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From people to Panthera: Natural SARS-CoV-2 infection in tigers and lions at the Bronx Zoo

Our take —

In this preprint yet to be peer reviewed, the authors report on SARS-CoV-2 infection among four tigers, three lions, and their human keepers at the Bronx Zoo. Animals presented only mild clinical signs and were presumably infected by their keepers who had developed symptoms prior to detection in animals. However, human-to-animal sequencing was only confirmed from tiger keepers to tigers; the origin of the infections in lions could not be determined and may have originated from another asymptomatic person. During March, keepers did not wear PPE. Now there are additional protocols in place for PPE use in enclosures for carnivores and bats, which has been formalized in recommendations to other zoo and wildlife organizations.

Study design

Case Series, Ecological

Study population and setting

The population includes tigers (Panthera tigris) and lions (Panthera leo) at the Bronx Zoo in New York, USA experiencing respiratory illness starting March 27, 2020. In addition to animals, researchers tested ten zookeepers and two managers that had close contact with tigers and lions between March 16 and April 1, 2020.

Summary of Main Findings

Four tigers and three lions (housed in a separate building from the tigers) developed mild respiratory signs when New York was experiencing a widespread outbreak of COVID-19 – the first tiger became ill on March 27, 2020. All respiratory signs resolved within one week except one tiger, whose signs lasted for 16 days. Examination in the first ill tiger revealed evidence of tracheitis in tracheal washes and in situ hybridization showed the presence of SARS-CoV-2 RNA in necrotic and inflammatory cells. Evidence of SARS-CoV-2 RNA was detected in respiratory secretions and/or feces from all seven animals; virus was isolated from three animals using Vero cells inoculated with tracheal wash fluid or fecal samples. Four staff (two tiger and two lion keepers) reported mild respiratory symptoms during this period. After oropharyngeal and blood samples were collected from symptomatic keepers on April 6, 2020, viral RNA or antibodies were detected in all four keepers. Nine SARS-CoV-2 genomes were sequenced (four from tigers, three from lions, and two from keepers) and eight full-length spike sequences (seven from symptomatic animals and one from an asymptomatic animal) were generated directly from respiratory and/or fecal samples. Viral whole genome sequencing revealed two genetic variants in animals, suggesting separate viral introductions to the animal populations. The tiger viruses were similar to those in their keepers, but lion viruses were of unknown origin.

Study Strengths

The research team performed virus isolation and full genome sequencing to determine the origin of the outbreaks in animals.


Sampling of tracheal washes, nasal/oropharyngeal swabs, and fecal samples was opportunistic and not repeated over multiple time points, so the dynamics of infection (viral loads in each sample type) are unknown. A virus neutralization assay was only performed on one tiger, so the seroconversion status of other animals is unknown. The researcher also did not test keepers and managers that reported no symptoms, so asymptomatic transmission from these workers to animals cannot be excluded. While it is assumed that infection of tigers was through the ill keepers, it is unclear from the investigation whether all tigers were infected by keepers or if some tiger-to-tiger transmission occurred. Finally, while lion keepers were positive for SARS-CoV-2 antibodies, no viral RNA could be sequenced, so infection of lions by keepers could not be assessed.

Value added

This report details an outbreak of SARS-CoV-2 in captive lions and tigers at the Bronx Zoo in late March, 2020, representing the first outbreak of the virus in non-human animals.

This review was posted on: 16 October 2020