When to see a vet urgently
The general condition of a cat with confirmed or suspected fever determines how urgently help is needed:
- breathing becomes difficult or your cat breathes with their mouth open;
- your cat collapses, is very weak or responds poorly;
- seizures or severe pain occur;
- your cat vomits repeatedly and does not eat or drink normally;
- a raised temperature follows time in a hot environment and your cat deteriorates rapidly.
Do not wait for the fever to become prolonged. Find a vet near you for emergency care.
Overview
A cat's normal body temperature is around 38–39 °C. With fever, it rises above this and remains high for several days. If initial investigations, such as blood tests, X-rays and ultrasound, find no cause, fever of unknown origin is recorded as the diagnosis. This means a disease process is occurring that routine methods have not yet identified.
One raised reading does not establish prolonged fever. Measurement conditions and anxiety caused by a veterinary visit, for example, can also affect temperature. Veterinary medicine has no single universally used definition or time limit for FUO. The term is generally used when appropriate initial investigations do not establish the cause of true fever and it persists or recurs.
In fever, inflammatory signals cause the brain's temperature-regulating centre to raise its target temperature. This differs from heatstroke or overheating caused by seizures, for example, where heat production and heat loss become unbalanced without the same change in the target. The distinction affects treatment: cold bathing at home is not the correct treatment for ordinary fever.
Causes
Fever of unknown origin can be associated with many conditions:
- Infections caused by bacteria, viruses, fungi or parasites.
- Immune-mediated diseases, including autoimmune diseases and systemic inflammatory responses.
- Tumours, including lymphoma and leukaemia.
- Rare hormonal and metabolic disorders.
- Chronic inflammation, such as granulomatous diseases.
Infections are important possibilities when investigating fever in cats, but inflammation is not always caused by a microorganism. Pancreatitis and some immune-mediated diseases, for example, can cause fever without bacterial infection. Medicines may also be associated with a raised temperature in some cats, so all current medicines and supplements are reviewed.
A temperature rise associated with hormonal or metabolic disorders is not necessarily true fever. A broad, non-specific list therefore does not establish which investigation will help a particular cat. Sometimes early signs are so subtle that even a common disease is found only on repeat examination.
Risk factors
- Cats from rescue centres and breeding establishments, where hidden infections are more likely.
- Cats that meet other cats outdoors.
- Cats that test positive for FIV or FeLV.
- Older cats with chronic diseases.
- Cats that have recently experienced stress or surgery.
Risks vary with the underlying disease. There is no strong feline FUO evidence establishing every situation above as an independent risk factor. A temporary temperature rise associated with stress does not, on its own, show a hidden infection. Fever starting after surgery is investigated according to the procedure, wound and other findings.
Young cats can also have an important cause of fever. Other diseases are sought in cats positive for FIV or FeLV too, rather than automatically attributing every symptom to the infection. Travel, import history, bites and contact with sick animals may change which infections need investigating.
Typical symptoms
The main sign is persistent fever, often above 39.5 °C. Other symptoms depend on the hidden cause:
- tiredness and listlessness;
- reduced appetite and weight loss;
- faster breathing or heart rate;
- intermittent episodes of weakness;
- sometimes, pain on examination by touch or hidden sites of inflammation.
Your cat may hide, sleep more than usual or resist being lifted and touched. Some cats shiver or move stiffly. Fever is confirmed by measuring temperature and interpreting the finding; subdued behaviour alone does not establish a raised temperature. Breathing difficulty should not be dismissed as a harmless effect of fever either.
Record the timing and progression of symptoms at home. Explain whether your cat sometimes seems completely normal, whether illness returns and which medicines were given before a change. Temporary improvement alone does not show what caused the fever or which treatment it responded to.
Complications and consequences
If the cause is not found and treated, prolonged fever can lead to:
- wasting;
- dehydration and anaemia;
- organ damage during chronic inflammation;
- progression of a hidden infection or tumour;
- death in severe systemic diseases.
Not all these problems result directly from fever. Anaemia or organ damage may primarily relate to the disease causing it. Nevertheless, persistent loss of appetite and fluid deficiency can worsen the cat's condition before a final diagnosis. Nutrition and fluid balance are therefore addressed during investigations.
Diagnosis at the vet
As this is a diagnosis of exclusion, the cat is investigated as thoroughly as possible:
- a complete blood count and blood biochemistry;
- viral tests for FeLV, FIV, coronavirus and other viruses;
- X-rays and ultrasound of the chest and abdomen;
- bacterial and fungal cultures;
- cell and tissue samples from suspicious tissues or lymph nodes;
- MRI or CT where necessary to look for hidden tumours.
