FIP Relapse: How to Spot Early Signs in the 84-Day Observation Window
- DVM Vien
- May 12
- 3 min read
Quick answer: Approximately 10-15% of cats relapse during the 84-day observation period after GS-441524 treatment ends. Most relapses occur between days 14 and 56 post-treatment. Early warning signs are a return of fever (over 39.5°C), reduced appetite for more than 24 hours, lethargy, or any new neurological sign. If you suspect relapse, restart GS-441524 at +2 mg/kg above the original dose for another 84 days. Cats that complete a second course achieve approximately 90% sustained remission.
What FIP relapse actually means
A relapse is the return of FIP clinical signs after the 84-day treatment course has ended. It is not the same as treatment failure (which means the cat never responded). Relapse typically reflects either residual virus that the immune system couldn't fully clear, or virus in a poorly-penetrated body compartment (most commonly the central nervous system) re-emerging.
The 84-day post-treatment observation is built specifically to catch relapse early. During this window, do not declare your cat cured. Continue weight tracking, watch for the warning signs below, and consider a bloodwork panel at days 28 and 84 post-treatment.
When does FIP relapse usually happen?
Days 1-14 post-treatment: rare, but possible if the original course was incomplete or under-dosed
Days 14-56 post-treatment: peak relapse window, accounting for approximately 70% of relapses
Days 56-84 post-treatment: less common, but still worth watching
After day 84 post-treatment: very rare. Cats symptom-free at this point are generally considered to be in sustained remission.
Warning signs by category
General signs (any FIP form)
Return of fever over 39.5°C — the single most reliable early sign
Decreased appetite for more than 24 hours
Unexplained weight loss after stable post-treatment recovery
Lethargy or behaviour change — hiding, reduced grooming, less interaction
Wet FIP relapse signs
Returning fluid in the abdomen (visibly distended belly)
Returning breathing difficulty or rapid breathing (chest fluid)
Dry / ocular FIP relapse signs
Eye changes: cloudiness, colour shift in the iris, asymmetric pupils
Newly palpable lymph nodes or organ enlargement
Return of jaundice
Neurological relapse signs
Neurological relapse is the most dangerous and the most easily missed because signs can be subtle:
Wobbly walking, reluctance to jump, or paw placement errors
Head tilt, circling behaviour, or unusual head movements
Seizures, tremors, or sudden vocalisation episodes
Behaviour changes: aggression, withdrawal, disorientation
What to do if you suspect relapse
Take the cat's temperature. Anything over 39.5°C is significant.
Weigh the cat and compare to the last 2 weeks of weight log. Sudden drop is significant.
Contact our support team immediately. Send the symptom timeline, temperature log, and weight history.
Book bloodwork with your local vet: CBC, biochemistry, A:G ratio. Compare to the day-84 baseline.
If clinical signs are confirmed, restart treatment at +2 mg/kg above the original dose, for another full 84 days.
Retreatment protocol
Standard retreatment protocol is the original dose +2 mg/kg/day, given subcutaneously for a fresh 84 days. For neurological relapse specifically, some protocols recommend +4 mg/kg over the original dose because of the blood-brain barrier challenge. Your consulting vet decides based on the specific case.
Original wet FIP at 6 mg/kg → retreat at 8 mg/kg
Original dry FIP at 8 mg/kg → retreat at 10 mg/kg
Original ocular or neuro at 10 mg/kg → retreat at 12-14 mg/kg
For full survival rate context, see our FIP survival rate analysis. For injection technique reminders during retreatment, see the injection guide.
Frequently Asked Questions
How likely is my cat to relapse?
Approximately 10-15% of cats that completed the 84-day protocol at correct dose relapse. The number rises sharply for cats with shortened or under-dosed courses. Neurological FIP has the highest baseline relapse rate (around 20%).
Should I do prophylactic treatment during observation?
No. The 84-day observation period is treatment-free by design. Resuming GS-441524 without clinical signs of relapse adds cost and stress without clinical benefit. Wait, watch, and act fast if signs appear.
What if relapse signs are ambiguous?
Bloodwork is the tie-breaker. A globulin spike with falling albumin or rising AGP strongly suggests relapse. A normal bloodwork panel with a one-day fever might be an unrelated viral infection. When in doubt, contact our support team — we'll review and recommend.
Can a cat relapse more than once?
Yes, though uncommon. Cats that relapse after a second course typically face a more difficult treatment trajectory. Third-course success is still possible with high-dose protocols (often 15+ mg/kg), but the decision involves cost, quality of life, and the specific clinical picture. Always consult your vet for a multi-relapse case.
Informational only — not veterinary advice. All retreatment decisions belong to your consulting/treating veterinarian.
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