Back in August, I wrote about a high-profile but poorly described “outbreak” of feline infectious peritonitis (FIP) in cats in Cyprus. Now, a preprint publication (Attipa et al. 2023) has come out with some interesting information, but lots of questions still remain. That’s the nature of emerging diseases – a single report gets us farther down the path but rarely answers all of the important questions.

Here’s a quick refresher about FIP: Feline infectious peritonitis is a nasty disease caused by feline coronavirus (FCoV), a very common virus in cats that circulates widely in the cat population. Whenever you have a large group of cats, you can be pretty sure there’s FCoV circulating somewhere in the group. It’s usually not a problem because the virus doesn’t typically make most cats sick. However, FIP develops in a small percentage of infected cats when there’s a random mutation in the spike protein of the virus that occurs after the cat is infected. So FIP is not spread cat-to-cat, it’s FCoV that spreads between cats, with an unpredictable change to the virulent version of the virus causing FIP in some cats.

In the past, FIP has been considered an almost invariably fatal disease. There are some new treatment options, but those are mainly black market-sourced in North America. I’ll get back to that topic later as there are some new developments.

Back to the cat deaths in Cyprus…

Reports came out of Cyprus earlier this year of thousands of cats dying from what they suspected was FIP. The numbers varied a lot and tended to decrease when people started asking questions, but still, there were lots of cats dying. It’s tough to assess the significance of reports like that when we have little data about what’s going on now and little historical data for comparison. There are lots of cats on Cyprus and most are feral. If you have a million feral cats, I’d expect a hundred thousand (or more) deaths per year in that population under normal circumstances, many due to FIP among other things, but we almost never hear about those deaths in the population because people don’t look, count or test. However, I also don’t dismiss boots-on-the-ground observations, so when people start talking about remarkable increases in deaths, I pay attention.

The preprint (Attipa et al. 2023) provides some more epidemiological and clinical information:

  • They only had 165 confirmed FIP cases, which is a very small number for a country with so many feral cats, but since diagnosis requires lab testing which costs money, only a subset of cats ever get tested.
  • This rate is considerably higher than the 3-4 cats per year in Cyprus that are normally diagnosed with FIP, but we don’t know if that’s because of a marked increase in disease, a marked increase in testing, or a combination of both.
  • Disease in the confirmed cases was pretty typical for FIP (i.e. no unusual clinical findings).

So was this a huge outbreak or not?

When there’s more discussion and awareness of a disease, you get more diagnoses of endemic disease that may have been there all along. Often these “outbreaks” are combination of a small overall increase or a small local cluster of disease and increased testing and reporting, leading to over-estimation of the true increase in the disease rate. Only a few cats a year were diagnosed with FIP in Cyprus before this, but how many did they test? If you start to look, you find things that have always been there.

However, the genomics data included in the paper support concerns that this could be a true, new, large FIP outbreak. The virus they found appears to be a recombination of a feline coronavirus with a pantropic canine coronavirus strain (see diagram below). Those are both alphacoronaviruses, which are different from SARS-CoV-2, which is a betacoronavirus. This kind of recombination event can happen, so it’s not a shocker. What it means and whether it will play a role in changes in disease or disease patterns is unclear, but it will need to be studied.

The big question is whether this new strain might be transmissible between animals as an FIP virus, versus the typical situation where a random mutation needs to occur in the FeCoV in a previously infected cat in order to cause FIP.

  • Based on some genomic characteristic and the very close similarity of different virus isolates, the authors suggest that this could be a virus that is transmissible in a FIP-causing form. If so, that would be a big concern, and could plausibly cause a true, large outbreak of FIP.

It’s also unclear if this is a new issue or whether it’s a newly discovered issue that’s already been there for some time. If it’s driving an outbreak, we’d expect that it’s a new emergence. However, due to lack of surveillance before the outbreak, we can’t be sure it wasn’t there before. I’d guess it’s fairly new based on the similarity between the strains they tested, as viruses tend to develop more population diversity over time (e.g. you find a wider variety of strains the longer the virus has been circulating, but all the strains they found were very similar).

This virus strain was also found in a couple of cats imported from Cyprus to the UK. That’s not surprising since lots of cats get moved around and they bring pathogens like this with them. It raises concern about whether this could result in seeding of a new, more virulent, strain in other countries around the world.

It’s still too early to make a definitive call on whether this is a transmissible FIP virus, whether it’s likely to cause other outbreaks or whether it’s spread beyond Cyprus. Unfortunately, as is often the case with infectious diseases, it’s a “time will tell” scenario, but we need to keep investigating to figure out what’s going on, what the risks are and, if this is an issue, what we can do to reduce there risk and impacts.

They also used molnupiravir in lots of cats during this outbreak, as they were using up stockpiled and expiring COVID-19 drugs for people. That scares the hell out of me given that drug’s crap-tastic performance as a COVID-19 drug and its tendency to cause more viral mutations. But, that’s a separate story…