Multispecies cluster of SARS-CoV-2 infections associated with an infected African lion

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The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was suspected to be capable of infecting some animal species as well as humans. Early on, evidence appeared of SARS-CoV-2 infection in domestic or pet animals, and later, in farmed mink and some wild animal species. However, a recent paper described, for the first time, the transmission of this virus from an animal in a zoo to zoo employees.

Study: Probable transmission of SARS-CoV-2 from an African lion to zoo employees. Image Credit: mojoeks/Shutterstock
Study: Probable transmission of SARS-CoV-2 from an African lion to zoo employees. Image Credit: mojoeks/Shutterstock

This underlines the need to observe safety practices to prevent zoonotic disease from spreading in either direction in zoos.


SARS-CoV-2 was identified as a zoonotic infection because of its presence in multiple mammals. The virus has a broad host range, and many animals in zoos and other enclosed areas harboring non-domestic animal species have been reported to have become infected naturally. The most at-risk appear to be the big cats, but ferrets, mink, non-human primates (NHPs), and others have also been frequently infected.

In most cases, the origin of the infection is a zoo employee. However, in the current paper, posted on the medRxiv* preprint server, the transmission occurred in the reverse direction. This has not been reported so far in a zoo environment.

The setting

The zoo is in Indiana, part of the Association of Zoos and Aquariums (AZA). The infected animal was an African lion (Panthera leo) that probably acquired the virus from a fully vaccinated employee with a breakthrough infection who was asymptomatic throughout. This was in December 2021, and the zoo was closed over the holidays.

The lion was alone in a building, about 30 feet from any other animal enclosures. Being very old, with physical limitations, it was fed and cared for by staff assigned this specific duty. This included treatment of its spine degeneration, and hand feeding, besides other medications and supplements.

The zoo also housed other big cats, including Amur leopards and tigers, a snow leopard, North American river otters, and several NHP species. All the species mentioned above had been doubly vaccinated with the ZOEMC (Zoetis Experimental Mink Coronavirus Vaccine) from September to November 2021.

All employees followed preventive measures recommended by the Zoo and Aquarium All Hazards Partnership, including the temporary cessation of guided tours behind the scenes and behavioral training, for species at risk of infection with the virus.

Zookeepers also had to be fully vaccinated, maintain social distancing, remain home if ill, and look out for symptoms of infection. They walked through disinfectant-filled tubs when passing in or out of enclosures and used cleaning methods less likely to produce infectious aerosols. Surgical masks and gloves were required gear for animal feedings.

The course of the outbreak

The lion became ill on December 18, 2021, and was diagnosed with SARS-CoV-2 infection from nasal swabs by a lateral flow immunoassay. It recovered but showed increasing spine stiffness and was euthanized on December 23, 2021.

The lion then passed the infection to three other fully vaccinated employees at the zoo. No other animal showed evidence of infection.

The outbreak was studied by an epidemiologist from the Indiana Department of Health (IDOH), the records being used for the current study. Lateral flow immunoassay was used to screen for human infection, followed by polymerase chain reaction (PCR) for confirmation. Genomic sequencing was carried out on the lion sample as well as 2/3 human samples (the third could not be sequenced).

The sequences from the lion and the two employees were genetically identical. They were compared with ten other samples from all over Indiana that showed a close relationship with the zoo sequences but were collected between August 2021 and February 2022. They showed differences of 12-24 nucleotides from the zoo cluster sequences.

The findings suggest that all four infections in this multispecies infection cluster were from a single source, probably one of the employees taking care of the lion when it became ill. All were asymptomatic, and all tested negative on screening tests on the same day. None became ill after that.

The three infected employees were probably not the source of infection as they became infectious only after the lion’s diagnosis. Only one employee had contact with the lion’s head (feeding) during the probable period when the animal was infected and was probably the asymptomatic carrier.

No human-to-human transmission pathway could be identified from the data, nor did the two employees with identical infections have identifiable community transmission pathways. Conversely, the lion breathed and roared and coughed while the infected humans were close by. Symptoms appeared in these individuals within three days of such exposure, making the lion-to-human transmission route the most probable.

What are the implications?

Close contact with large cats puts humans at risk for zoonotic SARS-CoV-2 transmission, making this an occupational risk for staff involved in the care of susceptible animals. In this case, the infection spread from and to fully vaccinated individuals, whether the lion or the humans.

While the epidemiologic investigation was made easier by multiple factors, including zoo holidays, the safety measures in place, and the lion’s situation, further procedural guidelines must be put in place to allow for timely investigation of such outbreaks, which may not have the same setting. Rapid antigen tests for animals need to be developed and validated for biosecurity reasons in such situations.

A One Health approach, which connects the health of people, animals, and environment, should be used to respond to susceptible zoo animals, particularly those requiring close human contact, that develop clinical signs compatible with SARS-CoV-2 infection.”

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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