1999 established it as a novel endotheliotropic herpesvirus specific to elephants [5]. Over the past 20 years, at least 90 lethal EEHV cases in elephant calves worldwide have been reported [6], and it is now considered a cause of high mortality in young Asian elephants in western zoos [7] and throughout Asia [1,812]. Despite many reports of confirmed EEHV cases, there is no evidence of epidemic outbreaks of EEHV. Rather, it is considered a sporadic rather than an endemic disease [1,1315] that affects both Asian and African elephant species [1,1012,16,17]. The taxonomic classification of EEHV has it in the genus Proboscivirus, subfamily Betaherpesvirinae, family Herpesviridae, order Herpesviruses [6]. However, more recently it has been proposed that EEHV should be in a new Deltaherpesvirinae subfamily based on the DNA polymerase and glycoprotein B gene [1,7,18]. Seven genotypes of EEHV have been identified so far, with EEHV1A and EEHV1B being the most frequently reported in young calves with acute hemorrhagic disease [7,8,19]. To date, most EEHV studies have focused on viral genetic diversity [8,14,17,2023], clinical pathology [4,2427], rapid diagnostic techniques [11,19,2830], infection confirmation methods [31,32], and treatment protocols [12,33]. Far less attention has been paid to understanding disease prevalence, risk factor identification, epidemiological modeling [34] or changes within the host following infection, all of which are needed to help understand the pathogenesis of this disease.

Collaboration with relevant organizations in Thailand, such as the EEHV Task Force, National Elephant Institute, and Chiang Mai and Kasetsart Universities over the last 10 years has provided a mechanism for data gathering and knowledge sharing that is beginning to shed light on the demographics and clinical characteristics of EEHV in this country, how the elephant responds to infection, and factors influencing disease progression. The aim of this report was to characterize the occurrence of EEHV cases in Thailand between 2006–2018 and analyze data using survival statistical survival models. We believe that this information will provide valuable information to assist in the diagnosis, treatment, and management of this rapidly fatal disease.

Materials and methods

Elephants and data collection

We utilized veterinary records of 58 confirmed cases of EEHV in captive elephants reported in Thailand between January 2006 and August 2018, as well as interviews of elephant owners and veterinarians involved with suspected cases. The elephants in this study participated in routine health services and an EEHV surveillance program, which involved several of the authors and was coordinated by the Center of Excellence in Elephant and Wildlife Research, Chiang Mai University (KB, PT, CT, CS) in collaboration with partner institutions: The National Elephant Institute (TA); The Faculty of Veterinary Medicine, Kasetsart University (SS), The Zoological Park Organization; and the Livestock Animal Department of Thailand. Activities are guided by the Thailand EEHV Task Force, chaired by co-author, C. Thitaram. Blood samples were collected by qualified veterinarians associated with the EEHV Task Force from elephants exhibiting signs of EEHV infection at the request of camp owners to confirm diagnosis and arrange treatment, if necessary. Samples were analyzed at the Chiang Mai University Veterinary Diagnosis Center or Kasetsart University Veterinary Diagnosis Center, and all collection procedures were approved by the elephant’s owner and in-charge veterinarian. No blood samples were collected outside of the health care services program for this study. Because only veterinary record data were utilized, Chiang Mai University does not require ethics committee approval.

Data were collected using methods similar to those from studies of Ebola viral outbreaks in humans [3537], and included demographic information on age, sex, and region of suspected or confirmed EEHV cases. Data on the host response to infection and progression of disease included age (in months) of the first clinical presentation, and the specific time (hour) from clinical presentation to death or recovery. Physical examinations were conducted and based on the Elephant Endotheliotropic Herpesvirus (EEHV) protocol by Cracknell [38] and the Guidelines for Management of Elephant Endotheliotropic Herpesvirus (EEHV) in Asia by Luz and Howard [39]. Clinical signs were recorded as follows: 1) lethargy—a lowered level of consciousness, with drowsiness, listlessness, and apathy; 2) anorexia—lack or loss of appetite; 3) fever—rectal temperature over 38°C; 4) facial edema—excess of watery fluid collecting in the head, neck, and trunk; 5) tongue cyanosis—petechial or patchy hemorrhage starting at the tip of the tongue and moving caudally; 6) diarrhea—loose and watery feces; and 7) bloody diarrhea—red coloration and smell of blood in the feces. Influencing factors included the subtype of EEHV, level (severity) of clinical signs, treatment procedures, antiviral medication (where used), weaning status, and training history that refers to whether elephants were trained for medical intervention by veterinarians, usually with the aid of the mahout. Most untrained elephants were calves less than 2.5 years of age, which limited treatment options, and any potential predisposing factors, such as seasons that were defined by The Climate of Thailand, Meteorological Department of Thailand ( and consisted of summer (mid-February to mid-May), rainy (mid-May to mid-October) and winter (mid-October to mid- February) periods or other clinical signs exhibited by the animal. EEHV treatments followed established guidelines [38,39].

