Introduction

Rapid human-to-human transmission of the novel coronavirus SARS-CoV-2 resulted in the global COVID-19 pandemic. Since its emergence in late 2019, the virus has been responsible for more than 7,000,000 documented human deaths,¹ and it is known to cause both acute respiratory and long-term disease in humans. Natural infections, some with associated disease, have also been reported in several nonhuman mammal species in both wild and managed care settings.^2–11 In addition to experiencing morbidity and mortality, some susceptible animal species may serve as reservoirs or bridge hosts that can transmit the virus to humans, potentially in the form of novel variants.^8–16 For these reasons, further investigation of SARS-CoV-2 infections in animals is needed to safeguard both human and animal health.

Tigers (Panthera tigris ssp) are classified as endangered by the International Union for the Conservation of Nature, and extensive global efforts are in place to ensure the continued survival of wild and captive populations of this iconic species.¹⁷ There have been many reports (both published and informal) of SARS-CoV-2 infections in tigers under human care.^{2–4,7,18–24} While numerous publications have described subsets of these cases and subsequent epidemiologic and genomic investigations in detail, overarching data regarding the incidence of SARS-CoV-2 infections in tigers in North American zoos and treatment and prevention measures remain limited.

The Association of Zoos and Aquariums (AZA) Tiger Species Survival Plan (SSP) Program is a cooperative population management initiative to ensure genetic soundness and longevity of ex situ Amur (Panthera tigris altaica), Malayan (Panthera tigris jacksoni), and Sumatran (Panthera tigris sumatrae) tiger populations. The Tiger SSP also oversees a small number (< 20 as of June 2023) of “generic” tigers of unknown pedigree, which are not recommended for breeding. The primary objective of this retrospective study was to evaluate SARS-CoV-2 morbidity and mortality in tigers housed in North American zoological institutions participating in the Tiger SSP from January 2020 to June 2023. Additional aims included characterizing SARS-CoV-2 infections in tigers and describing treatment interventions, testing methods, and vaccination practices implemented by participating institutions.

Methods

The subjects of this study were Amur, Sumatran, and Malayan tigers housed at North American zoological institutions that participated in the Tiger SSP from January 2020 to June 2023. An online survey (REDCap; Vanderbilt University) was developed and distributed via email to representatives from all 108 North American zoological institutions that housed SSP tigers between January 2020 and June 2023. These included primarily AZA-accredited facilities but also several non–AZA-accredited zoos. Most institutions housed multiple tigers, but 31 housed only 1 SSP tiger for at least a portion of the 42-month study period. Responses were collected from June 2022 until June 2023. Veterinarians were the primary target respondents, but other representatives (eg, veterinary technicians, curators, directors) were also contacted. The names of individual respondents, their occupations, and the institutions they represented were recorded to allow for exclusion of any duplicate results and permit follow-up questions. An introductory letter was included with the survey explaining its purpose and that collected data would be kept anonymous and only presented in a collated manner. Institutions were encouraged to resubmit a response to the survey if SARS-CoV-2 was diagnosed in any of their tigers after they had already completed the survey. The survey was approved by the North Carolina State University Institutional Review Board (21-105) and the Tiger SSP. The survey included dichotomous (yes/no), multiple-choice, checkbox, and open-ended questions. When applicable, the option to select “other” and type a free-text response answer was included with multiple-choice and checkbox questions. The survey utilized branching logic so that some items appeared based on answers to previous questions.

The aim of the first part of the survey was to collect data regarding SARS-CoV-2 cases in tigers. Respondents were asked whether any tigers at their institution had ever tested positive for SARS-CoV-2 infection. If “yes” was selected, they were asked how many tigers had tested positive. Additional items then appeared asking respondents to indicate signalment (age, sex, species), preexisting conditions, clinical signs observed, and case severity (asymptomatic, mild, moderate, or severe) for each positive tiger. Asymptomatic cases were defined as having no clinical signs and triggering no treatment intervention. Mild cases were defined as having clinical signs but triggering no treatment intervention. Moderate cases were defined as having clinical signs and triggering some treatment intervention. Severe cases were defined as having clinical signs and triggering intensive treatment intervention (eg, multiple treatments under anesthesia). A series of checklists and open-ended questions asked respondents to describe diagnostics and treatments pursued for each SARS-CoV-2–positive tiger.

A separate item asked respondents if any tigers at their institution were suspected to have been infected with SARS-CoV-2 but were not tested to confirm. If “yes” was selected, the same series of items asked about confirmed positive cases appeared in order to collect information for each suspect case. Respondents were also asked if any tigers died due to SARS-CoV-2. If “yes” was selected, additional items appeared requesting information about tiger signalment, clinical presentation, and gross and histologic lesions on necropsy.

