Almost every cat meets feline coronavirus (FCoV) at some point, usually with nothing worse than a few days of mild diarrhoea. But in roughly 5 to 10% of infected cats – most often those under three years old – the virus mutates inside the body into a form that turns the cat’s own immune system against it. That disease is feline infectious peritonitis, or FIP, and until recently it was nearly always fatal. Today the story has changed, and the single biggest factor in survival is how early you catch it.
One Virus, Four Presentations
| Form | Hallmark sign | What is happening |
|---|---|---|
| Wet (effusive) | Swollen, fluid-filled belly or laboured breathing | Inflammatory fluid leaking into abdomen or chest |
| Dry (non-effusive) | Persistent fever, weight loss, no fluid | Granulomas forming on kidneys, liver, lymph nodes |
| Ocular | Eye colour change, cloudiness, unequal pupils | Inflammation inside the eye |
| Neurological | Wobbly gait, tremors, seizures, behaviour change | Disease crossing the blood-brain barrier |
Kittens more often develop the wet form; adults lean dry or organ-specific. Both respond well to antiviral therapy when treatment starts early.
Early Signs Worth a Vet Visit This Week
- A fever that keeps coming back and does not respond to antibiotics
- Fading appetite and energy – a young cat that hides more and grooms less
- Weight loss with a pot-bellied outline, the classic wet-FIP contradiction
- Yellow tinge to gums or ears, suggesting liver involvement
- Eye changes or a new head tilt and stumble with no obvious cause
- Stunted growth or persistent unthrifty look in a kitten from a multi-cat or cattery background
None of these signs is unique to FIP, which is exactly why the disease gets missed. But if a young cat combines a stubborn fever with fading appetite for more than a few days, FIP belongs on the list of suspects.
How Vets Confirm It
FIP is diagnosed by pattern, not by one test. Key findings include anaemia and low lymphocytes on a blood count, high globulin levels, and above all a low albumin-to-globulin (A:G) ratio – below 0.6 is highly suggestive, below 0.4 strongly so. In wet cases, a quick Rivalta test on the chest or abdominal fluid has around 91% sensitivity, and PCR can detect viral genetic material. Ultrasound locates the fluid or organ lesions. No single result is definitive, but combined findings push diagnostic confidence above 90% without needing a biopsy.
The Treatment Revolution
Before 2018 there was no effective therapy. Then Dr. Niels Pedersen’s landmark study at UC Davis showed that GS-441524, a nucleoside analogue that blocks the virus from copying itself, could cure the majority of cases: 25 of 31 cats – about 81% – achieved full recovery on an 84-day protocol, and later data put success rates above 85% with early intervention. The standard course runs 84 consecutive days, followed by a 12-week observation period to catch relapses, with higher doses needed for neurological and ocular forms.
One myth is worth killing: the mutated FIP virus itself is not contagious between cats. The ordinary coronavirus spreads through shared litter trays, but only a small minority of carriers ever develop FIP, so householdmates need monitoring, not panic.
If your young cat is running a stubborn fever, wasting while its belly swells, or wobbling for no clear reason, ask your vet about FIP directly. Weeks matter – treatment that starts in the first stage of illness is the difference between a routine recovery and a fight.