Diagnostic methods

Blood, oral swabs, and tissue samples were collected by veterinarians with the Thailand EEHV Task Force from elephants that presented with signs of EEHV infection for molecular confirmation and EEHV subtype identification. All positive EEHV cases were confirmed by either conventional PCR or qualitative PCR [11,40,41] at the Faculty of Veterinary Medicine, Chiang Mai University or Kasetsart University, Thailand. DNA was extracted using NucleoSpin Blood (Macherey-Nagel, Germany) kits, and samples were screened for EEHV using DNA polymerase (PANPOL) PCR primers [29]. Standard terminase (TER) primers were used for subtype specification of EEHV1A/EEHV1B and EEHV3/EEHV4 [5]. Thirty-five cycles of PCR amplification were completed under the following conditions: 94°C for 1 minute, 50°C for 1 minute, and 72°C for 1 minute followed by 72°C for 7 minutes. The PCR product was visualized on 1% agarose gel stained with ethidium bromide. Positive EEHV TER PCR products were sequenced using an ABI 3700 Automated DNA sequencer (Applied Biosystems, CA, USA) and then analyzed using biological sequence alignment software (Bioedit v.7.2.6). The phylogenetic relationship was generated by multiple alignments of the nucleic acid program (ClustalX). The virus particle was measured by qPCR at Kasetsart University. Although it was not possible to conduct quantitative real-time PCR during the course of clinical signs, blood samples were collected and stored for retrospective testing using EEHV1 and EEHV3/4 specific TaqMan real-time PCR [32]. Samples testing positive for EEHV3/4 are assumed to be positive for EEHV4 and not EEHV3 because EEHV3 is found in African elephants only [18].

Treatment methods

EEHV treatment protocols were based on established guidelines [38,39] and were as follows: 1) no treatment with sudden death of the elephant; 2) supportive treatment only; and 3) supportive treatment with antiviral medication. Supportive treatments included: i.v. fluid therapy (at least 20 liters per day) with normal saline (0.9%) and dextrose (5%); i.v. antibiotics (e.g., Penicillin G 50,000 IU/kg, S.I.D) intravenously; vitamin C [(ascorbic acid) 6 mg/kg, S.I.D]; and plasma transfusion. Antiviral treatment included acyclovir (12–30 mg/kg B.I.D oral or intravenous) or famciclovir (12 mg/mg B.I.D oral or per rectal) administration.

Statistical analysis

Demographic and clinical characteristics of EEHV are presented as frequencies and percentages. Survival analyses were conducted using Kaplan-Meier tests and Cox’s regression models [4244]. Clinical cases included elephants that survived or died within an observation time period, which was set at a maximum of 168 hours (7 days) after onset of clinical signs, and was based on previous studies [1,26,29]. Elephants that were still alive after the survival time or had died from other causes were considered as censored data. Kaplan-Meier analysis was used to estimate survival time of EEHV cases [45,46]. Differences in survival curves for variables were determined by the log-rank test [42]. A Cox’s regression model was applied to estimate the hazard ratio for factors and the occurrence of EEHV. The hazard ratio is an estimate of the ratio of the hazard rate in a treated group compared to controls. In a clinical trial where disease resolution is the endpoint, the hazard ratio indicates the relative likelihood of disease resolution in treated versus control subjects at any given point in time [47]. Because the objective of the analysis was to explore potential factors associated with EEHV, Cox’s regression model was fitted with all variables including sex, subtype, severity, treatment procedure, antivirals, weaning and training [48,49]. All statistical analyses were performed using R statistical software Version 1.1.414 [50].


Status of EEHV in Thailand 2006–2018

Over a 12-year period, 58 animals were confirmed positive by PCR (Fig 1). The first PCR confirmed case of EEHV occurred in 2006 [11], the first year of this study. Between 2006 and 2016, 1–5 cases were confirmed each year. Since then, the number of cases has increased to over 15 per year, reflecting the increased use of PCR confirmation tests in clinically suspected cases.

Fig 1. Numbers of confirmed EEHV cases in each year from January 2006 to August 2018, both survivors and non-survivors.