The aim of the second part of the survey was to identify SARS-CoV-2 testing methods and vaccination protocols implemented by responding institutions. Respondents were asked whether their institution had tested any tigers for SARS-CoV-2 and, if so, what kinds of samples were collected and what tests were used. Respondents were also asked whether tigers at their institution were vaccinated for SARS-CoV-2 using the experimental veterinary recombinant vaccine developed by Zoetis Animal Health^24–27 and, if so, how many doses they had received. If a respondent had previously indicated that any tigers at their institution had tested positive for SARS-CoV-2, they were also separately asked if those tigers had been vaccinated for SARS-CoV-2 prior to infection and, if so, how many doses of vaccine they had received prior to the positive diagnosis.

In the third and final part of the survey, respondents were asked to describe personal protective equipment protocols used by staff working with tigers at the time of survey response and at the time of SARS-CoV-2 infections (if applicable) for Tiger SSP internal use. All data were collated, and descriptive statistics were generated in REDCap and Excel (Microsoft Corp).

Tiger SSP studbook records were manually reviewed to determine the total number of SSP tigers housed in North America and the total number of SSP tigers housed at institutions that responded to the survey during the study period (January 2020 to June 2023). The latter was used in calculating incidence of SARS-CoV-2 infection within the study group.

Results

Representatives from 55 of 108 institutions completed the survey for a response rate of 51%. The total number of tigers housed at responding institutions between January 2020 and June 2023 was 162 (86 Amur, 50 Sumatran, 19 Malayan, 7 “generic”). This represented 51% of the total number of SSP Tigers housed in North America between January 2020 and June 2023 (n = 317 [132 Amur, 92 Sumatran, 63 Malayan, 30 “generic”]). Ten responding institutions housed only 1 tiger for at least a portion of the 42-month study period. Most survey responses were completed by veterinarians (n = 43), with the remainder completed by directors (5), curators or lead keepers (6), or veterinary technicians (1). Two institutions diagnosed SARS-CoV-2 in their tigers after having already completed the survey and submitted a second replacement response.

Confirmed SARS-CoV-2 infections were reported in 39 tigers (17 males, 22 females) from 15 institutions, for a minimum study group incidence of 24% over a 42-month period. Four institutions reported infection in a single tiger, while the remaining 11 institutions reported multiple infected animals. The 4 institutions reporting infection in a single tiger each housed multiple tigers at the time of diagnosis. Infected animals included 20 Amur, 8 Sumatran, and 11 Malayan. Median age was 10 years (range, 1 to 18 years). Clinical signs identified in these animals, severity of cases, and treatments administered are summarized in Tables 1 through 3, respectively. Two asymptomatic SARS-CoV-2 infections were identified at 2 separate institutions when testing was implemented for all tigers after a conspecific tested positive for SARS-CoV-2. No cases in cubs under 1 year of age or “generic” (unknown pedigree) tigers were reported.

The number of Zoetis SARS-CoV-2 recombinant vaccine doses administered to tigers by each of the 55 responding institutions at the time of the survey are presented in Table 4. Of the 39 SARS-CoV-2–positive tigers, 8 had been vaccinated once and an additional 9 had been vaccinated twice (initial Zoetis recombinant vaccine plus booster) for SARS-CoV-2 prior to infection. Of the 8 confirmed SARS-CoV-2 cases in tigers that had received a single SARS-CoV-2 vaccine prior to infection, 1 was classified as asymptomatic, 6 were classified as mild, and 1 was classified as moderate. Of the 9 confirmed SARS-CoV-2 cases in tigers that had received 2 SARS-CoV-2 vaccine doses prior to infection, 4 were classified as mild and 5 were classified as moderate. Of the 22 tigers that had not received any SARS-CoV-2 vaccine doses prior to infection, 1 was classified as asymptomatic, 12 were classified as mild, 8 were classified as moderate, and 1 was classified as severe.

Twenty-two institutions (40%) reported testing tigers for SARS-CoV-2. Sample types and methods for antemortem SARS-CoV-2 testing and characterization in tigers at these institutions are presented in Tables 5 and 6. No cases of tigers with suspected but unconfirmed SARS-CoV-2 infections were reported. No deaths due to SARS-CoV-2 were recorded in animals in the study population. One tiger died 2 days after testing positive for SARS-CoV-2; however, gross and histologic lesions were not consistent with SARS-CoV-2, and pulmonary tissue samples were negative for SARS-CoV-2 with real-time reverse transcriptase PCR testing. Preexisting conditions reported in positive tigers at the time of SARS-CoV-2 infection included osteoarthritis (n = 3), mammary adenocarcinoma (1), early renal disease (1), suspected vaginitis (1), and inflammatory bowel disease with chronic cholangiohepatitis (1). For the majority of tigers diagnosed with SARS-CoV-2 (n = 32), no known preexisting conditions were reported.

