Fever in cats is most often caused by an infection, whether viral, bacterial, or fungal. It can also result from inflammation, immune system disorders, or cancer. A cat’s normal body temperature ranges from 100.5°F to 102.5°F, and anything above that upper limit is considered a fever. Once the temperature climbs past 106°F, there’s a real risk of organ damage.
How Fever Works in a Cat’s Body
Fever isn’t the same as simply overheating. When a cat has a fever, something has triggered the immune system to deliberately raise the body’s internal thermostat. White blood cells detect a threat, whether it’s a virus, bacteria, or damaged tissue, and release signaling molecules that travel to the brain. The brain’s temperature-control center then resets to a higher target, and the body starts generating and conserving heat to reach it. Shivering and blood vessel constriction are two of the main ways the body drives temperature up.
This is fundamentally different from hyperthermia, which happens when a cat’s body temperature rises from external causes like a hot environment, overexertion, or an overactive thyroid. In hyperthermia, the brain’s thermostat hasn’t changed. The body is just absorbing or producing more heat than it can shed. Heat stroke and seizures can cause dangerously high temperatures without involving the fever pathway at all. The distinction matters because fever and hyperthermia require different responses.
Viral Infections
Viruses are among the most common fever triggers in cats. Feline immunodeficiency virus (FIV) is a well-known example. During the acute phase, which typically hits one to three months after exposure, the virus spreads through lymph nodes and reproduces inside white blood cells. This causes temporary swelling of the lymph nodes, often accompanied by fever, depression, and loss of appetite. Cats with FIV can go years without symptoms after that initial phase, but persistent fever and weight loss later on generally signal a worse prognosis.
Other viral culprits include feline leukemia virus (FeLV), feline infectious peritonitis (FIP), and common upper respiratory viruses like feline herpesvirus and calicivirus. Upper respiratory infections are especially common in kittens and shelter cats, and fever is one of the earliest signs alongside sneezing, nasal discharge, and eye inflammation.
Bacterial and Parasitic Infections
Bacterial infections can trigger fever in many ways. Bite wound abscesses are one of the most frequent causes, particularly in outdoor cats that get into fights. Bacteria from the bite get trapped under the skin, multiply, and cause a localized infection that the immune system responds to with fever. Urinary tract infections, dental infections, and pneumonia are other common bacterial sources.
Some bacteria are more specific to cats. Bartonella henselae, the bacterium behind cat scratch disease in humans, usually doesn’t cause obvious illness in cats. When it does produce symptoms, the result is typically a brief, self-limiting fever lasting two to three days. Infections carried by fleas and ticks, including certain blood parasites, can also cause fever that may be harder to pin down without targeted testing.
Fungal infections, though less common, are another infectious cause. Cats in certain geographic regions may inhale fungal spores that establish infections in the lungs or spread throughout the body, producing persistent fever alongside weight loss and respiratory symptoms.
Non-Infectious Causes
Not every fever points to an infection. The immune system can misfire and attack the cat’s own tissues, producing inflammation and fever without any invading organism. These immune-mediated diseases cause the same cascade of signaling molecules that reset the brain’s thermostat, so the fever looks identical from the outside.
Cancer is another significant cause. Tumors can trigger fever either by provoking an immune response or by causing tissue breakdown that activates white blood cells. Lymphoma, one of the more common cancers in cats, frequently presents with fever as an early sign. Pancreatitis, which involves inflammation of the pancreas, and significant tissue trauma from injuries or surgery can also drive fevers through the same inflammatory pathway.
Fever of Unknown Origin
Sometimes a cat runs a fever for days or weeks and standard tests don’t immediately reveal a cause. Veterinarians call this a fever of unknown origin. The underlying problem is still one of the usual categories (infection, inflammation, immune-mediated disease, or cancer) but it takes more extensive investigation to identify it. Diagnostic workups for these cases are designed to cast a wide net, screening for bacterial, viral, and parasitic causes alongside markers of inflammation that help narrow down the problem. A blood sample is often banked early so it can be compared with a later sample to track changes in the immune response over time.
Signs Your Cat May Have a Fever
You can’t reliably tell a cat has a fever just by feeling their ears or nose. The only accurate method is a rectal temperature reading, which is how veterinarians confirm it. But there are behavioral clues that suggest something is off. Most cats with a fever become lethargic and reluctant to move. They lose interest in food, and their heart rate and breathing rate increase. Dehydration is common since feverish cats often stop drinking enough water. Some cats shiver visibly or move stiffly, and you may notice them hiding more than usual.
These signs overlap with many other conditions, which is part of what makes fever tricky to identify at home. A cat that’s unusually still, refusing food, and feels warm may simply be stressed or in pain. But if these behaviors persist for more than a day, a temperature check at the vet can quickly confirm whether fever is part of the picture.
How Veterinarians Find the Cause
Identifying why a cat has a fever usually starts with a physical exam and a thorough history. Your vet will want to know whether the cat goes outdoors, has had recent contact with other animals, is up to date on vaccines, and whether any symptoms appeared before the fever. From there, blood work is the first diagnostic step, looking at white blood cell counts and organ function to spot signs of infection or inflammation.
If blood work doesn’t tell the full story, the next steps may include urine testing, X-rays, or ultrasound to look for abscesses, tumors, or fluid buildup. Specific tests for viruses like FIV and FeLV are straightforward and often run early in the process. For more elusive causes, testing may expand to include cultures of blood or tissue samples, screening for tick-borne diseases, or even biopsies. The goal is to move from broad screening toward a specific diagnosis so treatment can target the actual cause rather than just suppressing the fever itself.