Demographics of EEHV confirmed cases

The demographic characteristics of the 58 cases confirmed by PCR are shown in Table 1. Most cases occurred in elephants less than 2.5 years of age, with a similar distribution between males and females. Cases occurred primarily in the northern part of Thailand, particularly in Chiang Mai province, which had the highest frequency of disease incidence and also the largest captive elephant population. Four EEHV subtypes were identified, with EEHV1A predominating, followed by EEHV4. Two elephants that died were coinfected with EEHV1A and EEHV4.

Table 1. Demographic characteristic of 58 confirmed EEHV cases in Thailand between 2006 and 2018.

Characteristic features of EEHV confirmed cases

As shown in Fig 2, 48 of the 58 cases (82.76%) had recorded clinical signs that included lethargy in all cases (100%), bloody diarrhea in 20 cases (41.67%), diarrhea in 10 cases (44.67%), fever in 16 cases (33.33%), facial edema in 16 cases (33.33%), and tongue cyanosis in six cases (12.5%).

Kaplan-Meier survival curve

Of the 58 cases with confirmed EEHV infection, 40 died; the fatality rate of all positive cases was 68.97%. The survival curve was derived from 21 elephants for which there were data from the time of clinical presentation to death, and indicated a median survival time after clinical presentation of 36 hours, by which time 50% of the cases were resolved (Fig 3).

Fig 3. Survival curve by Kaplan-Meier presented median survival time at 36 hours (n = 21).

Comparison of survival curves: Log rank test

Complete data for statistical analysis of each variable from the 58 positive cases were obtained as follows: (1) variables that included sex, subtype, weaning status, training history and season yielded 21 cases with complete data; (2) medical procedures and antiviral therapy yielded 20 cases with complete data. In the univariable analysis, there were no differences in survival curves between levels (categories) of each variable (Table 2); for example, the survival curve between female and male elephants was not different.

Table 2. Median survival time and univariable analysis.

Data are from 21 cases with complete histories from the time of clinical presentation to death.

Cox’s regression model

Cox regression modeling (Fig 4) found the hazard ratio of ‘no medical procedure’ was correlated with a higher risk of death compared to when medical intervention was provided. Moreover, animals that were given acyclovir had a statistically significant decreased risk of death compared with those that received no antiviral therapy. Other factors, including sex, viral subtype, and training were not statistically correlated with the risk of death.

Fig 4. Hazard ratio estimated along with confidence intervals and p-values for each variable.


Elephant endotheliotropic herpes virus is a global problem that affects young Asian elephants in a wide range of countries throughout North America, Europe, Asia, and Africa. The virus causes rapid death, especially in young calves [34]. We present here the first report of a retrospective observational study of data collected in Thailand between 2006 and 2018 in an EEHV positive population of captive elephants using a statistical model to describe the response pattern in all confirmed cases. Initially, many young elephants died without being diagnosed, or had clinical signs confused with other hemorrhagic diseases, such as hemorrhagic septicemia. These limitations may have led to relatively low numbers of EEHV clinical cases being identified in neighboring countries as well, such as Cambodia [51], Laos [52] and Myanmar [1]. In Thailand, our data showed that in the first and second quarters of the study, the number of cases was low, presumably reflecting less awareness by any level of domesticated elephant stakeholder in Thailand, e.g. mahouts, elephant owners, camps, veterinarian, academics and researchers. Over the past 15 years, however, EEHV diagnostic techniques, including PCR, have improved substantially [5] and veterinarians are now more capable of diagnosing EEHV infection. In 2016, local and regional EEHV task forces were established in Thailand and Singapore, respectively, to monitor this rapidly fatal disease, provide updated disease occurrences, and share information with field veterinarians involved in elephant care and laboratory testing facilities following published guidelines [39]. Additionally, a systematic recording system was set up to aid in future epidemiological analyses, and laboratory facilities were established to allow for more rapid diagnosis to confirm infection in suspected cases. Last, education about this disease has been enhanced and directed at mahouts, elephant owners, elephant caretakers and camp managers to improve reporting of suspected EEHV cases. Thus, the increase in confirmed cases during the last 2 years of the study is presumed to reflect improvements in disease recognition and diagnosis, especially through the use of PCR. However, we must acknowledge that there are as yet no empirical data to support this assumption, so we cannot rule out the possibility that there also may have been an actual increase in the incidence of the disease over this time period.

