The Diagnostic Approach to Fever of Unknown Origin in Cats*
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3 CE CREDITS CE Article 2 The Diagnostic Approach to Fever of Unknown Origin in Cats* ❯❯ Julie Flood, DVM, DACVIM Abstract: Identifying the cause of fever of unknown origin (FUO) in cats is a diagnostic challenge, Antech Diagnostics just as it is in dogs. Infection is the most common cause of FUO in cats. As in dogs, the diagnostic Irvine, California workup can be frustrating, but most FUO causes can eventually be determined. This article address- es the potential diagnostic tests for, and the differential diagnosis and treatment of, FUO in cats. rue fever (pyrexia) is defined as an during the initial workup or responds to increase in body temperature due to antibiotic treatment; therefore, most cats T an elevation of the thermal set point do not have a true FUO.4 in the anterior hypothalamus secondary to the release of pyrogens.1 With hyperther- Differential Diagnosis mic conditions other than true fever, the Information regarding FUO in cats is hypothalamic set point is not adjusted.1 extremely limited, and there are no retro- At a Glance Nonfebrile hyperthermia occurs when heat spective studies. Fevers are common in cats, gain exceeds heat loss, such as with inade- and most diseases associated with FUO Differential Diagnosis quate heat dissipation, exercise, and patho- in cats are infectious.5 Neoplasia is a less Page 26 logic or pharmacologic causes.1 common cause of FUO in cats, and FUO Clinical Approach Cats with true fever typically have body due to immune-mediated disease is rare in Page 26 temperatures between 103°F and 106°F cats.6 FUO causes are often separated into Potential Causes of Fever (39.5°C to 41.1°C).2 Cats are less likely than groups based on the underlying disease of Unknown Origin in Cats dogs to succumb to the dangerous effects mechanism.2,3,7 Most FUOs are caused by a Page 27 of body temperatures greater than 106°F, common disease presenting in an obscure 8 Staged Diagnostic which are usually seen with nonfebrile fashion. BOX 1 lists some causes of FUO in 3 Approach to Fever of causes of hyperthermia. Temperatures cats. It is thought that about 10% to 15% of Unknown Origin in Cats less than 106°F are unlikely to be harm- FUOs in cats remain undiagnosed despite 4 Page 28 ful in cats and may be somewhat benefi- thorough diagnostic evaluation. cial because they constitute a protective T r e a t m e n t 1,4 a Page 30 response to inflammation. Clinical Approach The term fever of unknown origin (FUO) As in dogs, the diagnostic approach to FUO is used liberally in veterinary medicine. It in cats must be targeted to each patient. It should be used to identify a fever that does should be guided by history and physi- not resolve spontaneously, that does not cal examination findings, laboratory test respond to treatment with antibiotics, and results, and the potential causes common for which the diagnosis remains uncertain to the geographic location.9,10 A three- after an initial diagnostic workup.4 Along stage approach, such as the one presented with a thorough history and physical in BOX 2, is commonly used.2–4 The goal of examination, initial diagnostics include investigating an FUO is to promptly estab- WEB a complete blood count (CBC), an FeLV lish a definitive diagnosis while minimiz- EXCLUSIVE antigen test, an FIV antibody test, a serum ing patient discomfort, client expense, biochemistry profile, and urinalysis with antimicrobial culture. The cause of fever aFor more information on the clinical ap- Supplemental material to this in most cats is infection that either is found proach to cats with FUO, please refer to the article is available at clinical approach section in the article starting CompendiumVet.com. *A companion article about fever of unknown on page 14. Many of the same tests used in origin in dogs begins on page 14. dogs can also be used in cats. 26 Compendium: Continuing Education for Veterinarians® | January 2009 | CompendiumVet.com FREE The Diagnostic Approach to FUO in Cats CE and invasive diagnostic tests.2 Communication History and Physical Examination with the owner is of utmost importance to Obtaining a thorough history is the first step ensure understanding of the time and finan- to a successful diagnostic approach. The vac- cial commitment that may be required in order cination history should be ascertained because to obtain a definitive diagnosis. vaccines can cause immune-mediated fevers If possible, all medications should be dis- in cats during the immediate postvaccination continued early in the evaluation to help rule period, and modified live virus vaccines can out a drug-induced fever. If the fever persists induce local lymphoid replication of the atten- beyond 72 hours after cessation of the medi- uated