Thorough investigation does not mean performing every test at once. Tests are staged according to findings, symptoms and the cat's condition. A repeat clinical examination may reveal a previously unnoticed bite abscess, painful joint, enlarged lymph node or oral change. Previous samples and treatment records are also reviewed.
Blood count and biochemistry changes are often non-specific: they may indicate inflammation or organ stress without naming the cause. Urine testing and, where necessary, culture help look for a urinary tract infection. Imaging can locate an area where a cell, tissue or fluid sample would provide more information. MRI may be justified by neurological findings, for example, but is not performed automatically in every cat with fever.
Interpreting coronavirus tests requires a distinction between exposure and FIP. An ordinary antibody result alone does not confirm FIP. Possible FIP is assessed using a combination of symptoms, laboratory values and investigations of fluid or abnormal tissue, for example. Positive FeLV or FIV results, or negative results early in infection, may also need confirmation.
Where the cat's condition allows, a culture sample is taken from an appropriate site before starting a new antimicrobial medicine. Previous medication and sample quality can affect the result. Screening for every possible microorganism without a reason based on exposure or symptoms does not always help establish a diagnosis.
Treatment and follow-up
Dehydration, nausea, pain and inadequate eating are treated during investigations. A cat in poor condition may need hospital monitoring and intravenous fluids. The final cause need not be known before essential supportive care starts. The aim is to support the cat's condition while investigations proceed.
Antibiotics are not an automatic answer to every fever because the cause may be a virus, other inflammation or a tumour. Medicines are chosen according to a justified suspicion or diagnosis. If a treatment trial is considered without a confirmed cause, the signs to monitor and assessment method are agreed beforehand. A trial can affect later investigations, so mention a planned referral before new medicines are started.
Do not give your cat human fever medicines or painkillers on your own. Many can cause poisoning. The need for a symptom-relieving medicine suitable for cats and its safety are assessed separately. Do not change or stop a prescribed medicine because of suspected drug fever without discussing it with the vet.
At home, monitor appetite, drinking, urination, breathing and alertness, as well as any new local signs. If temperature is monitored at home, measure it only as instructed and without unnecessary stress. Report a clear decline in appetite because prolonged failure to eat puts cats at risk of fatty liver disease. An agreed follow-up visit is needed even if the cat's condition fluctuates.
Prognosis
The outlook depends on the underlying cause and treatment response. Failure to find a cause at the first visit does not, in itself, indicate an incurable disease. Further investigations identify a treatable cause in some cats; others receive no definite diagnosis despite extensive investigation. Findings in referral patients cannot directly establish the prognosis of every cat with fever or how common different diseases are in your country.
Prevention
- Keep vaccination against viral diseases up to date.
- Maintain regular parasite control.
- Have chronic diseases treated promptly.
- Support immunity with a varied diet.
- Attend regular health checks and arrange tests even for small abnormalities.
There is no single way to prevent FUO because many different diseases can underlie it. Vaccines are chosen according to infection risk and previous protection; they do not protect against every cause of fever. The need for investigation is assessed according to symptoms, and not every small, isolated abnormality automatically requires the same extensive testing.
Important for cat owners
Fever of unknown origin is not a disease in itself but a warning sign of a hidden problem. If your cat's fever continues for more than 2–3 days without an apparent cause, take them to a vet promptly. The earlier thorough investigations are performed, the more likely the true cause can be found and appropriate treatment started.
The 2–3 days mentioned are not a safe waiting period for a sick cat. Failure to eat, breathing difficulty, pain or worsening general health warrants earlier contact. Staging investigations and targeting them to findings may be more useful than performing as many tests as possible immediately.
This article provides general information and does not replace veterinary care. If you notice signs of illness in your cat, contact a vet.
Frequently asked questions
Is a high temperature always fever caused by infection?
No. True fever can also accompany other inflammation or a tumour. Temperature can also rise in a hot environment or because of anxiety, for example. The vet assesses the measurement alongside symptoms and circumstances.
Does a coronavirus test confirm FIP as the cause of fever?
Not on its own. Coronavirus antibodies can be present without FIP, and diagnosis relies on several findings. Samples are chosen according to the changes found in the cat.
Why does the vet examine my cat again?
Symptoms and findings can change over time. New local pain, swelling or another change may guide sampling. Repeat examination is part of a systematic investigation, and normal initial findings do not rule out every disease.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
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