Fever of unknown origin (Proceedings)

Cats with fevers (103° F-106° F) are a common occurrence in everyday practice. Most cases respond to antibiotic therapy or are self-limiting (abscesses, viral infections, post-surgical fevers). However, the most frustrating case is one in which a routine course of antibiotics does not improve the clinical condition of the cat, routine diagnostics do not identify the cause and the fever is ongoing. This is the fever of unknown origin (FUO) case that requires a methodical approach to discover the exact cause of the fever so that proper therapy can be instituted. Your approach may vary based on the clinical presentation of each cat, as well as the patient's geographic location/travel history. You must also consider the cost/benefit of the diagnostic testing as well as the invasiveness of the tests performed. Tests may need to be repeated as the case progresses.

The Approach:

• Obtain a complete history, including travel, vaccination, drug/supplements.

• Perform a complete physical examination (repeat this step often).

• Collect the minimum database.

-CBC, Biochemistry profile, Urinalysis, FeLV/FIV test

-Thoracic and abdominal radiographs

Cause of the fever is still not defined, so the hunt continues....

• Culture

• Urine – aerobic and Mycoplasma, even when urine sediment is inactive. May need to culture urine repeatedly if history, clinical signs or other findings suggest urinary tract concerns.

• Blood – aerobic, anaerobic, Mycoplasma. Consider volume and timing of blood draws.

• Joint – aerobic, anaerobic, Mycoplasma

• Other (BAL fluid, feces, effusions)

• Also consider culturing for: Mycobacteria, other atypical bacteria, fungi

-PCR tests also available for some organisms (Mycoplasma)

• Imaging

-Ultrasound (Abdominal, Echocardiography, areas of swelling)

-CT/MRI – may reduce the need for exploratory surgery

Follow any leads noted on history, physical examination or localizing signs.

• Lameness – Joint fluid for cytology, cultures (aerobic, anaerobic, Mycoplasma), joint radiographs

• Swellings/ lymph nodes/ effusions/ mass lesions – needle aspirate for cytology/ cultures

• Bone marrow aspirates

• Respiratory signs – Airway wash, lung aspirate

• Infectious disease titers – consider specificity and sensitivity of the tests, disease prevalence, use of acute and convalescent titers

• Immune panels – Antinuclear antibody (ANA), rheumatoid factor (RF), Coombs test

• Tissue biopsies

While performing the hunt, don't forget to treat the patient.

• Therapeutic drug trials – Weigh risks versus benefits.

• Broad spectrum antibiotic therapy: Baytril, clindamycin

• Unusual bacteria, rickettsial, Mycoplasma: Doxycycline

• Fungal: Fluconazole or other antifungal

• Immune/neoplasia: Corticosteroid therapy (prednisolone)

The fever should break within 3 days if the drug therapy is going to work for that case...

• Maintain hydration – fluid therapy as needed.

• Encourage appetite. Appetite will typically return once the fever breaks.

Examples of Diseases that Cause Fever in Cats:

Infectious diseases (localized or systemic)

• Bacterial: bacteremia, discospondylitis, septic arthritis, osteomyelitis, pyothorax, peritonitis, pancreatitis, pyelonephritis, tooth root or other abscess, septic meningitis, mycobacterial infections, L-form infections, bartonellosis, plague

• Viral: FeLV, FIP, FIV, Hemorrhagic calicivirus

• Fungal: Histoplasmosis, cryptococcosis, blastomycosis, phycomycosis, coccidioidomycosis, sporotrichosis

• Protozoal: Toxoplasmosis, Cytauxzoonosis, Babesia spp

• Rickettsial & Myoplasma: Ehrlichiosis, Mycoplasma

• Parasitic: Dirofilaria immitis

Inflammatory diseases: pansteatitis, nodular panniculitis, granulomatosis, pancreatitis

Immune-medicated diseases: Immune-mediated polyarthropathy, Pemphigus foliaceus

Neoplastic conditions: Lymphoma, Leukemia, hepatic tumors, gastric tumors, lung tumors, necrotic masses

Miscellaneous conditions: Drug (tetracycline, penicillins, levamisole) or toxin reactions, hyperthyroidism

Points to consider:

• Many FUOs are caused by infections.

• The fever itself may be beneficial and is rarely harmful to the patient.

• Investigation of FUO may be time-consuming and expensive.

• Many diagnostic tests may be necessary.

• Tests may often be repeated several times.

• A diagnosis is ultimately obtained in most cases.

• Many cases of FUO prove to be treatable or manageable.

Case studies will be utilized in this presentation.

Selected Readings

Dunn JK, Gorman NT: Fever of unknown origin in dogs and cats. J Small Anim Pract 28:167-181, 1987.

Feldman BF: Fever of undetermined origin. Compend Contin Educ Pract Vet 2(12)970-77, 1980.

Johannes CM, Cohn LA: A clinical approach to patients with fever of unknown origin. Vet Med 95:633-642, 2000.

Lappin MR: Feline Fevers of Unknown Origin I, II, III. Western Veterinary Conference 2002.

Lappin MR: Fever of Unknown Origin I and II. Western Veterinary Conference 2003.

Lappin MR: Infectious causes of fever in cats. J Vet Int Med 16:366, 2002.

Lunn KF: Fever of unknown origin: a systematic approach to diagnosis. Compend Contin Educ Pract Vet 23(11):976-992, 2001.

McReynolds C, Macy D: Feline infectious peritonitis. Part I, Etiology and diagnosis. Compend Contin Educ Pract Vet 19(9):1007-1016, 1997.

Stubbs CJ, Holland CJ, Reif JS, et al. Feline ehrlichiosis; literature review and serologic survey. Comp Contin Educ Pract Vet 22:307-317, 2000.