SARS-CoV-2 Appears to Be Declining in Animals, Yale Researchers Report
During the early years of the COVID-19 pandemic, SARS-CoV-2 was detected in a growing number of animal species, including wildlife and companion animals such as dogs and cats. In some species—most notably white-tailed deer and mink—the virus spread efficiently, accumulated mutations, and spilled back into humans, raising concern about animals serving as long-term reservoirs for novel variants.
A new study from Yale University suggests that risk may now be decreasing.
In surveillance and laboratory studies conducted across the northeastern United States, researchers found that while SARS-CoV-2 continues to evolve in humans, its presence and transmission in animals appears to be waning.
“This is good news as this reduces the chance of spillback of newly evolved animal variants into humans,” said Caroline Zeiss, professor of comparative medicine and of ophthalmology and visual science at Yale School of Medicine (YSM) and senior author of the study.
The findings were published in Scientific Reports.
What the Surveillance Found
Coronaviruses are known for their broad host range and ability to cross species barriers—an ability that enabled SARS-CoV-2 to spark a global pandemic. Although earlier studies documented sustained transmission in certain animal populations, recent declines in reported animal infections raised questions about whether prevalence was truly decreasing or simply underreported due to reduced surveillance.
To address this, Yale researchers conducted broad coronaviral surveillance across domestic and wild mammals in the Northeast, including cats, dogs, ferrets, cattle, woodchucks, and other species. Samples (fecal, oral, or stool) were collected from 889 animals through a collaboration involving Yale University, the Connecticut Veterinary Medical Diagnostic Laboratory, and the Connecticut Agricultural Experiment Station.
Samples were screened using a broad-range coronavirus assay capable of detecting multiple coronavirus species. Positive samples underwent sequencing and genetic analysis for confirmation.
The result: multiple animal coronaviruses were detected across seven species—but none were SARS-CoV-2.
Experimental Infection and Transmission Studies
To further assess risk, researchers conducted controlled infections in white-footed mice, a common wild rodent with known relevance in zoonotic disease ecology. Mice were exposed to both the original SARS-CoV-2 strain and the more recent omicron variant.
“Our goal was to determine whether both variants could infect white-footed mice and whether they could transmit the virus to each another or to another species,” Zeiss said.
While mice were susceptible to infection with both variants, important differences emerged:
Omicron-infected mice shed less virus
Omicron did not transmit between mice
The original strain transmitted between mice
Neither variant transmitted to hamsters, despite exposure to contaminated bedding and cages
What This Means for Veterinarians and Pet Owners
Taken together, the data suggest that as SARS-CoV-2 continues adapting to humans, newer variants may be losing their ability to efficiently infect or spread among 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.”
However, researchers emphasize that surveillance remains critical.
“Ongoing monitoring is essential, particularly for viruses that are capable of infecting many species, such as corona and influenza viruses,” Zeiss added. “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.”
Why This Matters Clinically
For veterinarians, the findings support current guidance that SARS-CoV-2 poses minimal ongoing risk to companion animals, while reinforcing the importance of:
Zoonotic disease surveillance
One Health collaboration
Monitoring emerging pathogens beyond SARS-CoV-2
As pandemic-era concerns fade, the study highlights how viral evolution can narrow—not expand—host range over time.
Publication Details
Ibemgbo S, et al. The coronaviral landscape across diverse mammalian species in the Northeastern United States.Scientific Reports (2025).
DOI: 10.1038/s41598-025-32849-3
Source: Yale University