Discussion

This study describes SARS-CoV-2 morbidity, clinical signs, treatment interventions, testing methods, and vaccination practices in Amur, Sumatran, and Malayan tigers housed at 55 North American zoos participating in the Tiger SSP. Infections with SARS-CoV-2 were reported in 39 tigers in the study group of 162 tigers over a 42-month period (January 2020 to June 2023). As no prior studies have assessed incidence and clinical manifestations of SARS-CoV-2 infections in a large study group of zoo-housed tigers in North America, these data provided valuable new knowledge to advance understanding of SARS-CoV-2 in this species.

The daily close contact with animal care staff in the early stages of the pandemic was the source of SARS-CoV-2 infection in farmed mink and some zoo-housed animals.^3,7,15,28 Subsequently, despite heightened awareness of interspecies transmission, widespread asymptomatic infections in humans, and recommended precautionary measures, cases of both human-to-animal and animal-to-animal transmission of SARS-CoV-2 have been described in managed care settings.^{2,4–6,10,15,19,28,29} Several investigations of SARS-CoV-2 cases in captive tigers have provided genomic and epidemiologic evidence supportive of animal caretaker-to-tiger transmission specifically.^{2,4,7,18,20–24} To the authors’ knowledge, no previous reports have confirmed tiger-to-tiger transmission, and it is suspected that most cases in the present study were human-to-tiger transmission events. However, given that 11 of 15 institutions with SARS-CoV-2 cases in the present study confirmed infections in multiple tigers and that interanimal transmission has been documented in other species, tiger-to-tiger transmission remains a possibility.^{8,14–16,28,30,31}

The incidence of SARS-CoV-2 infections in this study group was 24%. This could be an underestimate given the potential for asymptomatic infections and that 60% of participating institutions reported never testing any of their tigers. While clinical signs were reported for most positive cases in this study, 2 infected tigers at 2 different institutions were asymptomatic, and SARS-CoV-2 was identified only because testing was implemented for all tigers following observation of symptoms in conspecifics. Thus, asymptomatic infections may be more widespread than reported in zoo-housed tigers. Of note, SARS-CoV-2 surveillance studies^{6,19,29,30,32} in domestic cats and dogs reported that asymptomatic infections were common. Similar studies in tigers are lacking. As survey responses were collected over 12 months, it is also possible that institutions may have responded to the survey, subsequently diagnosed SARS-CoV-2 in their tigers, and failed to submit a replacement response before June 2023. Two institutions did submit replacement responses after diagnosing SARS-CoV-2–positive tigers. Additional cases of unreported SARS-CoV-2 in tigers within the study group are considered unlikely, as 3 authors of this study (TMH, EC, and DM) hold active leadership roles as Tiger SSP veterinary and pathology advisors and would have been aware of recent, additional SARS-CoV-2–positive cases that needed to be reported.

Most SARS-CoV-2 cases in this report were classified as mild or moderate. Only a single case was classified as severe and necessitating intensive treatment intervention, after which the tiger recovered completely. No SARS-CoV-2 deaths were reported in this study group. Thus, the risk of death or severe disease from SARS-CoV-2 infection appears low in tigers; however, given the limited sample size of positive cases and continued evolution of the virus, these risks should not be discounted, particularly in animals with existing respiratory or other severe disease processes. While not included in the present study, there is at least 1 published report⁴ of rapid decline and subsequent euthanasia due to poor prognosis in a geriatric (17-year-old) female zoo-housed tiger infected with SARS-CoV-2 in Sweden. On necropsy, severe respiratory pathology associated with SARS-CoV-2 antigen on immunohistochemistry was identified. This animal was also found to have several significant comorbidities (hepatic and cervical neoplasia, pyometra, hemometra) that likely exacerbated decline, and thus this case does meet criteria for establishing SARS-CoV-2 as the primary reason for death.^4,33

While clinical signs observed in SARS-CoV-2–positive tigers varied considerably, a dry cough was the prevailing clinical sign (82%). Though nonspecific, inappetence and lethargy were each reported in more than half of positive tigers (64% and 62%, respectively). Nasal discharge and wheezing, seen in 46% and 31% of reported tigers, respectively, are more specific clinical signs that may be of greater use in recognizing SARS-CoV-2 infections in tigers. The 8 other clinical signs reported were observed in < 20% of cases and therefore may not be as reliable for detecting potential cases of SARS-CoV-2 in tigers.

In just over half of confirmed SARS-CoV-2 cases in the study population, at least 1 medical treatment was implemented. Due to the relatively small sample size, complete clinical recovery of all animals, and retrospective nature of the present study, the benefits of treatment interventions could not be effectively evaluated. Nonetheless, given the current paucity of published information that exists related to treatments, these data still provide practical information for zoo practitioners to consider when treating cases of known or suspected SARS-CoV-2 in tigers and potentially other felids.