Several studies have reported clinical signs, presentations and outcomes following EEHV infection in other range counties [5153], with results similar to ours. The survival time in the present study was only 1.5 days, which agrees with previous reports that fatality occurs after clinical signs are present for 1–7 days [29,54]. The mortality rate in the present study was 70%, not unlike the fatality rate of 80% in North America [9,29]. Descriptive analysis of demographic outcomes showed a high incidence of EEHV in northern Thailand, which also is the region with the highest numbers of captive elephants [55]. The most common EEHV subtype was EEHV1A, which agrees with findings in North America, Europe, India and Thailand [10,34,54,56,57]. Testing for EEHV5 by PCR was not performed because we lacked the specific primer in our laboratory, but this subtype is known to occur in Asian elephants [41,58,59] and should be investigated. Clinical signs in our study also reflected the findings of others, with the most frequent being lethargy, fever, facial edema, and tongue cyanosis [2,34,52].

According to EEHV treatment guidelines, antiviral drugs should be given in suspected EEHV-HD cases. Acyclovir is commonly used and is easier to obtain than famciclovir in Thailand. There were several clinical cases in this study where acyclovir was not administered because of sudden death of the calf. But others that were treated with antiviral drugs, antibiotics and supportive fluids did survive. Fluid therapy is viewed as an important part of EEHV-HD treatment [60]. Intravenous fluids are supportive by maintaining intravenous volume and electrolyte balance, protecting against hypovolemic shock [12]. Crystalloids (0.9% normal saline, ringers etc.) as an i.v. bolus (0.3–4 mL/kg) followed by rectal fluids is now recommended [61] and has been used in successful treatment of EEHV-HD cases. In addition, plasma and colloids may also be used to support oncotic pressure [12].

We used survival analysis of descriptive data, as described for medical analyses of “time to an event” data [44]. In the present study, a key event was the time from EEHV clinical presentation to death. The analysis allowed us to examine changes over time and factors that influenced survival, which are critically important in understanding host responses and the progression of the disease to survival [43]. Analysis of survival data frequently uses the Kaplan-Meier method, the Cox proportional hazard model, and the log-rank test to generate survival curves, test differences among survival curves for each variable, and determine risk factors [62]. In our study, survival times for each variable showed no statistically significant differences; for every variable, the disease pattern progressed the same over time.

Results from the Cox’s regression model showed that two factors influenced survival time: medical procedures and antiviral therapy. Some elephants that received intensive care still died, possibly because those cases presented with high severity signs or treatment was not initiated in time. As expected, antiviral use reduced the risk of death in accordance with several other reports that some young affected elephants survive through the use of antiviral medication, namely acyclovir or famciclovir [1,12,33]. Information from statistical models can inform and offer guidance for increased intensive monitoring and provision of better management procedures for elephants at risk, as well as indicate the need for future studies. For example, it has been suggested that stress may increase the risk of EEHV-HD by compromising the immune system [56,57,63]; however, there are as yet no empirical data to support that theory, so it should be explored.

According to EEHV Task Force guidelines, young calves under the age of 10 years should be closely monitored. Today, the recommended protocol is to begin treatment before clinical signs are observed, otherwise it is usually too late. Thus, all calves should be monitored routinely, although this requires real-time PCR, which is not always available. Thus, more support is needed to improve elephant mobile veterinary services, increase the number and capacity of diagnostic laboratory facilities, and educate elephant owners, camp managers, elephant curators, veterinarian assistants and mahouts about this disease and how to monitor and treat it. Data on confirmed EEHV cases also should be included in a global database so that future statistical analyses can be conducted to provide epidemiological data related to EEHV, as has been done with studies of Ebola [3537]. For example, to identify risk factors associated with EEHV-HD, information on husbandry and management, weather conditions, weaning, and nutrition, health and immune status, should be routinely collected.

The limitations of the present investigation were the retrospective study design, missing information on some variables, and the relatively small number of cases. For 37 cases, the data were incomplete because initially we lacked knowledge about EEHV pathogenesis, including clinical signs, diagnostic capability and adequate data recording. Moreover, some cases occurred rapidly in remote areas of the country or in elephant camps where access made it difficult to collect samples or interview mahouts to get accurate information. Nevertheless, this study is significant as it provides the first assessment of factors related to survival in EEHV-infected elephants.


The findings from this study indicate that infected elephants had a median survival time of only 36 hours. Factors that were linked to this short survival time indicate the urgent need for intensive observation and prompt treatment to reduce case mortality. As diagnostic capabilities improve in elephant range countries like Thailand, more intensive monitoring will be possible, allowing veterinarians to treat cases more effectively.

Supporting information

S1 Table. All information of 58 confirmed EEHV cases during January 2006 –July 2018.



