‘Long-FIP’: residual effects and latest research

Overview

The successful introduction of antiviral treatment with GS-441524 has transformed feline infectious peritonitis (FIP) from an almost invariably fatal disease into a potentially curable condition. Most cats appear to achieve sustained clinical remission, and prospective follow-up studies have demonstrated excellent long-term outcomes. Nevertheless, a spectrum of unexpected clinical, laboratory, and imaging findings has been observed during long-term follow-up. These observations have raised the question of whether some cats experience persistent or delayed consequences of previous FIP, its associated immune dysregulation and tissue damage, or antiviral treatment itself – an emerging concept provisionally termed “Long-FIP”.In particular, abdominal lymphadenomegaly can persist in otherwise clinically healthy cats for months after clinical recovery. Such structural changes could reflect residual inflammation, immune activation, or tissue remodelling rather than ongoing FCoV replication. However, an important recent observation is the occurrence of large-cell lymphoma (LCL) following successful treatment of FIP. Four cats from a cohort of 202 cats successfully treated with oral GS-441524 developed LCL within two years of their FIP diagnosis.

At present, this observation does not establish a causal relationship between FIP, GS-441524 treatment and subsequent lymphoma development. Several hypotheses can nevertheless be considered. FIP is characterized by profound immune activation and dysregulation, and prolonged antigenic stimulation or inflammatory processes could theoretically contribute to lymphomagenesis. A direct oncogenic effect of GS-441524 has not been demonstrated. The observation is nevertheless clinically important because lymphoma and recurrent FIP can produce overlapping clinical, laboratory and imaging findings. Unexpected neurological abnormalities have also been observed following apparently successful FIP treatment. Transient signs resembling feline hyperaesthesia syndrome have been reported months after completion of GS-441524 therapy in cats without other evidence of FIP relapse.

The pathogenesis of these abnormalities remains unknown. Potential explanations include residual inflammatory or neurological damage caused by previous FIP, immune-mediated mechanisms, effects associated with antiviral treatment, persistent infection within a tissue compartment, or an entirely unrelated neurological disorder. Current evidence does not yet establish “Long-FIP” as a defined disease entity. Rather, the term encompasses a growing spectrum of observations made possible by the unprecedented survival of cats that previously would not have survived FIP. These observations could ultimately represent several biologically distinct phenomena, such as residual tissue damage or remodelling following severe inflammatory disease; persistent immune or inflammatory alterations; consequences of prolonged antiviral exposure; recurrent or newly acquired FCoV infection; and potentially an altered risk of subsequent diseases, including lymphoma.

Learning outcomes

By the end of this webinar, participants should be able to:

Describe the currently recognised long-term clinical, laboratory, and imaging findings in cats following successful antiviral treatment of FIP.
Differentiate residual post-FIP abnormalities from true FIP relapse and recognise that persistent or newly developing abnormalities after successful treatment do not necessarily indicate FIP.
Recognise newly emerging diseases in FIP survivors, including lymphoma, and incorporate appropriate differential diagnoses when cats develop clinical signs following successful FIP treatment.
Develop a rational approach to long-term follow-up of FIP survivors, including appropriate investigation of persistent or new abnormalities.