The necessity of a One Health approach to surveillance, investigation, response, and prevention of emerging zoonotic diseases is well recognized.^3,7,19,26 Many United States (US) agencies define One Health as “a collaborative, multisectoral, and trans-disciplinary approach with the goal of achieving optimal health outcomes recognizing the interconnection between people, animals, plants, and their shared environment.”⁷ On a global scale, the COVID-19 pandemic has greatly impacted animal populations and subsequent management,^8,11,24 thereby providing an opportunity to operationalize One Health collaborations.¹⁷ Early coordination and communication between local, state, and federal animal and public health partners are critical to address zoonotic SARS-CoV-2 events, where transmission occurs between people and animals, including animals in human care in zoos, sanctuaries, and aquariums.^{2,10,11,15,17,24,28,31,32}

In the US, zoo animals with respiratory symptoms or suspected SARS-CoV-2 infection are tested following the onset of clinical signs or known exposure to a person with COVID-19.^{2,8,10,15,29,31,34} The majority of SARS-CoV-2–positive zoo animals experienced mild, self-limiting illness and have recovered, although a small proportion have died. More information is needed to understand the role of SARS-CoV-2 infection in fatal outcomes.^{2,10,15,22,28,31,33,34} As of 09 January 2024, 42% (n = 171) of all individual confirmed positive animal cases have occurred in zoo animals, primarily tigers, lions, snow leopards, otters, and gorillas.³³ In addition to heightening measures to mitigate viral spread, many US zoos have initiated One Health investigations with local, state, and federal governmental partners, as well as with academic and nonprofit entities. These partnerships have been essential for investigation and response to SARS-CoV-2 outbreaks in zoos.²⁸

Beginning in 2021, the Arizona Department of Health Services (ADHS) and the Translational Genomics Research Institute (TGen) launched several One Health studies to understand better the impacts of SARS-CoV-2 on animals in the state, including companion animals, zoo animals, and free-ranging wildlife.³⁷ In partnership with local wildlife agencies, zoos, wildlife rehabilitation centers, and state and federal governments, the program provides testing and epidemiologic support to expedite detection of and response to zoonotic SARS-CoV-2 infection in animals.

Reid Park Zoo staff consulted with TGen and ADHS regarding an epidemiologic investigation to inform public health measures. The investigation included contacting all staff in close proximity to the squirrel monkey and reviewing all recent reports of staff illness. Engagement of critical One Health partners occurred 28–30 June. Figure 2 depicts the timeline of key events and actions carried out by the One Health partner agencies involved in this investigation.

One of the animal caretakers reported development of mild respiratory symptoms, including sneezing, coughing, and sinus drainage, beginning around 04 June. Multiple at-home COVID-19 antigen tests were negative in subsequent days. Upon returning to work on 08 June (11 days before the squirrel monkey’s death), the animal caretaker continued to follow infection-prevention measures outlined in zoo policy, including wearing a KN95 mask and gloves and using a foot bath prior to entering the squirrel monkey enclosure. The animal caretaker visited urgent care on 16 June for continued intermittent mild symptoms and again tested negative by a COVID-19 antigen test. The caretaker returned to work on 17 June (2 days prior to the squirrel monkey’s death). On the morning of 19 June, the same day of the animal’s death, the animal caretaker yielded a positive result for COVID-19 concurrent with another positive, symptomatic adult in the household. Samples from the caretaker and family member were discarded and not available for genomic sequencing.

Two asymptomatic adult squirrel monkeys housed in the same enclosure tested negative for SARS-CoV-2 by rRT-PCR on serial fecal samples. The three squirrel monkeys were not included in the first round of Zoetisvaccinations^29,38 because of routine health monitoring concerns (e.g., preexisting health conditions in one female, and a suspected pregnancy in the other female). Serum samples collected 3 mon later (October 2022) from the other two squirrel monkeys tested negative for SARS-CoV-2 neutralizing antibodies; these monkeys have since been immunized. There were no other animals housed in the squirrel monkey enclosure. Other nearby exhibits were greater than 6 ft away and tended to by different caretakers.

