Warning: long post. TL;DR? We can drop the antiviral treatment duration for feline infectious peritonitis (FIP) to 42 days in many situations.

In veterinary medicine, a lot of our recommended durations for treatment for different diseases have little scientific backing. We “inherited” a lot of these treatment regimens from the days when the drugs were first used, when we had to try something, and as we tend to be risk averse and err on what we assumed was the side of caution, the initial treatment durations were longer more often than not. Unfortunately, rarely are subsequent studies done to refine such treatment durations to see how short we can make them while still being effective. People (not just in veterinary medicine) are often hesitant to try shorter treatments because we are risk averse and don’t like to change. There’s often pushback against using treatments that are shorter than the “standard” duration, despite the fact that the standard duration was originally just a guess that seemed to work. Duration of treatment is a big issue that I deal with every day with antibiotic treatment, but it’s also an issue with the use of other drugs, including antivirals.

Antiviral treatment with GS-441524 (GS) and remdesivir for feline infectious peritonitis (FIP) has been truly game changing. It’s taken an almost invariably fatal disease and made it usually successfully treatable (as long as we don’t start treating too late). As of 2024, these drugs are now also legally accessible to veterinarians in North America.

The current “standard” treatment duration for FIP is 84 days (12 weeks). Why 12 weeks? Ultimately a duration had to be picked to start, and that’s the number that was chosen. We’ve stuck with it not because cats necessarily need 84 days of treatment, but because 84 days seems to work.

• However, it would be better to shorten the treatment because the drugs are expensive, and treatment can be stressful on the cat (and sometimes the owner).
• GS is a very safe drug, but remdesivir can cause nasty injection site reactions, so there’s a safety issue with longer treatment too.
• Cost may be the biggest driver, as a lot of infected cats are in the care of rescues and shelters, which typically have very limited resources.

A recent study from Germany that we’ve been waiting to see in press (Zuzzi-Krebitz et al. 2024) has shown that we can likely reduce the duration of treatment to 42 days, at least in some cats. It was a randomized controlled trial (yay! we don’t get these done very often in veterinary medicine) that took cats diagnosed with FIP and randomly assigned them to get either 42 or 84 days of treatment with GS-441524 (15 mg/kg once a day). The cats were followed for 168 days to look at their response to treatment and to look for relapses. They enrolled 40 cats, which is a fairly small sample size, but not surprising since these studies are hard to do. There were 20 cats in each treatment group.

• One cat in each treatment group died (one was on day 3 and one on day 31) so the duration of treatment had no impact on survival. (This also shows how game-changing this treatment is, resulting in a 95% survival rate for a devastating disease that when untreated can have an equally high fatality rate.)
• All cats in both groups that survived to the end of their treatment course were in complete remission on day 168. That’s not a guarantee that none of them relapsed later, but 168 days is a pretty solid follow-up period.

The study used legally produced, compounded GS-441524 from BOVA in the UK. (That’s the same source used for importing these drugs to treat cats in Canada; BOVA has also partnered with Stokes in the US to provide access to the drug there too as of this summer). Using legitimate, pharmaceutical grade drug (versus black market or questionable compounding of research grade molecule) is another strong point of this study. It used a drug formulation in which we can have confidence in the quality and consistency, and that applies to what we’re using to treat cats here in Canada. BUT I would not extrapolate these results to treatment of cats with black market FIP drugs.

Does the drug dose matter?

I would also be a bit wary of extrapolating these results to cats treated with at the 10 mg/kg/day dose, which is a common treatment regimen for non-neurological and non-ocular FIP. Ideally we’d have data on duration of treatment for other dosing regimens too, but that’s probably not going to happen in the short term. That raises the question of whether we should move to the 15 mg/kg/day used in this study. How these results relate to the newer suggestion of using twice daily dosing is also unclear. I’d be more comfortable extrapolating this to 7.5 mg/kg twice daily (which is the same total daily dosage) than 10 mg/kg/day.

So, if someone is considering using a 42 day treatment instead of 84 days, I’d definitely use 15 mg/kg/day or 7.5 mg/kg twice daily.

Also note that this study focused on cats with effusive (wet) FIP.

There was one cat with neurological FIP and one with ocular FIP (one in each group) and they both survived. This could suggest that short durations are okay for these other forms of the infection, but I’d be really wary of making that call on a single case in the 42 day group. So, I would be hesitant to extrapolate these results to other forms of FIP. If there were compelling reasons to do so in a particular case, I’d be okay with a 42 day course in a cat with neurological or ocular FIP that responded really well to treatment. However, I’m a bit wary of saying we should move all of these severe cases to 42 days of treatment without more data and experience.

Finally, we need to remember that each patient is different. Response to treatment has to be part of the decision. If response is slow and incomplete at 42 days, that’s probably not a good stopping point.

As a small study, it certainly has some limitations, but this is a key study that gives us support to treat some (if not most) cats with FIP for a shorter time. That will be critical for situations in which cost is a barrier, both for shelters/rescues and homed cats. It’s one more step to making FIP a more easily treatable disease.

Thanks to the authors for doing the study.

Thanks to the cat owners for being willing to be part of the study, in which their cat might have been assigned to the group that received a shorter-than-typical treatment course.

• That’s a big deal, because people can be reluctant to sign onto a study where they are worried about the effectiveness of one of the approaches.
• If the study is looking at a new drug versus an old drug, there’s often optimism about the new drug, and people often hope to be be assigned to that group.
• For studies about shorter treatment durations, there’s less motivation to be in the non-standard group, especially if the drug costs are covered either way, so there’s no cost benefit to the cat owner. However, cost can also be a motivator to participate in a study like this if the owner could not afford or didn’t want to pay for treatment of any duration.