My cat has a fever — could it be FIP? How to tell, and when to worry

Short answer: a persistent fever that doesn't respond to antibiotics is one of the classic early signs of FIP — but fever has many causes, and most feverish cats do not have FIP. What raises suspicion is the pattern: a young cat, a fever that keeps coming back, and weight loss or fading appetite alongside it. Only a vet can tell for sure, and the sooner the better.

If you're here because your cat feels hot, is off their food, and you've read the word "FIP," take a breath. This page is the honest version of how worried to be — and what actually distinguishes an FIP fever from the many ordinary ones.

What a normal cat temperature is

A healthy cat runs warmer than a human — around 38.1–39.2°C (100.5–102.5°F). A true fever is generally above 39.4°C (103°F). A warm-feeling cat is not proof of a fever; only a thermometer confirms it, and "hot ears" or a "warm nose" are not reliable.

The fever pattern that points toward FIP

FIP fever has a recognisable character that separates it from a passing bug:

It persists or keeps returning — days on end, or a fever that fluctuates up and down rather than breaking.
It does not respond to antibiotics — this is one of the strongest clues, because antibiotics do nothing to a virus.
It comes with other signs — weight loss, fading appetite, lethargy, and sometimes a swollen belly, jaundice (yellow gums or eyes), or eye changes.
The cat is young and/or from a multi-cat background — most FIP cases are in cats under two, and shelters, catteries, and multi-cat homes see more of it.

A one-off fever in an otherwise bright, eating cat that resolves in a day or two is far more likely something ordinary.

The many things that are NOT FIP

Fever is the body's response to all sorts of problems, most of them not FIP:

A cat-bite abscess — extremely common, especially in cats that go outside; a fight wound seals over and forms a hot, painful swelling.
Viral upper-respiratory infection (cat flu) — sneezing, runny eyes, fever.
Other infections — dental, urinary, wounds.
Inflammation, pancreatitis, and other illnesses.

The point is not to be reassured into waiting — it's that a fever means "see a vet," not "assume the worst."

What to do

See a vet — don't wait it out. FIP moves, and so do the treatable things that mimic it.
Expect bloodwork. The blood pattern that supports FIP — high globulin, a low albumin-to-globulin (A:G) ratio, high bilirubin, anaemia — helps separate it from other causes. No single test is proof; the vet builds the picture. See why FIP diagnosis is hard.
Mention the context — the cat's age, whether it came from a multi-cat environment, and any weight loss or appetite change. It genuinely changes how a vet weighs the possibilities.

If it does turn out to be FIP, the news is far better than it once was: it is now a treatable disease with cure rates above 80%. But the first job is a diagnosis, not a guess.

Worried and want to talk it through? A vet on WhatsApp can help you decide how urgently to act — details below. It's a conversation, not a diagnosis.

Not veterinary advice. A fever always warrants a veterinarian who can examine your cat.

References

Cornell Feline Health Center — Feline Infectious Peritonitis. vet.cornell.edu
Efficacy of GS-441524 for feline infectious peritonitis: a systematic review (2018–2024). Read on PMC