SARS-CoV-2 Serological Surveillance of Both Vaccinated and Unvaccinated Zoo Animals with the Identification of a Sloth Bear and a Tapir with Previous Infection

Since its discovery in 2019, SARS-CoV-2 has continued to be detected in both humans and animals worldwide. Currently there is limited research focusing on serological surveillance of wildlife under human care. Here we tested 230 serum samples of 134 animals from two zoological institutions collected between 2015 and 2024. To assess prior exposure and antibody responses from natural infection or vaccination, we used three serological assays: a nucleocapsid protein-based ELISA (N-ELISA), a surrogate virus neutralization test (sVNT) for spike (S) protein and a neutralization assay with S-pseudotyped viral particles. Among the 114 samples collected from 58 animals at Fort Wayne Zoo in Indiana, 37 samples from 20 vaccinated animals were sVNT-positive, and 2 of the positive animals had 2 samples prior to vaccination that tested positive by N-ELISA. Of the 116 samples from 76 animals at Brookfield Zoo in Illinois, 20 samples of 20 animals were sVNT-positive, and 19 of the positive animals had been vaccinated. Among these 20 sVNT-positive samples, only one sample from a South American Tapir was positive from prior to vaccination and 1 sample from a sloth bear was also positive by N-ELISA, marking the first documented cases of SARS-CoV-2 exposure in both species. Neutralization assays with S-pseudotyped virus revealed that some of the sVNT-positive samples have strong activity against the WH1-S pseudovirus but showed significantly reduced neutralization against the Omicron LP.8.1-S pseudovirus. These results underscore the need for updated vaccines tailored to emerging variants. Overall, our findings highlight the importance of continued serological surveillance across multiple species to detect new SARS-CoV-2 exposures and monitor vaccine-induced immunity in captive animal populations.