One of the more frustrating conditions veterinarians have to deal with goes by the self-descriptive name "Fever of Unknown Origin," or FUO. Typically, an owner will bring in a dog, cat, chinchilla, what have you, complaining that he or she just isn’t quite right. The pet may be lethargic, perhaps isn’t eating well, but really doesn’t have any symptoms that point in a particular direction.

In these cases, a pet’s history is very important. An intact female in her heat/breeding cycle could point to uterine infections as a potential cause. Travel can raise suspicions about diseases that are not often diagnosed in a pet’s home range. Even taking certain types medications (e.g., tetracyclines in cats and sulfa drugs in dogs) can induce a fever.

Next comes the physical exam. The veterinarian should pay special attention to parts of the body where diseases can "hide." Pain that involves multiple joints, bones, muscles, or the vertebral column can be difficult to detect since pets often react by simply being less active than normal. An ophthalmic exam could reveal infection or other conditions within the eye. Careful auscultation of the chest in a quiet room can expose subtle changes in heart and lung sounds. A thorough palpation of the abdomen may identify small masses or slight changes in the size of organs.

If any abnormalities are uncovered during the physical examination, subsequent diagnostics are aimed in that direction. Unfortunately, a pet’s history and physical exam are often unremarkable in cases of FUO (otherwise the origin of the fever really isn’t unknown), making laboratory tests the next logical step.

Most veterinarians start with the following:

• a complete blood count (CBC) to look for evidence of inflammation, infection, and abnormally low cell counts
• a serum chemistry panel to investigate the possibility of organ dysfunction (e.g., kidney or liver disease) and biochemical changes like high blood calcium levels that can be seen with certain types of infections or cancer
• a urinalysis to diagnose infection and other disorders of the urinary tract
• a fecal examination to look for parasites and abnormal bacterial numbers/types within the gastrointestinal tract

When I’m drawing blood for a CBC/chemistry and getting a urine sample, I always take more than I need and save the "extra." The tests outlined above may very well be inconclusive, and in those cases I like to be able to run tests (serology, culture, etc.) for infectious diseases like cryptococcosis, ehrlichiosis, heartworm, feline leukemia and immunodeficiency viruses, and toxoplasmosis without stressing the pet with additional blood draws or a second cystocentesis.

If I still don’t have an answer as to what is causing a pet’s fever, it’s on to imaging. X-rays and/or ultrasound of the lungs, heart, abdomen, spine, and limbs may all be necessary.

I hate to admit it, but I’ve worked up a few patients to this degree (or had to stop earlier due to financial constraints) and still haven’t been able to diagnose the cause of the pet’s fever. I usually try to refer at this point, but when the owners decline, we’ll often resort to what is euphemistically referred to as an antibiotic response test. Basically, I prescribe a broad spectrum antibiotic and cross my fingers. If the disease is bacterial in origin and the bacteria in question are susceptible to the drug I picked, we just might be successful. Otherwise, it’s back to the drawing board.

Dr. Jennifer Coates

Image: tired tula by megan ann / via Flickr