New feline coronavirus blamed for thousands of cat deaths in Cyprus

When thousands of cats started to die this year on the Mediterranean island of Cyprus, nicknamed the “island of cats” for its 1-million-strong feline population, the crisis made international news. The animals had fevers, swollen bellies, and lethargy—symptoms that pointed to feline infectious peritonitis (FIP), a common condition caused by a type of cat coronavirus. But scientists struggled to explain the apparent explosion in cases.

Now, researchers have identified a possible culprit: a new strain of feline coronavirus that has coopted key RNA sequences from a highly virulent dog pathogen called pantropic canine coronavirus (pCCoV). The findings, posted as a preprint last week on bioRxiv , could help explain how severe illness managed to spread so widely among cats on the island.

They ve done a great job in identifying what looks to be a very interesting and concerning virus, says Gary Whittaker, a virologist at the Cornell University College of Veterinary Medicine who was not involved in the research. Although canine-feline coronavirus crossovers have been reported before, he says, this is the first documented case of a cat coronavirus combining with pCCoV, apparently leading to a perfect storm of both disease and transmissibility.

Veterinarians in Cyprus raised the alarm early this year about increased cases of FIP, which is not related to COVID-19 and does not affect humans. By July, animal activists and media outlets had reported nearly 300,000 deaths, although local veterinarians revised that figure dramatically downward, to about 8000. In August, the Cypriot government agreed to the veterinary use of the human SARS-CoV-2 medication molnupiravir, which blocks coronavirus replication and appears to be an effective treatment for FIP.

The rapid rise in cases presented scientists with a puzzle. Most feline coronaviruses infect the gut, where they cause mild infections that don t escalate to FIP. These strains are easily transmitted from cat to cat through feces. They sometimes mutate into a more dangerous form called FIP virus (FIPV) that instead infects immune cells and triggers serious disease. But unlike the intestinal strains, FIPV typically isn t transmitted between animals.

To determine what was causing the new infections, researchers at the University of Edinburgh and colleagues collected fluid samples from the abdomens and spines of sick cats admitted to clinics in Cyprus and used RNA sequencing to look for viral genetic information. What they found was a previously undescribed feline coronavirus, which they dubbed FCoV-23, that contains a large chunk of RNA from the dog virus pCCoV. (The “pantropic” in its name means that, unlike regular intestinal canine coronaviruses, pCCoV infects lots of different tissues.)

FCoV-23 appears to have arisen when a feline coronavirus encountered pCCoV in an unidentified animal host and coopted the latter s spike protein —the structure coronaviruses use to gain access to host cells, explains study co-author Christine Tait-Burkard, a virologist at the University of Edinburgh’s Roslin Institute. This and other genetic tweaks may have allowed FCoV-23 to cause severe FIP while still infecting the intestines and spreading through feces, she says. The team also speculates that the spike protein changes could have made FCoV-23 more stable outside an animal host, increasing the chance of transmission via contact with contaminated feces .

a veterinarian holding up a cat to show its belly
A veterinarian holds up a cat infected by FCoV-23, showing a swollen belly and missing fur. Pet Priority Medical Care/Sotira, Famagousta, Cyprus

It s not yet clear how far FCoV-23 has spread, although the team did identify one case in the United Kingdom in a cat that had been imported from Cyprus. The general risk to cats outside the island remains low, Tait-Burkard says.

Margaret Hosie, a virologist at the University of Glasgow and president of the European Advisory Board on Cat Diseases, says it s exciting to see virological data emerging from the Cypriot population. However, she cautions that there are many open questions about how FCoV-23 spreads and causes disease. For example, it will take more epidemiological data to be sure the virus is transmitted directly from cat to cat through feces.

The apparent rise in FIP cases this year could partly be the result of increased awareness of the condition, she notes. We don t know the numbers previously, so we can t say there s been a huge outbreak. Feline coronaviruses and pCCoV have coexisted in the Mediterranean region for years, she adds, so it s possible that the genetic crossover happened some time ago.

Tait-Burkard and colleagues are now collaborating with researchers in Cyprus to test local cats for FCoV-23 and get a better idea of its prevalence and fatality rate. They and others are also working to improve diagnostic tools for the virus and are planning lab studies to probe how it infects different cell types. The team additionally wants to investigate whether unique features of FCoV-23 can explain the resulting disease s apparently high rate of neurological symptoms—present in 28% of cases compared with 14% of typical FIP.

In the meantime, the discovery of this mixed cat-dog coronavirus highlights the importance of taking a broad, cross-species approach to understanding viral evolution, Whittaker says. This feline coronavirus has got huge potential for us to understand what goes on in general in coronavirus virology.