The authors thank Chiang Mai University for financial support. We also thank organizations that provided valuable information for this study, including the National Elephant Institute (Thai Elephant Conservation Center and Southern Elephant Hospital, Thailand), the Zoological Park Organization of Thailand, Kasetsart University (Thailand), Chiang Mai University (Thailand), Dr. Thittaya Janyamathakul D.V.M, Pattara Elephant Farm (Thailand), Dr. Channarong Saisaard D.V.M, Elephant Nature Park (Chiang Mai, Thailand), Dr. Pratthana Inthawong D.V.M, Elephant Kingdom, (Surin, Thailand), Dr. Crueahtong Kayan D.V.M, Dr. Panida Muanghong D.V.M, Maetaeng Elephant Park & Clinic (Chiang Mai, Thailand), and veterinarians at the Friends of Asian Elephant Foundation Elephant Hospital (Lampang, Thailand). We are extremely grateful to Dr. Andrew Higgins BVetMed MSc Ph.D. FRSB MRCVS, scientific advisor to Phang Nga Elephant Park (Thailand) for assistance in editing this paper.


  1. 1. Long SY, Latimer EM, Hayward GS. Review of elephant endotheliotropic herpesviruses and acute hemorrhagic disease. ILAR J. 2016;56: 283–296. pmid:26912715
  2. 2. Dastjerdi A, Seilern-Moy K, Darpel K, Steinbach F, Molenaar F. Surviving and fatal elephant endotheliotropic herpesvirus-1A infections in juvenile Asian elephants–lessons learned and recommendations on anti-herpesviral therapy. BMC Vet Res. 2016;12. pmid:27567895
  3. 3. McCully RM. Herpes nodules in the lung of the African elephant (Loxodonta africana). Onderstepoort J Vet Res. 1971;38(4): 225–236.3. pmid:4368768
  4. 4. Ossent P, Guscetti F, Metzler AE, Lang EM, Rübel A, Hauser B. Acute and fatal herpesvirus infection in a young Asian elephant (Elephas maximus). Vet Pathol. 1990;27: 131–133. pmid:2161138
  5. 5. Richman LK, Montali RJ, Garber RL, Kennedy MA, Lehnhardt J, Hildebrandt T, et al. Novel endotheliotropic herpesviruses fatal for Asian and African elephants. Science. 1999;283: 1171–1176. pmid:10024244
  6. 6. Kotila-Row A. Detection of elephant endotheliotropic herpesvirus (EEHV) in Asian (Elephas maximus) and African elephants (Loxodonta africana). 2015. Available from:
  7. 7. Ehlers B. Endotheliotropic elephant herpesvirus, the first betaherpesvirus with a thymidine kinase gene. J Gen Viro. 2006;87: 2781–2789.
  8. 8. Fickel J, Richman LK, Montali R, Schaftenaar W, Göritz F, Hildebrandt TB, et al. A variant of the endotheliotropic herpesvirus in Asian elephants (Elephas maximus) in European zoos. Vet Microbiol. 2001;82: 103–109. pmid:11423201
  9. 9. Hayward GS. Conservation: clarifying the risk from herpesvirus to captive Asian elephants. Vet Rec. 2012;170: 202–203. pmid:22368209
  10. 10. Zachariah A, Zong J-C, Long SY, Latimer EM, Heaggans SY, Richman LK, et al. Fatal herpesvirus hemorrhagic disease in wild and orphan Asian elephants in southern India. J Wildl Dis. 2013;49: 381–393. pmid:23568914
  11. 11. Sripiboon S, Tankaew P, Lungka G, Thitaram C. The occurrence of elephant endotheliotropic herpesvirus in captive Asian elephants (Elephas maximus): first case of EEHV4 in Asia. J Zoo Wildl Med. 2013;44: 100–104. pmid:23505709
  12. 12. Sripiboon S, Angkawanish T, Boonprasert K, Sombutputorn P, Langkaphin W, Ditcham W, et al. Successful treatment of a clinical elephant endotheliotropic herpesvirus infection: the dynamics of viral load, genotype analysis, and treatment with acyclovir. J Zoo Wildl Med. 2017;48: 1254–1259. pmid:29297830
  13. 13. Zong J-C, Latimer EM, Heaggans S, Richman R., Hayward G. Pathogenesis and molecular epidemiology of fatal elephant endotheliotropic disease associated with the expanding proboscivirus genus of the betaherpesvirinae. Proceedings of the International Elephant Conservation and Research Symposium. Florida; 2008.
