Cornell researchers have discovered that a lethal variant of feline coronavirus, previously thought to be limited to a devastating 2023 outbreak in Cyprus that killed thousands of cats, has in fact appeared in the United States, raising concerns about future large-scale outbreaks in vulnerable cat populations. The findings were published in the March 6 issue of the Journal of Feline Medicine and Surgery by a cohort of researchers at Cornell University’s College of Veterinary Medicine. “The findings are significant and are a result of our ongoing work in surveillance of pathogenic diseases of veterinary relevance in the U.S,” said Dr. Ximena A. Olarte-Castillo, post-doctoral associate in the Goodman lab at the Baker Institute for Animal Health and first author on the paper.

Feline coronavirus is common in cats and typically causes mild or asymptomatic infections. However, in some cases, it can mutate within the cat’s body and cause feline infectious peritonitis (FIP), a severe disease that is almost routinely fatal if not treated. The most common form of the virus, FCoV-1, tends to be relatively benign and highly transmissible or highly pathogenic but unable to spread easily. A less common variant, FCoV-2 — thought to be a hybrid of a feline and a canine coronavirus — accounts for only about five percent of FIP cases worldwide; the specific strain of FCoV-2 that impacted the cat population on Cyprus is known as FCoV-23, or Cyprus-23.

Whittaker and his team conducted a retrospective analysis of three archived samples of FCoV-2 from 2013 and 2016. Using whole-genome sequencing and viral analysis, the researchers examined 20 samples of tissue, feces and abdominal fluid (ascites). They identified deletions in a region of the virus’s spike protein — known as “domain zero” — in two of the cats that developed prolonged disease. The offspring of one of the cats, which had a shorter disease course, was found to have a long version of the spike protein.

“Our data suggest that the transmissible form of the virus carries the long spike protein,” Whittaker said. “Once inside the cat, the virus can undergo a deletion to produce a short spike version, which is associated with systemic infection and the development of FIP. One big question is what controls transmissibility of these viruses. So, I think that may be down to this domain zero. What makes type 2 viruses particularly concerning is that they have the potential to be both highly transmissible and highly pathogenic. That’s what made the Cyprus outbreak so impactful, and why we’re paying much closer attention now. This work shows that what happened in Cyprus is not a one-off event."

Despite the presence of these high-risk viral features in U.S. samples, outbreaks on the scale of Cyprus have not occurred here thus far. Researchers believe this is largely due to differences in cat population structure.

“In the U.S., many of the cases we studied came from single-cat households,” Olarte-Castillo said. “Even if the virus had the ability to spread, it simply didn’t have the opportunity. In Cyprus, you had large, dense populations of free-ranging cats — that’s a completely different scenario.”

However, growing populations of shelter cats and free-roaming cats in the U.S. could change that. If a virus like this enters a large, connected population, the consequences could be significant.

The findings highlight an urgent need for improved diagnostic tools, surveillance, and awareness among veterinarians.

Currently, distinguishing between FCoV-1 and FCoV-2 in clinical settings is difficult. Whittaker’s team is working to develop rapid diagnostic tests, much like COVID tests, that could provide answers within minutes, enabling faster and more targeted responses.

“If a veterinarian identifies a type 2 infection, that should immediately raise an alarm,” Whittaker said. “It’s not just about treating the individual cat, it’s about preventing potential spread.”

Effective antiviral treatments for FIP are now available, making early detection even more valuable. “We’re in a much better place therapeutically than we were even a few years ago,” Whittaker said. “But timing is critical. If there’s any suspicion of a high-risk infection, treatment should begin immediately. So we need to try and be as ready as we can for that eventuality.” 

The urgency of finding answers to the FCoV-2 threat warranted funding from the Cornell Feline Health Center’s Rapid Response Fund, established in 2012 in order to fast-track research.

“This is exactly the kind of situation that justifies rapid response funding,” Bruce Kornreich, D.V.M. '92, Ph.D.'05, director of the Cornell Feline Health Center, said. “You have an emerging viral threat, new data coming in from around the world, and an urgent need to understand what it means for animal health here in the United States.

“Understanding how these viruses transmit, evolve and cause disease is the cornerstone of improving diagnosis, treatment and prevention,” Kornreich said. “This work doesn’t just answer current questions; it prepares us for future challenges.”

The study also underscores the importance of proactive surveillance and preparedness in animal health — what researchers describe as panzootic preparedness. “In order to head off health problems, you have to know they exist and are emerging,” said Kornreich. “And that's the beauty of this type of work.”

 Written by Christina Frank