Page 1451 - Small Animal Internal Medicine, 6th Edition

CHAPTER 90
VetBooks.ir

Fever of Undetermined

Origin

FEVER AND FEVER OF week of thorough in-hospital evaluation. If the term FUO
UNDETERMINED ORIGIN were to be used in the same way in animals as is recom-
mended for humans, few dogs and cats would actually have
The term fever refers to a syndrome of malaise or nonspecific it. Therefore, in this chapter, the discussion focuses on the
systemic clinical signs accompanied by pyrexia or hyperther- approach to a dog or cat with fever that does not respond to
mia. In this chapter, however, the terms fever and pyrexia are antibacterial antibiotic treatment and for which a diagnosis
used interchangeably. Fever constitutes a protective physi- is not obvious after a minimal workup has been performed
ologic response to infectious and noninfectious causes of (e.g., complete blood count [CBC], serum biochemistry
inflammation that enhances the host’s ability to eliminate a profile, urinalysis).
noxious agent. As a general rule, the clinician typically presumes that a
A variety of stimuli, including bacteria, endotoxins, dog or cat with fever has an infection until proved otherwise.
viruses, immune complexes, activated complement, and This appears to be true in reality, as shown by the fact that a
necrotic tissue, trigger the release of endogenous pyro- large proportion of dogs and cats with fever respond to non-
gens by the phagocytic system, mainly the mononuclear specific antibacterial treatment. No clinicopathologic evalu-
cells, or macrophages. These endogenous pyrogens include ation is performed in most of these animals because the fever
interleukin-1, tumor necrosis factor, and interleukin-6, responds promptly to treatment.
among others. They activate the preoptic nucleus of the
hypothalamus, raising the set point of the thermostat by DISORDERS ASSOCIATED WITH FEVER
generating heat through muscle contraction and shivering, OF UNDETERMINED ORIGIN
and conserving heat through vasoconstriction.
In humans several patterns of fever have been associated In humans, certain infectious, neoplastic, and immune-
with specific disorders; however, this does not appear to be mediated disorders are commonly associated with FUO.
the case in dogs and cats. In people with continuous fever, Approximately one third of patients have infectious dis-
the pyrexia is maintained for several days or weeks. This eases, one third have cancer (mainly hematologic malignan-
type of fever is associated with bacterial endocarditis, central cies, such as lymphoma and leukemia), and the remaining
nervous system lesions, tuberculosis, and some malignan- third have immune-mediated, granulomatous, or miscella-
cies. In people with intermittent fever, the body temperature neous disorders. In 10% to 15% of patients with FUO, the
decreases to normal but rises again for periods of 1 to 2 days; underlying disorder remains undiagnosed, despite intensive
this is seen in brucellosis and some malignancies. In remit- efforts. In a study of 66 dogs with fever, infectious diseases
tent fever the temperature varies markedly each day but is were diagnosed in 26% of the patients, immune-mediated
always above normal (39.2° C [103° F]); this type of fever is disease in 35%, neoplasia in 8%, and a diagnosis could not
associated with bacterial infections. The term relapsing fever be obtained in 23% (Battersby et al., 2006). In a more recent
is used to refer to febrile periods that alternate with variable study of 50 dogs with fever evaluated in a teaching hospital in
periods of normal body temperature, as seen in humans France, 48% of dogs were diagnosed with inflammatory non-
with malaria. infectious diseases, 18% with infectious disease, and 6% with
The term fever of undetermined (or unknown) origin neoplasia; a final diagnosis could not be obtained in 28% of
(FUO) is used liberally in veterinary medicine to refer to cases (Chervier et al., 2012). In this study, of the initial diag-
a febrile syndrome for which a diagnosis is not evident. In nostic procedures, hematology (23%), biochemistry (25%),
human medicine, FUO refers to a febrile syndrome of more and imaging (27%) were the most helpful in obtaining a
than 3 weeks’ duration that remains undiagnosed after 1 diagnosis, whereas immunology and bacteriology were the

1423