Deadly Cat Disease Now Treatable: CSU Study Shows 77% Success Rate

Colorado State University veterinarians have demonstrated that molnupiravir, an oral antiviral drug, achieves a 77% survival rate treating feline infectious peritonitis, a coronavirus disease in cats that was previously invariably fatal. The findings, published February 2, 2025, in the Journal of Feline Medicine and Surgery, represent the largest clinical trial of the treatment to date.

For Northern Colorado cat owners and veterinarians, this means FIP—once a death sentence—is now treatable through local veterinary compounding pharmacies. The research may also offer insights into long COVID in humans.

FIP impacts an estimated 5 to 10 percent of cats, especially younger cats and kittens. For years, nearly all cats that developed the disease died.

In the trial, 73 pet cats with naturally occurring FIP received oral treatment for 12 weeks and were monitored for six months. Of the surviving cats, 12 percent relapsed, but all recovered after a second round of treatment. No cats experienced side effects that required stopping the medication.

"Going from a 100% lethal disease to now having multiple options for drugs that can save the majority of these cats—it's one of the most exciting things that has happened in decades in feline medicine," said Dr. Petra Cerna, a feline medicine specialist who led the study.

Another antiviral, GS-441524, achieves about 80 percent effectiveness and also received FDA clearance for compounding in 2024. As of 2024, molnupiravir is FDA-cleared for veterinary prescription through compounding pharmacies, giving Northern Colorado vets and cat owners accessible treatment options against FIP.

Having two effective treatments matters: some cats may relapse on the first medication, and combination therapy—like HIV treatment in humans—reduces antiviral resistance risk. Vets can combine both drugs if a cat relapses on the first medication.

CSU's findings align with a global shift in FIP treatment. A global assessment released in 2025 confirmed this transformation: according to the FIP Update Guide from the Feline Veterinary Medical Association and iCatCare, response rates with recommended dosages now exceed 85 to 90 percent. The guide notes that "as global access to legal antiviral formulations expands, the prognosis for FIP has improved dramatically, transitioning from an invariably fatal disease to one with a high likelihood of long-term survival."

The research also offers a window into long COVID. Treated cats can develop systemic inflammation, including myocarditis and gastrointestinal issues—conditions also seen in severe human COVID.

"These cats used to all die, but now because they live longer, we're seeing similarities between FIP in cats and human coronaviruses," said Cerna, who has previously published work detailing this connection. "There is a lot of potential to study this naturally occurring disease in cats in ways that may also benefit humans."

CSU researchers hope studying how cats recover from FIP could reveal treatments for long COVID in humans. The university is testing whether shorter treatment windows and additional antivirals, including Paxlovid, can improve outcomes further.