QUARTERLY BEAT
APRIL 2026
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FIP DIAGNOSTICS UPDATES FROM 2025 Dr. Amy Kaplan-Zattler, cVMA, DACVECC, MRCVS CE Program Manager, VETgirl Now that antivirals have made FIP treatment a real option, how do we confidently diagnose FIP? This VETgirl article breaks down which cats are at risk, what FIP looks like “in the wild,” and how to stack supportive diagnostics so you can start therapy sooner. And if you’d prefer instead to listen about FIP diagnostic updates, you can also tune into our FIP in Real Practice- Updated Clinical Cues and Diagnostics podcast HERE!
Image courtesy of Dr. Dave Gordon.
In a small number of cats infected with FeCV (around 10%), the virus can mutate and begin replicating within macrophages and monocytes. At this stage, the newly mutated virus is called the feline infectious peritonitis virus (FIPV) and it essentially “hitchhikes” inside these white blood cells throughout the body, triggering an over-the-top inflammatory response centered around blood vessels (i.e., vasculitis), which is what ultimately drives the clinical symptoms of FIP. We still don’t fully understand how or why this mutation occurs in some cats but not in their siblings or in other cats living in the same environment. FIP is more often thought as a “young cat” disease (< 2 years of age), but don’t cross it off your differential list in adult cats; a small percentage are first diagnosed later in life. Intact males are overrepresented compared to females, and breed may also play a role as purebred cats have been overrepresented in some reports. Other risk factors include coinfection with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV), other concurrent disease, immunosuppression, and stress.
For years, feline infectious peritonitis (FIP) had been a diagnosis that made everyone’s stomach drop — the kind of case that felt as grim as the big “C” (cancer) diagnosis. But in the last few years, extra-label use of select antivirals has flipped the script, with reported response rates around 85–90%. With treatment now truly on the table, the priority shifts to rapid, confident diagnosis so cats can start therapy as early as possible for the best outcomes. In Part 1 of this 2-part blog, we focus on recognizing FIP in real life and how to build the strongest supportive diagnostic case. Check back next week for Part 2, where we dive into updated treatment strategies, including dosing, monitoring, and with a refreshed antiviral “cheat sheet” chart. Which Cats get FIP? Most young cats will pick up a feline enteric coronavirus (FeCV) at some point - often early in life - especially when living in close proximity of other cats such as in multi-cat households, catteries, or shelters. FeCV replicates within the apical columnar epithelial cells of the small intestine which can cause gastrointestinal signs such as diarrhea, some cats develop upper respiratory signs, and some remain asymptomatic. The virus is shed as early as 2-3 days after infection and cats may continue shedding for up to several weeks; a small handful of cats have been documented to shed lifelong. The diarrhea is usually short-lived, but if you have a kitten presenting with a history of persistent diarrhea for weeks to months, FeCV should definitely be among your differential diagnoses.
What Does FIP Look Like Clinically in Our Feline Patients? FIP is not a one-size-fits all disease – in fact it’s
quite the shape-shifter. Many times FIP presents as an “ADR” cat, producing
only nonspecific signs (e.g., lethargy, anorexia, weight
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