Summary of Section 10: Treatment of FIP  The availability of effective curative antiviral treatments for FIP, particularly the nucleoside analogue GS-441524, has totally changed the landscape of this previously fatal disease. These treatments act quickly, allowing for the diagnostic trial treatment of cats in which FIP is very likely. However, treatment is often expensive, not licensed and not available legally in many countries, which complicates its use. Some countries have access to veterinary compounded GS-441524 products. In others, owners source antivirals themselves online, but the quality, purity, and concentration of active ingredients in these preparations is unknown, although they are clearly effective, based on published studies.  Success rates of 81% to 100% have been reported in cats treated with different preparations of compounds believed, or known, to contain GS-441524. In initial studies, GS-441524 was administered by subcutaneous (SC) injection, which was often painful, but oral preparations are now available, which are very effective, are cheaper and better tolerated that SC injections. Most studies have used 84-day treatment courses, but shorter courses might be also effective. Non-clinically significant transient adverse effects of GS-441524 can include elevations in ALT (hepatoprotectants are sometimes given but are unlikely to be needed), lymphocytosis, and eosinophilia. Weight gain has been cited as a simple long-term measure of treatment efficacy too as it is easy to measure using paediatric weighing scales, every one to two weeks, allowing for an increased dose to be calculated to maintain the appropriate dosage despite weight gain during recovery. Hyperbilirubinaemia, hyperproteinaemia and leucocyte abnormalities typically normalise within a few weeks, but hyperglobulinaemia might take longer to normalise. Overall, a good appetite and/or activity level, a higher temperature, a lower bilirubin concentration and fast normalisation of α1-acid glycoprotein (AGP) appear to be prognostically useful to predict survival with GS-441524 treatment. FCoV antibody concentrations are not useful to track response to treatment. Abdominal lymphadenomegaly has been reported following effective GS-441524 treatment but does not signify FIP relapse.  Remdesivir is a nucleoside analogue and the prodrug of GS-441524. A human-licensed preparation is available, as well as a veterinary compounded product in some countries. Remdesivir is injected, either intravenously or SC, but SC administration is painful. Most veterinarians thus favour oral GS-441524 treatment for FIP, unless remdesivir is the only antiviral available and/or the cat is unable to tolerate oral medication (e.g., due to being very sick). No comparative controlled studies currently exist on the efficacy of remdesivir and GS-441524.  Molnupiravir is another oral nucleoside analogue. It has shown promising results as a first-line agent and a rescue agent for cases that relapse. A human-licensed preparation is available, but rules vary in different countries as to whether it may be used in cats. More comparative studies are required.  Protocols have emerged on how nucleoside analogues are used to treat FIP; these usually include recommendations for higher dosages in cats with ocular or neurological signs.  Vaccination and neutering have both been performed during, or following, successful treatment of FIP with nucleoside analogues, in cats in which these procedures have been deemed necessary. No relapse of FIP has been recorded although employment of feline-friendly methods is recommended to minimise stress.  GC376 is an injectable protease inhibitor that has been used successfully for the treatment of FIP. Dentition adverse effects were reported. No legal preparations are currently available although it is hoped that a cat licensed product will be available in the future.  Some veterinarians have used mefloquine, recombinant feline interferon-omega (rfIFN-ω) and/or polyprenyl immunostimulant (PI) for the treatment of FIP, although none of these are as effective as the nucleoside analogues.  Oral mefloquine is an affordable human-licensed product that has been used anecdotally for FIP, but no published FIP treatment studies exist. It might be useful as adjunct treatment or in cases where other more effective antivirals cannot be used due to cost or availability. It is given with food to avoid vomiting as a side effect.  rfIFN-ω is licensed for use in cats in some countries and, for FIP, it has been used most recently following antiviral therapy with GS-441524 to prevent relapse. However, controlled studies are needed to confirm efficacy of, and need for, rfIFN-ω, as many studies have shown excellent survival following nucleoside analogue (including GS-441524) treatment without follow-up rfIFN-ω.  PI might be helpful in the treatment of FIP without effusions although response to treatment is slow, over several months. It has been found that concurrent systemic glucocorticoid treatment should be avoided with PI, as this worsens prognosis.  Although more studies are needed, systemic glucocorticoids should probably be avoided in the treatment of FIP, although topical steroids for uveitis are permitted.  Veterinary supportive care (e.g., intravenous fluids, appetite stimulants, anti-emetics, analgesia, vitamin B12, non-steroidal anti-inflammatories) is very important in the recovery of cats that are very sick due to FIP. However, veterinary support is often not sought by owners who have obtained antiviral drugs illegally for their cats as veterinarians are unable to advise, obtain or prescribe illegal drugs, leading to a disconnect between owners and veterinarians. It is possible for vets to give supportive care to cats in this situation for welfare reasons, as long as documentation is created to confirm there has been no veterinary involvement in the illegal drug procurement or administration.  Further details on antiviral, immunomodulatory and supportive treatments for FIP (including dosages) are given in Table 2 and Table 5, which should be used in conjunction with this summary.