Horses might catch the COVID-19 virus, but they aren’t likely to show signs of illness.

A small percentage of healthy racehorses recently tested positive for SARS-CoV-2 following COVID-19 outbreak among racetrack personnel in California. The findings suggest horses might silently carry the virus and potentially contribute, albeit minimally, to its spread, said Nicola Pusterla, DVM, Dipl. ACVIM, professor of medicine and epidemiology at the University of California, Davis.

“If there’s one takeaway message I want each of you to consider for yourself (as veterinarians), but also for your horse owners, it is that if you have COVID-19, stay away from other humans—we know that—but also, stay away from animals,” Pusterla said, speaking at the 2022 American Association of Equine Practitioners (AAEP) Convention, held Nov. 18-22 in San Antonio, Texas.

“We don’t want the virus to suddenly adapt in a different species and then bounce back into human beings, and that is still a possibility,” he said.

Very Few Positives Among Horses Without Respiratory Signs

Scientists have already confirmed cases of SARS-CoV-2 transmission between humans and animals, mainly cats, dogs, and minks. But horses have a spike protein receptor that’s very similar to the one in humans, which suggests they might also be able to acquire and transmit the virus, Pusterla said.

To better understand that possibility, Pusterla and his fellow researchers ran quantitative polymerase chain reaction (qPCR) testing for SARS-CoV-2 on submitted nasal secretion samples of 667 horses that had experienced an acute onset of fever and respiratory signs January and December 2020. They also tested the samples for multiple common equine respiratory pathogens, including equine influenza virus, equine herpesviruses, equine rhinitis viruses and Streptococcus equi subspecies equi.

In addition, the team tested serum samples from 587 young Thoroughbred horses racing at Del Mar Racetrack in California after multiple jockeys and other track personnel tested qPCR-positive for SARS-CoV-2 in summer 2020. They looked for antibodies to SARS-CoV-2 using an enzyme-linked immunosorbent assay that targets the receptor-binding domain of the spike protein.

About one-third of the sick horses in the study tested positive for at least one common equine respiratory pathogen, he said. But none of them had a positive result for SARS-CoV-2.

By contrast, 35 (6%) of the healthy Thoroughbreds tested positive for SARS-CoV-2 antibodies, said Pusterla.

The team then compared those results to serum testing of 1,186 horses of various breeds and ages that had been hospitalized at his university clinic from February 2020 to March 2022. Forty-two horses (3.5%) in that group tested positive for SARS-CoV-2 antibodies.

“The way horses become infected with SARS-CoV-2 is likely through the spillover from human beings, with or without clinical COVID-19,” he said. “It’s interesting to know that populations of horses that have a lot of interaction with humans- especially these days where we have a more contagious variant of SARS-CoV-2 being transmitted- are more likely to test seropositive.”

An Evolving Coronavirus and its Unknown Threats

As SARS-CoV-2 shifts toward being more infectious than it was two years ago, the virus might target less likely hosts, said Pusterla. In other words, horses are probably more likely to acquire SARS-CoV-2 today than they were in 2020, when he collected most of his current study data.

“I would … argue that in 2020, the variant that was circulating wasn’t as contagious as the one we have today in 2022,” he said. “Food for thought.”

In particular, he said to keep in mind the risks to humans, horses, and other species as the virus continues to evolve and adapt to its environment.

“We have to protect these horses,” he said. “We don’t know what the future holds with this virus. So far it doesn’t seem to adapt to other animal species. But we shouldn’t try to challenge this dogma.”