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Ask a Vet: What Causes Sudden Fever and Malaise in Cats?

"Fever of unknown origin" has many causes, some way more serious than others.

Dr. Eric Barchas  |  Apr 26th 2016


I recently treated a cat who was feeling very badly. The four-year-old cat, a mostly indoor Siamese, had seemed fine at bedtime the night before. Her owner awoke to discover that the cat was very lethargic and did not want to get out of bed or eat breakfast. She made it to a chair in the living room, and she did not leave the chair for the rest of the day. The owner brought her to me at 3 p.m.

The first thing I noticed about the cat was depression. In veterinary medicine the word depressed means something different from its colloquial use for humans. In humans, depression usually refers to psychological depression. In cats, we use the term to describe physical depression. The cat was lethargic.

As I evaluated her further, I found that her gums were sticky. There was thick saliva in her mouth. Her eyes were sunken, and her skin was not as elastic as it should be. These are signs of dehydration. Her eyes were dull and glassy. Her coat was unkempt, and flea feces was present in her hair. Her heart and lungs sounded normal. There was no evidence of pain when I palpated her abdomen, and her organs felt normal. However, there was one other significant finding. Her temperature was 105.3 degrees Fahrenheit.

The normal temperature in cats is 100.0 to 102.5 degrees. The cat had a significant fever.

There is a name for the condition the cat was suffering, but it isn’t a diagnosis. In fact, the name itself implies that no diagnosis had been reached. The cat had fever of unknown origin.

Many things can cause fever in cats. Bacterial infections are the most common. Viral infections also are possible. Inflammatory conditions can trigger fever. Exposure to certain toxins might lead to fever as well. Pain can cause fever. Cancer, unfortunately, also is a cause of the syndrome. Foreign objects lodged in the intestines might trigger the condition.

The cat was young, so cancer was not likely. Cancer sometimes strikes four-year-old cats, but (mercifully) it isn’t common. The other causes were more probable.

Two things about the cat’s physical exam and history stood out to me. She was a mostly indoor cat, which meant that she occasionally went outside. And she had fleas.

Cats who go outside are more likely than their strictly indoor counterparts to suffer from fever of unknown origin. Outdoor cats are more likely to be exposed to infectious diseases. Also, outdoor cats are much more likely to be exposed to a very common menace to cats: other cats.

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Frequently, cats who go outside fight with other cats. I asked the owner about the cat’s history. The owner assured me that her pet did not look for fights. She tried to avoid other cats. That did not make much difference in my mind. Every neighborhood has tough resident cats who like to fight. Even if the cat in question didn’t pick a fight, someone else might have picked a fight with her.

When a cat is in a fight, she is likely to be bitten. There is an urban legend that holds that cats have “dirty mouths.” There is a common belief that cats have more bacteria in their mouths than other creatures. In fact, that is not true. However, cats’ mouths contain a 1-2 punch of features that can lead to aggressive infection when they bite. First, they have long, sharp teeth that penetrate deep into tissues. Second, their mouths tend to contain a bacteria, Pasteurella multocida, that thrives in environments that lack oxygen. When a cat bites, the teeth act like syringes that inject bacteria deep into tissues but leave only a very small wound on the skin. The skin wound closes almost immediately, leaving the bacteria in their preferred environment where there is no oxygen.

The fleas also were significant. Fleas carry a number of infectious diseases that can cause fever and sicken cats. Some of them, such as plague and feline infectious anemia, can be life threatening.

There were other possible causes for the fever. Infections of the kidneys are not uncommon. Pneumonia can strike cats. Pancreatitis sometimes causes the types of symptoms the cat was suffering. Inflammation of internal organs or the nervous system may trigger fever of unknown origin.

Because of the potential for serious illness, I recommended that the cat be hospitalized for treatment and diagnostic testing. We started intravenous fluids and antibiotics. Capstar, which kills fleas, was administered. Finally, we started pain killers to treat the generalized pain and cramping that fever can cause.

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Blood tests, urine tests, and X-rays were normal. This was a relief. Cats with fever of unknown origin, but without anemia or evidence of organ problems, usually respond to the treatment that had been started for my patient. The prognosis was good.

Nonetheless, the situation was tenuous for the rest of the afternoon. The cat’s temperature climbed as high as 106.5 degrees. If it got much higher she would be at risk of organ damage, and anti-inflammatory medications would be necessary.

Fortunately, 106.5 was the peak. The fever began to resolve, and dead fleas rained off of the cat’s body. I was happy to see the dead fleas, because truly the only good flea is a dead one.

Later in the evening, we discovered something that led to a final diagnosis. The cat had scratches on her abdomen compatible with a cat fight. The diagnosis was an infection from a cat fight. The scratches had not been visible during the initial exam. This is not uncommon. Cat scratches often take many hours or even days to become red and visible — this is a process in which the injuries “declare themselves” over time.

I am happy to report that the cat regained her appetite overnight, and the fever resolved by the morning. She went home on antibiotics and with instructions to stay indoors and to be tested in six weeks for the feline immunodeficiency virus, which is spread through fighting.

Any cat who develops severe lethargy should receive immediate veterinary attention. But if you wish to avoid a weekend trip to the emergency vet for treatment of a cat fight infection, I recommend that you keep your cat indoors.