For decades, feline infectious peritonitis (FIP) was considered fatal.

But in less than a decade, the disease has become a striking case study of feline therapeutic innovation.

Not only that: the first effective FIP antivirals emerged in a global grey market created by cat guardians, not within any traditional veterinary pathway.

This raises the question: are we seeing the emergence of a wider feline antiviral market?

FIP is caused by a mutated form of feline coronavirus that triggers systemic inflammatory disease. Treatment options traditionally involved supportive care, and mortality rates exceeded 90-95%.

This changed after data was published in 2018 demonstrating the efficacy of nucleoside analogue antivirals (including GS-441524) against viral replication. The research suggested survival rates of approximately 70–90% in treated cats.

Interestingly, GS-441524 is a close relation to Gilead Sciences’ remdesivir, which was later used to treat COVID-19 in humans.

This scientific breakthrough did not immediately lead to an approved FIP treatment.

Despite data published in 2018 from the Pedersen group work at UC Davis demonstrating GS-441524’s efficacy against FIP, Gilead Sciences declined to license the drug for veterinary use.

This sparked an extraordinary concerted global response: cat guardians across the world collaborated to find a workaround.

The result was the creation of a global parallel market for unlicensed GS-441524 for cats with FIP.

Over time, the market for unlicensed GS-441524 started to evolve.

Veterinarians became increasingly involved in managing treatment, in the face of growing clinical evidence that GS-441524 worked against FIP. Meanwhile, regulators in several countries began signalling greater leeway for the veterinary use of antiviral compounds.

The turning point: In May 2024, the FDA published new guidelines stating that it would not prioritise enforcement against compounded GS-441524. Pharmaceutical compounding is the preparation of customised medications by pharmacists who mix, alter or combine ingredients for a specific patient’s needs, when a commercially approved drug is unavailable or unsuitable.

This enabled veterinarians to access approved drug formulations through compounding pharmacies. Similar pathways also emerged in markets such as the UK, Canada and Australia.

The FIP treatment landscape is currently divided into three channels:

Compounded antivirals: Compounded formulations have played a crucial role in making antivirals available to treat cats with FIP. In several countries, veterinarians now access GS-441524 or remdesivir through licensed compounding pharmacies such as Bova Group and Stokes Pharmacy. However, compounding is unable to be carried out on a large scale, which means the bottleneck to wider access to FIP treatments remain regulatory constraints.

Informal and grey-market markets: Before compounding became widely available, many cat guardians acquired GS-441524 through informal networks and unlicensed suppliers. These channels still exist, but are shrinking as legal channels and access expand.

Emerging commercialisation efforts: Several companies are exploring formal development pathways for FIP antivirals, including Anivive Lifesciences, which has investigated protease inhibitor therapies targeting FIP.

The FIP grey market may be a prelude to the first modern antiviral treatment category for cats. This is particularly noteworthy as:

These factors also highlight a recurring characteristic of feline medicine: innovation is often driven from the bottom up by cat guardians using real world evidence, rather than by pharmaceutical companies or regulators.

In the case of FIP, cat guardians effectively built a parallel pharmaceutical market and then pressured regulators to respond.

There are several developments that indicate that a structured market is forming for FIP treatment. This is driven by:

In most cases, vaccines against infectious diseases usually appear before treatments. In the case of FIP, the reverse seems to be true, with treatments appearing before vaccines.

That said, global R&D into FIP vaccines is accelerating:

If antivirals attain formal regulatory approval, FIP could become one of the first cases where feline medicine has shifted so quickly from limited options to a structured pharmaceutical market.

In turn, the success of antivirals may spur further innovation in feline infectious disease. A lot depends, however, on regulators’ willingness to approve feline-specific antivirals, as well as pharma industry appetite to develop them.

The story behind the creation of a market for FIP antivirals is striking, but not altogether unusual in the feline space, where cat guardians (rather than pharma companies or regulators) are the drivers of therapeutic innovation.

While there are few other treatments that match the speed and scale of the FIP market, there are other cases where cat guardians have been key in accelerating therapeutic development:

Hypertrophic cardiomyopathy (HCM): HCM is the most common heart disease in cats, and for decades has lacked disease-modifying treatments. However, feline breed-focused groups, especially those with Maine Coon and Ragdoll cat guardians, have helped fund research into potential therapies, including sirolimus (rapamycin).

In 2025, the FDA granted conditional approval to TriviumVet’s Felycin-CA1 (sirolimus delayed-release tablets) for the treatment of subclinical HCM in cats.

Feline diabetes: Cat guardian communities, such as the Feline Diabetes Message Board, popularised home glucose monitoring using human glucometers and insulin protocols. Subsequent studies confirmed that cat guardians can reliably perform home monitoring outside the clinic.

Breed-specific diseases: Cat guardians have also helped to fund genetic research into breed-specific diseases, such as polycystic kidney disease in Persians.

These cases highlight a clear dynamic in feline medicine.

While pharmaceutical innovation in dogs often follows a traditional top-down model led by industry, progress in cats has sometimes emerged from grassroots demand.

The FIP antiviral story represents the most dramatic example to date; this indicates that engaged cat guardian groups can sometimes be seen as early signals of future therapeutic markets.