A common saying in veterinary medicine is that “cats are not small dogs”. Cats are a very different species with different diseases — some even joke that cats have an extraterrestrial origin — feline UFOs you might say.

We also see fever a bit differently in cats and dogs. Typically with dogs, if a diagnostic “net” is cast wide enough a diagnosis can be made but that is not always the case in cats. If enough parameters are met in investigating a cat fever without the cause being identified, we will call this “Fever of Unknown Origin” (FUO).

Although fevers are common in cats, information about FUO in cats is limited. Most often, feline FUOs are eventually determined to be due to an infection. Less often, cancer or immune-mediated disease is found to be the cause of the fever. Still, the origin of 10-15% of fevers of unknown origin in cats remain a mystery despite thorough diagnostic evaluation.

Outdoor vs. Indoor Cats

The considerations differ for an outdoor vs. an indoor cat. Indoor cats live a more sheltered life and therefore they are less likely to get into cat fights (if they are without roommates 😀, less likely to get FIV or feline leukemia, and less likely to get a bacterial infection from eating a bird (although some cats on balconies can still get into mischief).

The thorough investigation we are talking about when we explore a cat fever is a stepped approach until we have either found a cause, or exhausted all possibilities. We start with a physical examination, which might reveal something as externally obvious as an abscessed cat fight wound. If the examination is normal except for the fever, we will continue our search by running blood work (including checking FIV and FeLV status), and a urinalysis and culture. Advances in DNA and RNA testing mean that we can also check for evidence of feline calicivirus, herpes virus, and chlamydia. Fevers that are viral in origin do not have specific treatment but the patient sometimes needs supportive care. Bacterial infections may be the main cause of fever or secondary to inflammation of the gut, pancreas or other organs, or inflammation due to cancer. At this point in the investigation, we will likely include radiographs and an abdominal ultrasound.

Meet Kiwi — our most recent FUO

One such case was Kiwi.

Kiwi, an 11-year-old previously healthy cat was presented to our hospital in May for further evaluation and support of an ongoing fever and poor appetite despite some initial treatments. She was admitted and further diagnostic work up (as described above) was done.

One of the considerations of FUO is that cats do not do well with fasting and can develop life threatening liver failure. Fatty liver disease, when it develops, requires a long course of tube feeding because cats (unlike dogs) do not tolerate force-feeding well when they won’t eat because they feel sick or nauseated. In this patient, we wanted to place a feeding tube in her esophagus (which she could go home with) but worried her liver was already affected as crucial clotting factors (made by the liver) were not normal. Her fever had resolved with treatment but she remained uninterested in eating. Instead, a tube was placed in her nose (with some sedation) to her stomach. She tolerated her nasogastric tube feeding well and she started eating small amounts.

Kiwi soon got to go home and she is now happy and healthy. She spent her recovery enjoying catnip, relaxing on her bed and sitting out on the patio.