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Cyprus is home to 1 million or so free-roaming cats that wander its streets, parks, and even luxury resorts. They are about as numerous as people. So when a deadly cat outbreak began sweeping across the Mediterranean island this year, the humans quickly noticed something was terribly wrong.
Stray and feral cats died by the thousands. Pet cats died, too. Their bellies became swollen like bowling balls, a symptom characteristic of the disease feline infectious peritonitis, or FIP, that is almost 100 percent fatal left untreated. FIP is caused by a coronavirus that infects cats but not humans. (It is related to but distinct from SARS-CoV-2.) The disease can fester in small, indoor outbreaks, but it had never raced across an entire island, leaving thousands of dead outdoor cats in its path. In early 2023, lab-confirmed FIP cases in Cyprus shot up 20-fold. The unusualness of this outbreak frightened cat owners on the island and confounded veterinarians around the world.
It seemed like something about FIP must have changed, but what? Had a new strain appeared? Or, experts wondered, could the outbreak have something to do with the spread of COVID, also caused by a coronavirus and also known to infect cats?
Either way, the pandemic came with a silver lining for FIP: It made treating the cat disease a lot easier. The two coronaviruses share enough similarities that COVID antivirals are also effective against FIP—in fact, they are downright miracle cures. “It’s like bringing these cats back from the dead,” says Gary Whittaker, a virologist at Cornell who studies FIP.
The first FIP cure appeared a couple years before the pandemic, but for a long time cat owners’ could only buy black-market versions online. The response to COVID supercharged research into antivirals against coronaviruses, expanding the arsenal of drugs and making them more widely available. So Cyprus was oddly prepared for this new viral scourge. This summer, the government began releasing leftover stockpiles of the COVID antiviral molnupiravir for cats with FIP. If this outbreak had happened even a few years earlier, Cyprus would have had to watch helplessly as its cats died one by one.
In January, veterinarians on Cyprus began noticing an unusual number of FIP cases. Demetris Epaminondas, the vice president of the Pancyprian Veterinary Association, told me that his wife, also a veterinarian, hadn’t seen any cases of FIP the year before. This year, she’s seen more than 50. The earliest cases clustered in the capital city of Nicosia, but they soon began radiating outward, reaching the most rural, remote regions of Cyprus last—as if a novel pathogen was spreading from an initial epicenter.
The feline coronavirus is a perplexing virus that strikes different cats in radically different ways. In most cats, it infects the gut, causing only temporary diarrhea, if any symptoms at all. But in a small minority—perhaps those already under stress or with a genetic predisposition—the virus mutates into a new and unique strain inside the cat, infecting white blood cells that travel throughout the body causing inflammation. Notably, these mutated strains rarely spread to other cats. When the virus gains the ability to cause severe disease, it seems to lose the ability to infect new hosts. It keeps running smack into an evolutionary trade-off.
But in rare cases, the virus turns into a “hot strain,” says Danielle Gunn-Moore, a feline medicine specialist at the University of Edinburgh. These strains have just the right mutations to sicken cats with FIP and spread to new ones. Scientists closely documented one such outbreak in a shelter in Taiwan, where the same strain of FIP appeared to kill eight cats. Gunn-Moore and her colleagues in Edinburgh—including Charalampos Attipa, a vet originally from Cyprus who gathered some of the initial outbreak data—are now sequencing viruses to see if a particularly hot strain has developed the ability to spread on a massive scale. Piecing the genomes together has been technically challenging, she told me, because unlike with COVID, researchers have few fully sequenced feline coronaviruses to use as reference. They hope to analyze the results from about 100 samples soon. An unusual “hot strain”—perhaps combined with some genetic disposition in Cyprus’s cats—is the team’s top hypothesis.
It didn’t escape their attention, though, that this unusual outbreak comes on the heels of an outbreak of another coronavirus: SARS-CoV-2, of course. The team wondered if the feline coronavirus could have recombined with SARS-CoV-2 to create a virulent new coronavirus, though the two are divergent enough that this scenario would be unusual. Or might SARS-CoV-2 be involved but indirectly? FIP exhibits an unusual pattern where previous immunity from a vaccine can perversely make the disease even worse, Gunn-Moore told me. (This has foiled FIP vaccines for cats, though the pattern doesn’t show up with COVID vaccines for humans.) COVID has likely infected some of these cats recently, and she wonders if having had COVID might worsen FIP in a similar way—an unexpected consequence of the global pandemic.
Until quite recently, vets had no way to treat FIP. The turning point came in 2018, when a researcher at UC Davis tested a new drug chemically similar to the antiviral drug remdesivir in 10 cats with FIP. All 10 were cured. This was unheard of. News of the miracle cure began spreading by word of mouth, but because the company that patented the drug refused to license it, Chinese manufacturers began making very successful black-market versions. Remdesivir, which also works against FIP, has since become available through various means too.
In late 2020, as new COVID antivirals such as molnupiravir began generating buzz, Chinese manufacturers once again saw the potential for FIP. A company began selling unlicensed molnupiravir to overseas cat owners, touting an unpublished study in which 286 cats were all fully cured. Who knows how much stock to take in this, but remarkably, peer-reviewed research has since validated the effectiveness of the drug. In five years time, FIP has gone from the “worst diagnosis you could get” for cats to a disease fully curable with not one but three different drugs, says Nicole Jacque, a cat-rescue volunteer who has helped conduct research on molnupiravir. The interest that the pandemic spurred in coronavirus antivirals has been a boon for FIP. (Vets told me that Paxlovid could be a promising antiviral for FIP too, though no one seems to have tested it yet.)
When the FIP outbreak hit Cyprus, people there also began buying black-market drugs. The cost is incredibly high, running into thousands of U.S. dollars per cat depending on the exact drug. And there’s no guarantee that the products really contain the drug at the purity and the doses claimed. “We cannot rely on the black market,” Epaminondas says. Vets in Cyprus began asking the government for help earlier this year. By then, molnupiravir had been falling out of favor globally as a human drug, as Paxlovid had proved more effective. The Cypriot government had plenty of molnupiravir left over, and it agreed to release the first doses to veterinarians this summer.
Although molnupiravir can certainly treat FIP, it may not be the “best” drug for it. In lab studies, it seems toxic at lower amounts than the other options, so it could be easier to accidentally overdose. And because molnupiravir works by inducing lots of mutations in the virus—to the point where it should stop working—experts have wondered if it could speed up the emergence of new variants, a concern that previously dogged its human use.
But molnupiravir has big practical advantages too: It is the cheapest of the FIP drugs. And in the U.S., it is the most likely to be readily available, says Drew Weigner, a former president of the EveryCat Health Foundation, which has funded several FIP drug studies under his tenure. The drug is currently authorized for emergency use for COVID, but if it’s officially approved, vets could also prescribe it off-label to cats. And then America, too, might finally have a legal drug to treat FIP—in case the outbreak spreads globally, and even if not, for the many cats that still routinely get FIP every year.