Seventy-six percent of participating institutions reported vaccinating their tigers for SARS-CoV-2 (Zoetis recombinant vaccine), with the majority of these administering at least 2 doses (69%). In 2020, Zoetis, a global pharmaceutical company that produces animal medications and vaccines, developed an adjuvanted recombinant stabilized spike trimmer protein SARS-CoV-2 vaccine designed specifically for use in animals.^25–27 Following SARS-CoV-2 infections in zoo-housed gorillas in early 2021, Zoetis responded to requests from zoological institutions across North America for doses of this experimental vaccine for emergency use in susceptible taxa, including felids.^34,35 Similar to many of the human SARS-CoV-2 vaccines, an initial dose followed by a booster in 3 weeks is recommended.^25–27,34 To date, this vaccine has been reportedly administered to numerous different species with minimal perceived adverse effects and has successfully mounted a robust immune response in multiple studies^2,25–27,35 in domestic dogs and cats. Additionally, a challenge study performed in domestic cats documented a strong anamnestic response and reduced viral load and shedding in vaccinated cats compared to controls challenged with intranasal and oral SARS-CoV-2, supporting its efficacy.³⁶ Preliminary research investigating the Zoetis SARS-CoV-2 recombinant vaccine in several nondomestic felids, including tigers, found initial antibody response and increasing titers in most animals 3 months after vaccination, with titers decreasing at 6 months after vaccination.³⁷ Despite this limited evidence of initial antibody response in tigers, in the present study, 9 tigers that tested positive for SARS-CoV-2 (and developed clinical signs) had received a full 2-dose course of the vaccine prior to infection. Thus, analogous to SARS-CoV-2 vaccines authorized for use in humans, the Zoetis SARS-CoV-2 recombinant vaccine does not necessarily prevent infection in tigers. It remains unknown whether vaccination can significantly reduce infection incidence, viral load and shedding, or disease severity in tigers. No apparent trend was observed between vaccination status and case severity in the present study. Presently, the Felid Taxon Advisory Group recommends administering the 2-dose vaccine series, followed by additional boosters every 6 to 12 months.³⁷ Further prospective studies are needed to determine efficacy, appropriate dosing, and booster recommendations.

It is also important to consider that over the 42-month study period, there were dozens of SARS-CoV-2 variants of interest that emerged and circulated in humans, with 5 different major variants meeting criteria to be labeled variants of concern (Alpha, Beta, Gamma, Delta, Omicron) as defined by the World Health Organization.¹ Thus, it is likely that the cases described in this report represent tigers infected with differing SARS-CoV-2 variants of concern. Previous investigations have described Alpha and Delta variant infections in tigers housed in North American zoos.^2,21,38 Variants of concern differed in disease manifestations, severity, and outcomes in humans¹ and could have had similar differences in tigers. However, the small sample size and limited use of testing designed to distinguish between SARS-CoV-2 variants in tigers (eg, whole genome sequencing) used by responding institutions precluded comparisons between variants of concern. Similarly, the potential efficacy of the Zoetis recombinant veterinary vaccine in reducing disease severity and/or prevalence of SARS-CoV-2 in tigers may differ across variants. Furthermore, this vaccine was distributed in waves, and tigers across institutions were not vaccinated at the same time, which may also have impacted the findings of this study.³⁴

As data collection relied upon voluntary completion of a survey, only animals from 55 of 108 (51%) target institutions, or 162 of 317 (51%) tigers, were included in the study group. While the response rate was subjectively high for a voluntary survey, the data presented herein may not be entirely representative of all North American institutions participating in the Tiger SSP. Additionally, as the Tiger SSP population is constantly in flux with births, deaths, and transports, a small subset of tigers were not present in the study group or broader North American SSP population for the entirety of the 42-month study period. Lastly, due to the relatively small sample size of confirmed SARS-CoV-2 cases in tigers, inferential statistical analysis was not possible in the present study. Nevertheless, this study provides novel and practical information that enhances current understanding of SARS-CoV-2 infections in tigers.

In conclusion, this retrospective survey study describes 39 SARS-CoV-2 infections in zoo-housed tigers from 15 North American institutions from January 2020 to June 2023. Seventeen positive tigers had been vaccinated for SARS-CoV-2 (1 to 2 doses) prior to infection. Most infected tigers presented with mild to moderate clinical signs, including dry cough, nasal discharge, and wheezing. Two cases were asymptomatic. A single case was characterized as severe. No deaths due to SARS-CoV-2 were reported in the study group. Roughly half of cases received at least 1 treatment intervention. Approximately 70% of participating institutions fully vaccinated their tigers (≥ 2 doses). These findings, summarizing more than 3 years of SARS-CoV-2 infections and vaccination practices in tigers, will advance evidence-based management recommendations for the species.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The authors have nothing to disclose.