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334 Fever of Unknown Origin

Technician Tips SUGGESTED READING AUTHOR: Claire R. Sharp, BVMS, MS, DACVECC
A great deal of effort should be made to help Buffington CAT: Idiopathic cystitis in domestic EDITOR: Leah A. Cohn, DVM, PhD, DACVIM
VetBooks.ir appropriate litter box management. Med 25:784-796, 2011.
clients understand EE for indoor cats and
cats—beyond the lower urinary tract. J Vet Intern
Client Education
Client-oriented information is available at
www.indoorcat.org.

Fever of Unknown Origin Client Education
Sheet

BASIC INFORMATION GEOGRAPHY AND SEASONALITY interleukin-1 or exogenous pyrogens such
Some regions are endemic for particular infec- as lipopolysaccharides (LPSs) and other
Definition tious diseases. See specific individual topics. bacterial endotoxins.
• Fever is defined as an elevated body tempera-
ture (>103°F [>39.5°C]) due to an altered ASSOCIATED DISORDERS DIAGNOSIS
hypothalamic set point. Temperatures > 106°F (>41.1°C) may cause
• Fever of unknown origin (FUO) is defined multiple organ dysfunction (p. 665), dissemi- Diagnostic Overview
in human medicine as an illness of at least nated intravascular coagulation (p. 269), and FUO is not a disease diagnosis, but the result of
3 weeks’ duration with a fever, for which death. disease. Use of the term FUO is justified when an
a cause is not identified after 3 hospital- Clinical Presentation elevated body temperature has been documented
ized days or outpatient visits. Veterinary on several occasions (typically over several days)
equivalents are adaptations of this definition. HISTORY, CHIEF COMPLAINT in the absence of confounding factors such as
• Temperatures consistently > 105°F (>40.6°C) Depends on underlying cause but often is anxiety or warm ambient temperatures.
are uncommon in FUO, and temperatures associated with nonspecific clinical signs such
> 106°F (>41.1°C) are more common with as lethargy, depression, and anorexia. Individual Differential Diagnosis
nonfebrile hyperthermia (p. 421). and environmental influences may raise the • A detailed differential diagnosis of FUO is
body temperature of normal, healthy patients provided on p. 1223.
Synonyms and must be considered when interpreting a • Rule out nonfebrile causes of an elevated
• Pyrexia patient’s temperature, but elevated temperature body temperature.
• Fever is a subset of hyperthermia, not a over days to weeks is most often caused by
synonym. Nonfebrile hyperthermia (e.g., true fever. Initial Database
high body temperature due to physical exer- • CBC (with blood smear evaluation), serum
tion, heat stroke, muscle fasciculations) does PHYSICAL EXAM FINDINGS biochemistry profile, urinalysis: results vary
not involve alterations in the hypothalamic A thorough physical exam of all patients with with organ system involvement and disease
set point and is not treated with antipyretic FUO must include rectal palpation (dog), causation.
drugs. fundic, oral, orthopedic, and neurologic • Urine bacterial culture and sensitivity should
exams, meticulous examination of the skin, be performed in all cases of FUO, even if
Epidemiology and thoracic auscultation. urine sediment is inactive.
SPECIES, AGE, SEX • Depression/lethargy • Feline leukemia virus antigen and feline
• Any age, breed, or sex • Tachycardia immunodeficiency virus antibody testing
• Young patients are more likely to have • Tachypnea/hyperpnea should be done for all cats.
infectious causes. • Dehydration • Thoracic and abdominal radiographs,
• Middle-aged patients are more likely to • Lymphadenopathy with infectious or abdominal ultrasound: results vary with
have noninfectious inflammatory diseases, neoplastic disease organ system involvement.
including immune-mediated disorders. • Neck or back pain or central signs with men-
• Older patients are more likely to have ingitis, meningoencephalitis, discospondylitis Advanced or Confirmatory Testing
neoplastic causes. • Joint pain or swelling with monoarthritis or Further laboratory testing depends on history,
polyarthritis physical exam findings, and minimal database
GENETICS, BREED PREDISPOSITION • Heart murmur may indicate endocarditis, results.
• Shar-pei: possible cytokine abnormality especially if new in onset and/or diastolic. • Laboratory tests
• Gray collie: cyclic hematopoiesis • Chorioretinitis or uveitis with infectious ○ Blood cultures: to detect bacteremia associ-
• Irish setter: leukocyte adhesion deficiency disease (p. 1137) ated with discospondylitis, endocarditis, or
• Weimaraner: neutrophil function defect • Localized swelling and/or pain with cellulitis other foci of bacterial infection. A negative
• Blue Persian: Chédiak-Higashi syndrome or abscesses culture result does not rule out bacteremia.
○ Cytology and cultures of bile, cerebrospinal
RISK FACTORS Etiology and Pathophysiology fluid (CSF) or synovial fluid, as indicated
• Immunosuppression or immunodeficiency Pathophysiology: ○ Cytology of aspirates from lymph nodes
• Exposure to infectious agents or vectors • Fever occurs when the hypothalamic set point or affected organs
• Travel to endemic areas of disease is raised. ○ Serologic tests: antibody titers or antigen
• Inflammation and/or pathogens increase tests for specific infectious agents. If infec-
CONTAGION AND ZOONOSIS the hypothalamic set point by causing the tious disease is suspected and initial titers
Risk varies, depending on underlying cause release of endogenous pyrogens such as are negative, repeat in 2-4 weeks.

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