Feline infectious peritonitis develops when a common and usually harmless feline coronavirus mutates inside an individual cat. It is not caught from another cat as FIP itself. The classic picture is a young cat with a fluctuating fever that does not respond to antibiotics, weight loss and lethargy. It needs proper investigation rather than another course of treatment.
The most important thing to say first
If you search this condition you will find a great deal of writing, some of it recent and some of it a decade old, and the older material is uniformly bleak. Advice about FIP has changed substantially in the last few years, and information written before that is no longer an accurate guide to what can be done.
What has not changed is the value of getting a diagnosis rather than guessing. So if you have a young cat who is unwell in a vague, persistent way, the useful action is an appointment and a conversation about current options, not an evening reading old forum threads and drawing conclusions.
Where it comes from
Feline coronavirus is extremely common and mostly unremarkable. Many cats carry it, particularly those who have lived in shelters, catteries, rescues or multi-cat households, and in the great majority it causes nothing worse than mild transient diarrhoea, often in kittenhood, and is then shed in faeces and passed around via shared litter trays.
In a small proportion of cats, the virus mutates within that individual animal in a way that lets it infect and replicate inside immune cells. That mutated form spreads through the body and triggers an intense inflammatory response, and that is FIP.
The practical consequence of this is worth being clear about, because it causes a lot of unnecessary distress. A cat with FIP is not spreading FIP to your other cats. The common coronavirus circulates in a household, but the mutation is an individual event. Your other cats are not doomed because one has been diagnosed.
Risk is higher in cats under about two years old, in purebred cats, in multi-cat and high-density environments, and after a stressful event such as rehoming, surgery or a house move. It is also seen in older cats.
Two presentations, and both are easy to miss early
The early signs are frustratingly non-specific: a fluctuating fever that does not respond to antibiotics, poor appetite, weight loss or failure to grow, lethargy, and a general sense that a cat is simply not thriving. Many cats are treated for something else first, and that is understandable rather than negligent.
The effusive or wet form involves fluid accumulating in body cavities. The abdomen swells, sometimes quite quickly, or fluid collects around the lungs and causes laboured breathing. A kitten with a distended belly who is otherwise thin is a recognisable picture.
The non-effusive or dry form produces inflammatory lesions in organs instead, and it is the harder of the two to identify. Depending on where those lesions sit, it can cause eye changes such as cloudiness, colour change or inflammation inside the eye, or neurological signs including wobbliness, tremors, behaviour changes or seizures. A young cat with unexplained eye or neurological signs should have this considered.
Any cat with laboured breathing, a rapidly swelling abdomen, collapse or seizures needs same-day attention through our urgent care service regardless of what the underlying cause turns out to be.
Why diagnosis is genuinely difficult
There is no single blood test that returns a yes or no. This is the part owners find hardest, and it is worth explaining rather than glossing over.
A common coronavirus antibody test tells you a cat has been exposed to the ordinary virus, which enormous numbers of healthy cats have. It does not tell you the mutation has occurred, so on its own it cannot diagnose or rule out the disease.
A diagnosis is instead built from several pieces: the signalment and history, bloodwork patterns including changes in protein fractions, imaging, and where fluid is present, analysis of that fluid, which is often the most informative single sample available. Molecular testing on fluid or tissue adds further weight. Our diagnostic services cover this work, and our post on when your pet needs advanced assessment explains how layered testing of this kind is put together.
Because the picture is assembled rather than read off a machine, the conversation happens in a medical consultation where all of it can be considered together. Expect that to take more than one visit.
Treatment: ask, do not assume
For a long time this condition was considered untreatable, and that belief is still the first thing many owners encounter. The situation has changed considerably in recent years, antiviral approaches have altered what is possible, and the regulatory and availability picture has continued to move.
What that means for you is simple and specific: ask your veterinary team what the current options are for your cat, in this province, right now. Do not conclude anything from an article, this one included, and do not act on advice from online groups about obtaining medication through unofficial channels. Products sourced that way are of unverified content and quality, treatment needs proper veterinary monitoring to be done safely, and a cat being treated without that oversight can deteriorate without anyone recognising why.
Bring your questions to the appointment. This is an area where a current, individual answer is worth far more than a general one.
Reducing risk in a multi-cat home
You cannot prevent the mutation, but you can reduce coronavirus load and the stress that appears to contribute.
Litter trays are the main route, so provide one per cat plus one, keep them apart rather than in a row, scoop at least daily and clean thoroughly. Keep trays well away from food and water bowls. Avoid overcrowding, since density is the single clearest environmental factor.
Reduce stress where you can: enough vertical space, separate resting places, predictable routines, and a gentle introduction process for any new cat. Quarantine new arrivals initially, which also helps with everything else they might bring.
Where a household is grieving one cat and worrying about the others, the reasonable approach is monitoring rather than testing everyone. Watch for fever, weight loss and failure to thrive, and have any cat who becomes unwell examined promptly. Our guide to kitten care in Victoria covers the early-life checks where these problems most often surface, and ongoing monitoring sits within our wellness program. Because the inflammatory picture here overlaps with other immune-driven disease, our post on autoimmune disorders is a useful companion read on how such conditions are investigated.
Frequently asked questions
Is FIP contagious to my other cats?
Not as FIP. The common feline coronavirus does spread between cats through faeces and shared litter trays, but the mutation that causes FIP happens inside an individual cat and that mutated form is not readily transmitted. Your other cats are not fated to develop it because one has been diagnosed. Good litter hygiene and avoiding overcrowding remain worthwhile.
What are the early signs of FIP?
Vague ones, which is why it is missed: a fluctuating fever that does not respond to antibiotics, poor appetite, weight loss or failure to grow, and lethargy. Later signs depend on the form, with a swollen abdomen or laboured breathing in the effusive type, and eye or neurological changes in the non-effusive type. Young cats are most often affected.
Can a blood test confirm FIP?
No single test does. A coronavirus antibody result only shows exposure to the common virus, which most cats have had, so it cannot confirm or exclude the disease. Diagnosis is built from history, bloodwork patterns, imaging, analysis of any fluid present, and molecular testing. That is why it usually takes more than one visit.
Is FIP still always fatal?
The information most owners find first is out of date. What can be offered has changed considerably in recent years, and availability and regulation have kept moving. Rather than relying on any article for this, ask your veterinary team what the current options are for your cat in this province. That answer is specific and it changes.
Should I buy treatment online?
No. Medication obtained through unofficial channels is of unverified content and quality, and treatment for this condition requires proper veterinary monitoring to be carried out safely. A cat treated without that oversight can deteriorate without anyone identifying why. Have the conversation with your veterinarian instead, including about cost and practicalities.
Can I have my cats tested to see if they will get it?
There is no test that predicts it, because the mutation is an individual event that cannot be foreseen. Testing healthy housemates for coronavirus antibodies usually generates worry without changing anything. Monitoring is the better approach: watch for fever, weight loss and failure to thrive, and have any cat who becomes unwell examined promptly.