  14. 14. Zong J-C, Latimer E, Heaggans S, Richman L, Hayward G. Viral gene subtyping of eighteen North American cases of EEHV hemorrhagic disease. Proceedings of the International Elephant Conservation and Research Symposium. Bangkok, Thailand; 2009.
  15. 15. Barman NN, Choudhury B, Kumar V, Koul M, Gogoi SM, Khatoon E, et al. Incidence of elephant endotheliotropic herpesvirus in Asian elephants in India. Vet Microbiol. 2017;208: 159–163. pmid:28888631
  16. 16. Azab W, Damiani AM, Ochs A, Osterrieder N. Subclinical infection of a young captive Asian elephant with elephant endotheliotropic herpesvirus 1. Arch Virol. 2018;163: 495–500. pmid:29094239
  17. 17. Bronson E, McClure M, Sohl J, Wiedner E, Cox S, Latimer E, et al. Epidemiology evaluation of elephant endotheliotropic herpesvirus 3B infection in an African elephant (Loxodonta africana). J Zoo Wildl Med. 2017;48: 335–343. pmid:28749266
  18. 18. Zong J-C, Latimer EM, Long SY, Richman LK, Heaggans SY, Hayward GS. Comparative genome analysis of four elephant endotheliotropic herpesviruses, EEHV3, EEHV4, EEHV5, and EEHV6, from cases of hemorrhagic disease or viremia. J Virol. 2014;88: 13547–13569. pmid:25231309
  19. 19. Ehlers B, Burkhardt S, Goltz M, Bergmann V, Ochs A, Weiler H, et al. Genetic and ultrastructural characterization of a European isolate of the fatal endotheliotropic elephant herpesvirus. J Gen Virol. 2001;82: 475–482. pmid:11172087
  20. 20. Ling PD, Long SY, Fuery A, Peng R-S, Heaggans SY, Qin X, et al. Complete genome sequence of elephant endotheliotropic herpesvirus 4, the first example of a GC-rich branch proboscivirus. mSphere. 2016;1. pmid:27340695
  21. 21. Ling PD, Long SY, Zong J-C, Heaggans SY, Qin X, Hayward GS. Comparison of the gene coding contents and other unusual features of the GC-rich and AT-rich branch probosciviruses. mSphere. 2016;1: e00091–16. pmid:27340696
  22. 22. Richman LK, Zong J-C, Latimer EM, Lock J, Fleischer RC, Heaggans SY, et al. Elephant endotheliotropic herpesviruses EEHV1A, EEHV1B, and EEHV2 from cases of hemorrhagic disease are highly diverged from other Mammalian herpesviruses and may form a new subfamily. J Virol. 2014;88: 13523–13546. pmid:25231303
  23. 23. Wilkie GS, Davison AJ, Watson M, Kerr K, Sanderson S, Bouts T, et al. Complete genome sequences of elephant endotheliotropic herpesviruses 1A and 1B determined directly from fatal cases. J Virol. 2013;87: 6700–6712. pmid:23552421
  24. 24. Garner MM, Helmick K, Ochsenreiter J, Richman LK, Latimer E, Wise AG, et al. Clinico-pathologic features of fatal disease attributed to new variants of endotheliotropic herpesviruses in two Asian elephants (Elephas maximus). Vet Pathol. 2009;46: 97–104. pmid:19112123
  25. 25. Richman LK. Pathological and molecular aspects of fatal endotheliotropic herpesviruses of elephants. Ph. D, Thesis, The Johns Hopkins University. 2003. Available from: 2
  26. 26. Richman LK, Montali RJ, Cambre RC, Schmitt D, Hardy D, Hildbrandt T, et al. Clinical and pathological findings of a newly recognized disease of elephants caused by endotheliotropic herpesviruses. J Wildl Dis. 2000;36: 1–12. pmid:10682740
  27. 27. Seilern-Moy K, Haycock J, Dastjerdi A, Leifsson PS, Bertelsen MF, Perrin KL. Fatal elephant endotheliotropic herpesvirus-1 and -4 co-infection in a juvenile Asian elephant in Europe. JMM Case Reports. 2016;3.
  28. 28. Hardman K, Dastjerdi A, Gurrala R, Routh A, Banks M, Steinbach F, et al. Detection of elephant endotheliotropic herpesvirus type 1 in asymptomatic elephants using TaqMan real-time PCR. Vet Rec. 2012;170: 205–205. pmid:22186378
  29. 29. Latimer E, Zong J-C, Heaggans SY, Richman LK, Hayward GS. Detection and evaluation of novel herpesviruses in routine and pathological samples from Asian and African elephants: identification of two new probosciviruses (EEHV5 and EEHV6) and two new gammaherpesviruses (EGHV3B and EGHV5). Vet Microbiol. 2011;147: 28–41. pmid:20579821
