Fever of Unknown Origin in Cats: Systematic Investigation

Unexplained persistent fever in cats causes significant suffering and requires systematic investigation to identify treatable underlying causes.

Key Facts

FUO is defined as fever exceeding 39.4 degrees C on multiple occasions without identified cause after initial workup
Common causes include occult infections, immune-mediated disease, and neoplasia
Bartonella, mycoplasma, and deep mycoses cause persistent fever in cats in endemic regions
Immune-mediated polyarthritis causes fever, joint pain, and reluctance to move in affected cats
FIP causes refractory fever alongside other systemic signs - GS-441524 is now an effective treatment
Bone marrow aspirate identifies infiltrative neoplasia or aplastic anemia causing fever
Response to prednisolone (fever resolution) suggests immune-mediated cause in cats with negative infectious workup

Welfare Considerations

Persistent fever causes significant suffering - anorexia, lethargy, muscle weakness, and the discomfort of elevated body temperature impair quality of life substantially. Cats with FUO often experience protracted diagnostic workups during which they continue suffering without treatment. A systematic approach - thorough history, complete blood count, biochemistry, infectious disease screening, imaging, and targeted biopsy - identifies treatable causes in most cases. Treatment of identified causes produces dramatic welfare improvement. Empirical trials (treating assumed causes without diagnosis) can delay finding the actual cause and prolong unnecessary suffering.

What You Can Do

Document fever episodes accurately with home thermometer readings and timing
Provide a thorough history including travel, animal exposures, and recent medications
Request systematic infectious disease screening including Bartonella, mycoplasma, and regional pathogens
Ask about FIP testing and treatment eligibility if other causes are excluded - GS-441524 is highly effective
Pursue imaging (radiography, ultrasound) and bone marrow sampling if primary workup is unrevealing
Maintain appropriate nutrition during fever workup - assisted feeding if the cat is anorexic
Ask for clear communication about diagnostic steps and expected timelines