agent.5,12 Determining indoor/outdoor cation, a drug reaction can be ruled out.11 status, travel history, flea and tick control and Drugs that are known to induce fever in cats potential exposure to diseases transmitted by include tetracycline, sulfonamides, penicillins, parasites (e.g., hemotrophic mycoplasmosis, and levamisole. ehrlichiosis, bartonellosis, cytauxzoonosis), and contact with other cats is also impor- BOX 1 tant as many FUO causes are transmissable.5 Potential Causes of Fever Knowledge of ingestion of prey species may 2,4 be helpful because songbirds can carry sal- of Unknown Origin in Cats monellosis, rabbits can carry tularemia, and rodents can carry plague or toxoplasmosis.5 Bacterial infection (focal or systemic): Bac- Cats are frequently affected by stress hyper- teremia, infective endocarditis, septic arthritis, thermia, which must be ruled out before an osteomyelitis, diskospondylitis, septic menin- extensive diagnostic evaluation is pursued. As gitis, pyothorax, pyelonephritis, prostatitis, in dogs, FUO diagnostic clues in cats are gen- stump pyometra, peritonitis, abscess Bacterial diseases: Bartonellosis, borrelio- erally not readily apparent on physical exami- sis(?), mycoplasmosis (hemotrophic and non- nation, so repeated detailed examinations are 9 hemotrophic), tuberculosis and other myco- essential. The whole body should be carefully bacterial diseases, diseases caused by L-form palpated to detect subtle swelling or discom- bacteria (e.g., cellulitis or synovitis secondary fort, which may help localize the fever source. to bite wounds or surgical incisions) The thorax should be gently compressed Viral: FeLV, FIV, feline infectious peritonitis, to evaluate for a cranial mediastinal mass. feline calicivirusa Repeated fundic examination should be per- Rickettsial: Feline ehrlichiosis, anaplasmosis, formed because numerous infectious diseases QuickNotes Rocky Mountain spotted fever (e.g., FIP, FIV, FeLV) cause ocular changes. Fungal: Histoplasmosis, blastomycosis, cryp- Absence of ocular changes does not rule out Infectious diseases tococcosis, coccidioidomycosis infection with these diseases. Repeated neu- are the most com- Protozoal infections: Toxoplasmosis, cytaux- rologic and orthopedic examinations should mon causes of zoonosis, neosporosis(?), babesiosis(?), try- be performed, although they can be difficult fever and fever of panosomiasis(?) to interpret in an uncooperative cat. unknown origin in Immune-mediated diseases: Polyarthritis, cats. systemic lupus erythematosus, rheumatoid Feline Leukemia and Feline arthritis, vasculitis, meningitis, steroid-re- Immunodeficiency Virus sponsive neutropenia and fever FeLV antigen and FIV antibody blood tests Neoplastic: Lymphoma, leukemia, multiple should be conducted on every febrile cat. myeloma, necrotic solid tumors Noninfectious inflammatory diseases: These tests are rapid and reliable, but it is Lympha denitis, panniculitis, pansteatitis, important to understand how to interpret posi- 13,14 pancreatitis, granulomatosis tive results. Miscellaneous: Portosystemic shunt, drug reaction, toxin, hyperthyroidism, idiopathic Fecal Examinations causes Fecal samples should be obtained from cats with FUO. If diarrhea is discovered, rectal aHurley KE, Pesavento PA, Pedersen NC, et al. An cytology should also be conducted. Other outbreak of virulent systemic feline calicivirus dis- diagnostic tests to consider include fecal flota- ease. JAVMA 2004;224(2):241-249. tion with centrifugation, direct fecal examina- CompendiumVet.com | January 2009 | Compendium: Continuing Education for Veterinarians® 27 FREE CE The Diagnostic Approach to FUO in Cats BOX 2 tion, and fecal cultures. Cats can be bacteremic Staged Diagnostic Approach to Fever from Salmonella (and possibly Campylobacter) 2,3 of Unknown Origin in Cats infection without diarrhea, so fecal cultures should be submitted, especially if neutrophils Stage 1 are evident on rectal cytology.15-17 If clostridial Take a thorough history. spores are seen on cytology, samples should Stop all medications to rule out drug-induced fever. be submitted for Clostridium perfringens Perform a meticulous physical examination, including enterotoxin testing.5 fundic and neurologic examinations. Conduct FeLV and FIV testing. CBC and Serum Biochemistry Profile Obtain samples for CBC, blood smear, and serum chemistry profile. Typically, the changes seen on the CBC and Save serum for serology or other testing. serum chemistry profile in cats with FUO are Conduct a complete urinalysis and urine culture. nonspecific but can help suggest the next diag- Submit a sample for urine protein:creatinine ratio if nostic steps. A blood smear should always be proteinuria and inactive sediment are present. evaluated along with the CBC to help identify Conduct fecal centrifugation and fecal