Why Does My Cat Have a Persistent Fever?

Find out what "fever of unknown origin" means for your cat, and what tests your vet should run.

Q: My cat (about 2 ½: years old, 15-18 pounds, indoor cat) has had a fever of 103-105 degrees for the past week. He finally ate about 2 tablespoons of chicken yesterday morning. My vet ran bloodwork tests, took X-rays and did ultrasounds, and everything comes back negative. 

Is there anything you can suggest? Are there any odd cat issues out there that have gone unchecked due to rarity or is there something simple that gets overlooked?  

A: It sounds like your cat has a condition that we term fever of unknown origin (FUO). Identifying the cause of the fever can be quite the diagnostic challenge. The most common cause of fever of unknown origin is some kind of infection. When faced with a cat with FUO, veterinarians will run some basic diagnostic tests such as a complete blood count, chemistry panel urinalysis and feline leukemia virus and FIV test. If these initial tests don’t yield a diagnosis, additional tests such as X-rays and ultrasound may be warranted. It sounds like your vet has done a thorough workup, and I imagine he’s just as frustrated as you are. 

One test that is often overlooked is a urine culture. A simple urinalysis may show there is no urinary tract infection; however, a urine culture is the true test for determining whether there’s an infection, especially a kidney infection, which could definitely cause the fever. Blood cultures are another test that should be considered, to see if we can isolate bacteria from the bloodstream.

The list of things that can cause fever in cats is huge. Bacterial infections, viral infections, fungal infections, protozoal infections, immune-mediated diseases, inflammatory diseases and neoplastic disorders (cancer) can all cause fevers in cats. Most FUOs are caused by a common disease that presents in an uncommon fashion. There are several uncommon causes of FUO in cats, but to list all of the disease possibilities, and all of the potential tests for them, is beyond the scope of this column.

I’d suggest a trial course of antibiotics to see if the cat responds. A broad-spectrum antibiotic is recommended.  If the fever breaks, then a bacterial infection was probably the cause. If the fever persists, additional diagnostic tests will need to be performed to find the cause. Good luck with your cat!

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