Research finds SARS-CoV-2 declining in animals

At the beginning of the COVID-19 pandemic, SARS-CoV-2 was detected in a range of non-human animal species, both wild and domestic. Scientists discovered that in white-tailed deer and mink, the virus spread readily within the species and acquired mutations that eventually "spilled back" into humans. This raised concerns that the parallel evolution of SARS-CoV-2 in animals could create more transmissible or pathogenic variants, resulting in a resurgence of human infections.

In a new study, researchers at Yale University performed surveillance to understand the current coronaviral landscape of wild and domestic animals in the northeastern United States. They found that while the virus continues to evolve in humans, its grip seems to be weakening within the animal kingdom.

“This is good news, as it reduces the chance of spillback of newly evolved animal variants into humans,” said Caroline Zeiss, DACVP, DACLAM, professor of comparative medicine and of ophthalmology and visual science at Yale School of Medicine (YSM) and senior author of the study.¹

“Ongoing monitoring is essential, particularly for viruses that are capable of infecting many species, such as corona and influenza viruses. Long-term surveillance of wildlife and animals living close to humans gives us a unique chance to spot new pathogens that might spread to humans or affect animal health,” she continued.

The Surveillance of Viruses Coronaviruses have a very broad host range and can jump across species; this ability was at the root of the virus crossing the animal-human boundary to trigger the COVID-19 pandemic. Since then, the virus has undergone rapid evolution, with one variant succeeding another as it becomes increasingly transmissible in its predominant human host.

This progression has been accompanied by evidence of transmission between humans and animals, as well as sustained transmission within certain species. While the rates of SARS-CoV-2 prevalence in animal samples have declined since the pandemic's peak, it remained unclear whether this reflected a true decline or simply reduced resources for surveillance.

For this new study, titled "The coronaviral landscape across diverse mammalian species in the Northeastern United States" and published in Scientific Reports, researchers sought to understand the reasoning behind this decline.² They conducted a broad surveillance study of wild and domestic animals—including cats, dogs, ferrets, woodchucks, and cattle—located within the northeastern US. In total, the team collected fecal swabs, oral swabs, or stool specimens from 889 animals. This was a collaborative effort between Yale University, the Connecticut Veterinary Medical Diagnostic Laboratory at the University of Connecticut, and the Connecticut Agricultural Experiment Station.

The samples were analyzed for coronaviral RNA using an assay method capable of detecting a broad range of coronavirus types. Positive samples were sequenced to identify the specific coronavirus species, and researchers conducted genetic analysis on most positive samples to provide independent verification of the results.¹

To complement the surveillance, researchers conducted experimental infections of white-footed mice in the laboratory using the original strain of SARS-CoV-2 and the Omicron variant. They focused on white-footed mice because they are the most common rodent in the study area and have an established role in spreading pathogens between humans and other animals. To test cross-species transmission, the team exposed hamsters—known to be a susceptible model for infection—to contaminated bedding and cages from the infected mice.

“Our goal was to determine whether both variants could infect white-footed mice and whether they could transmit the virus to each other or to another species,” Zeiss said.¹

Keeping Pets Safe Through these studies, researchers detected several kinds of animal coronaviruses in samples from seven different species; however, none of them tested positive for SARS-CoV-2.²

When researchers infected the white-footed mice in the lab, the mice contracted both the original and Omicron variants. However, they did not shed as much of the virus with the Omicron variant and did not transmit it among themselves. Notably, while the original strain could spread among the mice, neither the original nor Omicron variants jumped to the hamsters.¹

Researchers say these findings suggest that as SARS-CoV-2 evolves in its preferred human host, successive variants may be losing their affinity for non-human animals.

“Therefore, the chance of spillback of newly evolved animal variants into humans is very low,” Zeiss said. “For most people, this is relevant to pet owners who live in close contact with animals.”

This study was supposed by the United States Department of Agriculture and the rodent capture occurred as part of integrated tick management studies funded by the CDC.²

References

1. Dalton M. SARS-CoV-2 is on the decline in animals, study finds. Yale News. January 9, 2026. Accessed January 14, 2026. https://news.yale.edu/2026/01/09/sars-cov-2-decline-animals-study-finds
2. Ibemgbo S, Compton S, Breban MI, et al. The coronaviral landscape across diverse mammalian species in the Northeastern United States. Scientific Reports. Published online December 20, 2025. doi:https://doi.org/10.1038/s41598-025-32849-3