A new study from the University of California, Davis, is offering new hope for cats diagnosed with feline infectious peritonitis (FIP), a disease that was once considered a death sentence. Caused by a mutated feline coronavirus, FIP has historically led to severe illness and high mortality, particularly in young cats living in shelters or catteries.

Published in STEM CELLS Translational Medicine, the new research explores the effects of combining antiviral medication with mesenchymal stromal cell (MSC) therapy. FIP occurs when a usually mild intestinal virus mutates and begins attacking a cat’s immune system, often resulting in either fluid buildup in the abdomen or inflammation in organs without fluid—known as wet and dry FIP, respectively.

While antiviral treatment alone, such as GS-441524, has been shown to eliminate the virus, researchers observed that some cats continue to experience immune system irregularities even after clinical recovery. These symptoms include enlarged lymph nodes, hinting at a prolonged immune response that mirrors long COVID in humans.

In the UC Davis clinical trial, cats received antiviral medication either alone or alongside MSC infusions. While all participants recovered from the infection, those that received MSC therapy showed clear signs of improved immune regulation. These cats had fewer hyperactive immune cells and increased numbers of regulatory T cells, indicating a more balanced immune response. Scientists also noted a restoration of exhausted immune cells and potential development of long-term immune memory.

The treatment was well tolerated, with no serious side effects reported.

Beyond helping cats recover more fully from FIP, the findings may offer broader insights into how the immune system recovers from severe coronavirus infections in both animals and humans. The research was supported by the NIH and UC Davis’ Center for Companion Animal Health.