• This article is an extract from “FIP – causes, signs, diagnosis, treatment and prevention update”, published in Vet Times 53.27.

Monitoring

Monitoring of cats on antiviral treatment for FIP is important. Most cats experience improvement of clinical signs within the first few days; however, some cats may need supportive/additional therapy, repeated drainage of pleural effusion (monitor respiratory rates and repeat point‑of‑care ultrasound), blood pressure support or antiseizure medications, for example.

It is essential that cats are regularly weighed (owners should ideally do that at least once a week at home on suitable cat/baby scales). This is needed to ensure the appropriate dose of remdesivir or GS-445124 is given; doses should be increased accordingly to the weight gain, which can occur quickly once the kitten responds positively to the treatment.

When dosing, it is recommended to round up to the next quarter of a tablet to avoid under‑treating.

Serial blood work (CBC and serum biochemical panel) should be performed during and after completion of the course of treatment; however, it should be tailored to the client’s finances and cat’s response to therapy.

Haematocrit, total protein (albumin and globulins), and other abnormal parameters should normalise during treatment.

Remdesivir is reported to cause renal and hepatotoxicity in people, but this appears to be very rare in cats and mainly associated with higher doses; this is probably because the human and animal versions have different carriers.

In the initial study with GS-441524, effusion resolved in the first one to two weeks of treatment, while anaemia may not resolve until after six to eight weeks of treatment (Pedersen et al, 2019). Monitoring serum AGP may also be useful.

Abnormal serum protein values usually improve progressively and reach normal levels after 8 to 10 weeks of treatment.

If the patient is not improving as expected then it is important to review the diagnosis and consider other differentials (Table 1), such as FIV, FeLV, toxoplasmosis, cholangiohepatitis, septic peritonitis, mycobacteriosis, lymphoma and haemoplasmosis.

Table 1. Potential differential diagnoses for FIP
Disease	Comparison and diagnostics to differentiate
Pancreatitis	Often cats involved in fights. Abdominal and even pleural effusions have high total proteins. With bacterial infection, cell counts are high, with degenerative neutrophils and intracellular bacteria. Culture can reveal bacterial infection. Abdominal ultrasound and/or measurement of feline pancreatic lipase immunoreactivity can be helpful.
Septic abdomen	Usually very sick, may present in shock. Effusions have high cell counts, with degenerative neutrophils and intracellular bacteria (although these may be few). Culture can reveal bacterial infection.
Pyothorax	Anorexia, vomiting and weight loss is common.
Lymphocytic cholangitis	Usually more chronic. Liver enzyme activities (such as alkaline phosphatase and alanine aminotransferase) are increased; fine-needle aspiration of bile or liver, and/or liver biopsy is diagnostic.
Toxoplasmosis	Can involve multiple organs. Diagnosis by high IgM antibodies or rising IgG titres.
Mycobacterial infections	Fever, lymphadenopathy, respiratory signs, abdominal masses and uveitis are common. Severe hyperglobulinaemia is less common. Cytology may show acid-fast bacteria in tissue samples. PCR and/or culture can be useful, but diagnosis can be challenging.
Congestive heart failure	Heart murmur and/or gallop may be present. Mostly modified transudate. Echocardiography is useful to make diagnosis.
Lymphoma	Multiple organ involvement. Cytology can reveal neoplastic cells.
Carcinomatosis	Can present as single or bicavitary effusion. Cytology can reveal neoplastic cells, but often challenging diagnosis antemortem.

If the diagnosis is robust and no improvement has been seen then the dose of remdesivir or GS-441524 should be increased by 3mg/kg to 5mg/kg per day to a maximum of 20mg/kg daily, with this given in twice daily doses for GS-441524.

Drug combinations

In humans, serious viral infections (such as HIV and hepatitis C virus), are treated with a combination of a number of antiviral drugs that work in different ways, plus at least one immunostimulant as antivirals drugs benefit from a strong immune system.

For example, highly active antiretroviral therapy is known as the anti‑HIV “cocktail” – it is a combination of three or more drugs, from at least two of the three classes of antiretroviral therapy, such as protease inhibitors, nucleoside analogues, RT inhibitors, and non-nucleoside analogue RT inhibitors.

It is, therefore, very likely that FIP treatment will develop to include a combination of drugs – potentially starting with two antiviral drugs, plus an immunostimulant.

Hopefully, these drug combinations will allow for much shorter courses of treatment with fewer relapses.

Treatment of cats with faecal shedding of coronavirus

FCoV is endemic in the feline population and faecal shedding is common.

Some concerning papers have been published regarding the use of GS‑441524 and similar analogues to stop faecal shedding. The authors strongly advise against this as the cat is likely to become reinfected, and the indiscriminate use of nucleoside analogues will result in viral resistance, making FIP more difficult or even impossible to treat.

Optimising environmental conditions, improving hygiene, and reducing stress and overcrowding are recommendations for all multi‑cat environments.

• Some drugs mentioned in this article series are not licensed for veterinary use.

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Also read:

• FIP: clinical features and diagnosis
• FIP: treatment with antivirals
• FIP: treatment with financial limitations, and black market antivirals warning