Relapses can occur after the completion of treatment if the FIP virus has not been completely eliminated. This can be attributed to various factors, including insufficient uptake of the antiviral into certain lesions, viral resistance to the antiviral, or inadequate duration or dosage of treatment.
It's important to note that relapses may not present with the same symptoms as the original diagnosis. For example, cats that previously had effusions may not exhibit any signs of effusion during a relapse. It is common for relapses to manifest with ocular or neurological symptoms, even if the cat had not previously shown such symptoms. Neurological and ocular symptoms can sometimes be subtle and may have been missed during the initial diagnosis or may not have visibly appeared yet, indicating that the selected dosage may have been too low, merely suppressing the virus. In other cases, the virus may have escaped to the eyes or brain during treatment.
The relapse rate appears to be relatively low for cats that have successfully completed a full and accurate treatment using high-quality antiviral preparations. Anecdotally, for cats treated with GS-441524, the relapse rate seems to be less than 10%. Cats with neurological and ocular involvement are more prone to relapse due to the added difficulty of overcoming the blood-brain and blood-eye barriers. Additionally, viral resistance to the antiviral can exacerbate the likelihood of relapse.
The most common timeframe for a cat to experience a relapse is within the first few weeks after treatment. While rare, there have been a few reported cases where cats diagnosed with FIP again after more than 3 months post-treatment. It remains unclear whether these instances are true relapses or cases of re-infection or re-mutation of FIP.
Treating a relapse typically involves reinitiating treatment for a minimum of 8 weeks, starting at a higher daily dosage than the previous round of treatment, following the guidelines for dosage increases used when there is a poor response to treatment.
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