Researchers at the University of California, Davis, have uncovered new details about how a once-deadly coronavirus disease in cats spreads through the immune system. The findings may help scientists better understand long COVID and other long-lasting inflammatory illnesses in people.

The disease, feline infectious peritonitis, or FIP, is caused by a form of feline coronavirus that changes inside some cats. If left untreated, it is almost always fatal. While FIP only affects cats, it shares many features with serious coronavirus-related conditions in humans, including severe inflammation that can damage multiple organs, as well as symptoms that can persist or return.

A broader attack on the immune system

For years, the prevailing belief was that the virus behind FIP infected just one type of immune cell. 

“What we found is that it actually infects a much broader range of immune cells, including those that are critical for fighting infection,” said lead author Amir Kol, associate professor with the UC Davis School of Veterinary Medicine.

The study was published in the journal Veterinary Microbiology.

The researchers examined lymph node samples from cats with naturally occurring FIP. Lymph nodes are key immune system hubs where white blood cells gather and coordinate responses to disease. The team found viral material inside several types of immune cells — including B lymphocytes, which produce antibodies, and T lymphocytes, which help the immune system recognize and eliminate infected cells.

They also found evidence that the virus was actively replicating itself inside these immune cells, rather than simply leaving behind harmless fragments. 

Why this matters beyond cats

In people with severe or long-lasting coronavirus illnesses, scientists suspect that the virus may persist in the body or continue to disrupt the immune system. Studying this directly in humans is difficult, because doctors rarely have access to immune tissues such as lymph nodes.

Cats with FIP offer a rare opportunity to study these processes up close.

“This is where cats give us a unique opportunity,” Kol said. “We can directly study infected immune tissues in a naturally occurring coronavirus disease — something that’s very difficult to do in people.”

The researchers also found that traces of the virus could remain in immune cells even after antiviral treatment ended and cats appeared healthy. Because some immune cells can live for years, this lingering infection could help explain long-term immune problems or disease relapse.

A model for long-term coronavirus disease

The findings suggest that FIP may serve as a valuable real-world model for understanding how coronaviruses interact with the immune system over time. Insights gained from cats could help guide future research into chronic inflammation and post-viral syndromes in humans, including long COVID.

By bridging veterinary and human medicine, the study highlights how naturally occurring diseases in animals can help answer critical questions about human health.

Other authors of the study include Aadhavan Balakumar, Patrawin Wanakumjorn, Kazuto Kimura, Ehren McLarty, Katherine Farrell, Terza Brostoff, Jully Pires, Tamar Cohen-Davidyan, Jennifer M. Cassano, Brian Murphy and Krystle Reagan of UC Davis.

Funding for the study was provided by the National Institutes of Health and the Sock-FIP fund at the Center for Companion Animal Health at the UC Davis School of Veterinary Medicine. It was also supported by the Faculty of Veterinary Medicine at Kasetsart University in Thailand.