Viral and serologic testing as part of the One Health and SARS-CoV-2 surveillance program was conducted on samples from 55 other animals at the Reid Park Zoo, including African lions, ring-tailed lemurs, spotted necked otters, macaques, and goats. All samples were negative for SARS-CoV-2.

Over the past decade, numerous successful multi-institutional One Health collaborations have been formed in Arizona to understand the circulation of endemic diseases better, to respond to zoonotic disease outbreaks, and to conduct surveillance of emerging pathogens.^5,18,23,37 Despite robust implementation of the One Health approach in Arizona, collaborations have previously overlooked the routine inclusion of zoos and wildlife facilities. Zoonotic transmission of SARS-CoV-2 presents novel concerns and challenges in zoo settings.^{2,10,15,28,34} In the beginning of the pandemic, zoos faced challenges in implementing effective biosecurity practices. The emergence of new variants and changing guidance further prompted shifts in policy governing general zoo operations.¹³ Close interactions with animal caretakers and the risk imposed by congregate housing continue to be implicated as important factors in SARS-CoV-2 outbreaks at zoos.^{2,8,10,15,28,31,34} Taken together, these issues highlight the need to include these partners in surveillance, preparedness, and response.

In this case study, necropsy and pathologic evaluation revealed necrotizing colitis and, together with fecal culture results, suggested toxemia due to Clostridium perfringens type A as the driver of the fatal outcome in the squirrel monkey.³⁰ Pathologic findings did not implicate SARS-CoV-2 as a causal or contributing factor in the animal’s illness and death, although its burden on the immune system may have limited the squirrel monkey’s ability to respond to the clostridium infection. Using the algorithm established through rigorous clinical, epidemiologic, and pathologic investigation by Carpenter et al.⁶ to assess the role of SARS-CoV-2 in an animal’s death, infection was deemed incidental to the death of the squirrel monkey. Additionally, given the rapid decline, it is plausible that SARS-CoV-2 infection never established itself systemically in the squirrel monkey. Lastly, although samples for sequencing from the animal caretaker were unavailable, the most plausible hypothesis is that the squirrel monkey was exposed to SARS-CoV-2, likely by a caretaker. This is supported by three lines of evidence: (1) other animals at the zoo, including nonhuman primates and the two cohabitating squirrel monkeys were negative for SARS-CoV-2, suggesting other animals were not the source of exposure; (2) public contact with the squirrel monkey was limited, suggesting a caretaker source; and (3) the animal was found to be infected with the Omicron variant, which was the predominant lineage circulating in the human population during the same timeframe and geographic location. The Omicron variant has also been identified in other animals in human care, including a gorilla and a tiger.^33,35

Little is known about natural infections of SARS-CoV-2 in nonhuman primates housed in zoos. Most infections in zoo settings to date have been documented in gorillas, with few reported natural infections in monkeys.^28,33 Squirrel monkeys (Saimiri spp.) are commonly found in zoological collections throughout the US,¹ and studies have shown that squirrel monkeys appear to have the least severe pathological changes when infected with SARS-CoV-2, in comparison to other nonhuman primates.⁹ This may in part be because of genetic differences in cellular receptors mediating the entry of SARS-CoV-2 in New World monkeys.²⁰ Given these experimental data, limited evidence of SARS-CoV-2 in neotropical nonhuman primates,²⁷ and the now reported infection in this particular species, further studies to understand the range of susceptible host and transmission factors are warranted. Moreover, it is also essential to consider the potential conservation impact SARS-CoV-2 and other pathogens transmitted from or between humans to animals can have on primate hosts.¹²

Overall, the establishment of an active One Health surveillance program for SARS-CoV-2 in Arizona enabled the rapid detection and investigation of SARS-CoV-2 in a novel species and prompt response to prevent spread to other people or animals. Establishing trusted partnerships between the zoo, ADHS, and TGen prior to this incident (April 2022) facilitated smooth and effective public health action. By continuing to foster relationships with multisectoral partners, additional collaborations to address critical One Health issues will be formed, enabling the expansion of zoonotic disease surveillance opportunities across the human–animal–environment interface.^17,25,26,32