  30. 30. Stanton JJ, Zong J-C, Eng C, Howard L, Flanagan J, Stevens M, et al. Kinetics of viral loads and genotypic analysis of elephant endotheliotropic herpesvirus-1 infection in captive Asian elephant (Elephas maximus). J Zoo Wildl Med. 2013;44: 42–54. pmid:23505702
  31. 31. Kochagul V, Srivorakul S, Boonsri K, Somgird C, Sthitmatee N, Thitaram C, et al. Production of antibody against elephant endotheliotropic herpesvirus (EEHV) unveils tissue tropisms and routes of viral transmission in EEHV-infected Asian elephants. Sci Rep. 2018;8: 4675. pmid:29549315
  32. 32. Kochakul V, Boonsri K, Tiwananthagorn S, Somgird C, Thitaram C, Pringproa K. Development of in situ hybridization for detection of elephant endotheliotropic herpesvirus in Asian elephants. J Vet Diagn Invest. 2018; 1040638718773810. pmid:29730973
  33. 33. Schmitt DL, Hardy DA, Montali RJ, Richman LK, Lindsay WA, Isaza R, et al. Use of famciclovir for the treatment of endotheliotropic herpesvirus infections in Asian elephants (Elephas maximus). J Zoo Wildl Med. 2000;31: 518–522. pmid:11428400
  34. 34. Sripiboon S, Jackson B, Ditcham W, Holyoake C, Robertson I, Thitaram C, et al. Molecular characterisation and genetic variation of elephant endotheliotropic herpesvirus infection in captive young Asian elephants in Thailand. Infect Genet Evo. 2016;44: 487–494. pmid:27503594
  35. 35. Li J, Duan H-J, Chen H-Y, Ji Y-J, Zhang X, Rong Y-H, et al. Age and Ebola viral load correlate with mortality and survival time in 288 ebola virus disease patients. Int J Infect Dis. 2016;42: 34–39. pmid:26523640
  36. 36. Waxman M, Aluisio AR, Rege S, Levine AC. Characteristics and survival of patients with ebola virus infection, malaria, or both in Sierra Leone: a retrospective cohort study. Lancet Infect Dis. 2017;17: 654–660. pmid:28258817
  37. 37. Weppelmann TA, Donewell B, Haque U, Hu W, Magalhaes RJS, Lubogo M, et al. Determinants of patient survival during the 2014 ebola virus disease outbreak in Bong County, Liberia. Glob Health Res Policy. 2016;1. pmid:29202055
  38. 38. Cracknell J. United Kingdom elephant health program: elephant endotheliotropic herpesvirus (EEHV) protocol. 2008;1–46. Available from:
  39. 39. Luz S, Howard LL. Guideline for management: Elephant endotheliotropic herpesvirus (EEHV) in Asia. Recommendations from the 1st Asian EEHV strategy Meeting. 2 nd ed. Wildlife Reserves Singapore Group; 2016.
  40. 40. Stanton JJ, Nofs SA, Zachariah A, Kalaivannan N, Ling PD. Detection of elephant endotheliotropic herpesvirus infection among healthy Asian elephants (Elephas maximus) in south India. J Wildl Dis. 2014;50: 279–287. pmid:24484479
  41. 41. Zong J-C, Heaggans SY, Long SY, Latimer EM, Nofs SA, Bronson E, et al. Detection of quiescent infections with multiple elephant endotheliotropic herpesviruses EEHV2, EEHV3, EEHV6 and EEHV7 within lymphoid lung nodules or lung and spleen tissue samples from five asymptomatic adult African elephants. J Virol. 2015; JVI.02936–15. pmid:26719245
  42. 42. Clark TG, Bradburn MJ, Love SB, Altman DG. Survival analysis part I: basic concepts and first analyses. Br J Cancer. 2003;89: 232–238. pmid:12865907
  43. 43. Dudley WN, Wickham R, Coombs N. An introduction to survival statistics: Kaplan-Meier analysis. J Adv Pract Oncol. 2016;7: 91–100. pmid:27713848
  44. 44. Singh R, Mukhopadhyay K. Survival analysis in clinical trials: Basics and must know areas. Perspect Clin Res. 2011;2: 145–148. pmid:22145125
  45. 45. Etikan I. The Kaplan-Meier estimate in survival analysis. Biom Biostat Int J. 2017;5.
  46. 46. Goel M, Khanna P, Kishore J. Understanding survival analysis: Kaplan-Meier estimate. Int J Ayurveda Res. 2010;1: 274–8. pmid:21455458
  47. 47. Spruance SL, Reid JE, Grace M, Samore M. Hazard ratio in clinical trials. Antimicrob Agents Chemother. 2004;48: 2787–2792. pmid:15273082
  48. 48. Bewick V, Cheek L, Ball J. Statistics review 12: Survival analysis. Crit Care. 2004;8: 389–394. pmid:15469602
  49. 49. Wiazzane N, Chauffert B, Ghiringhelli F. Retrospective analysis of survival benefits of chemotherapy for metastatic or non-resectable intrahepatic cholangiocarcinoma. Clin Res in Hepatol Gastroenterol. 2013;37: 614–618. pmid:23711827
  50. 50. R Development Core Team (2017). R: A language and environment for statistical computing Vienna: R Foundation for Statistical Computing. Available: Accessed 2018 August 15
  51. 51. Reid CE, Hildebrandt TB, Marx N, Hunt M, Thy N, Reynes JM, et al. Endotheliotropic elephant herpes virus (EEHV) infection. The first PCR‐confirmed fatal case in Asia. Vet Q. 2006;28: 61–64. pmid:16841568
  52. 52. Bouchard B, Xaymountry B, Thongtip N, Lertwatcharasarakul P, Wajjwalku W. First reported case of elephant endotheliotropic herpes virus infection in Laos. J Zoo Wildl Med. 2014;45: 704–707. pmid:25314848
  53. 53. Puyati B, Charoenphan P, Boonyasart B, Kanistanon K, Kampa J. Detection of elephant endotheliotropic herpesvirus in Asian elephant from organs and trunk swap samples. Chiang Mai Vet J. 2015; 13:153–164.
  54. 54. Stanton JJ, Zong J-C, Latimer E, Tan J, Herron A, Hayward GS, et al. Detection of pathogenic elephant endotheliotropic herpesvirus in routine trunk washes from healthy adult Asian elephants (Elephas maximus) by use of a real-time quantitative polymerase chain reaction assay. Am J Vet Res. 2010;71: 925–933. pmid:20673092
  55. 55. Godfrey A, Kongmuang C. Distribution, Demography and Basic Husbandry of the Asian Elephant in the Tourism Industry in Northern Thailand. 2009; 13–18.
  56. 56. Kendall R, Howard L, Masters N, Grant R. The impact of elephant endotheliotropic herpesvirus on the captive Asian elephant (Elephas maximus) population of the United Kingdom and Ireland (1995–2013). J Zoo Wildl Med. 2016;47: 405–418. pmid:27468010
  57. 57. Schaftenaar W, Reid C, Martina B, Fickel J, Osterhaus ADME. Nonfatal clinical presentation of elephant endotheliotropic herpes virus discovered in a group of captive Asian elephants (Elephas maximus). J Zoo Wildl Med. 2010;41: 626–632. pmid:21370642
  58. 58. Stanton JJ, Nofs S, Peng R, Hayward GS, Ling PD. Development and validation of quantitative real-time polymerase chain reaction assays to detect elephant endotheliotropic herpesviruses-2, 3, 4, 5, and 6. J Virol Methods. 2012;186: 73–77. pmid:22842286
  59. 59. Wilkie GS, Davison AJ, Kerr K, Stidworthy MF, Redrobe S, Steinbach F, et al. First fatality associated with elephant endotheliotropic herpesvirus 5 in an Asian elephant: pathological findings and complete viral genome sequence. Sci Rep. 2014;4. pmid:25199796
  60. 60. Wiedner E, Howard LL, Isaza R. Treatment of Elephant endotheliotropic herpesvirus (EEHV). In: Fowler ME, Miller RE, editors. Fowler’s zoo and wildlife medicine 7, Current Therapy. W.B. Saunders; 2012. pp. 537–543.
  61. 61. Howard LL, Schaftenaar W. Elephant endotheliotropic herpesvirus. In: Miller RE, Lamberski N, Calle PP, editors. Fowler’s zoo and wild animal medicine 9, current therapy. W.B. Saunders; 2019. pp. 672–679.–3
  62. 62. Zaman Q, Pfeiffer KP. Survival analysis in medical research. 2011. Available from:
  63. 63. Boonsri K, Somgird C, Noinafai P, Pringproa K, Janyamethakul T, Angkawanish T, et al. Elephant endotheliotropic herpesvirus associated with clostridium perfringens infection in two Asian elephant (Elephas maximus) calves. J Zoo Wildl Med. 2018;49: 178–182. pmid:29517457