The 2 major differentials for elevated body temperature (> 102.5 F) are fever (pyrexia) and hyperthermia. Hyperthermia results from increased muscle activity, increased environmental temperature, or increased metabolic rate (i.e.
hyperthyroidism). Fever develops when the thermoregulatory set point in the hypothalamus is increased, resulting in increased
body temperature from physiologic mechanisms inducing endogenous heat production or heat conservation. If the cause of fever
is not apparent for > 2 weeks, the case is classified as having fever of unknown origin.
Fever results when leukocytes, particularly mononuclear cells and neutrophils, are activated. Leukocytes are generally stimulated
by contact with bacterial, viral, fungal, and parasitic agents, neoplasia, tissue necrosis (extensive trauma and pancreatitis
included), and primary immune-mediated diseases like immune-mediated hemolytic anemia, immune-mediated thrombocytopenia, and
systemic lupus erythematosus. Activated leukocytes release a variety of soluble factors like interleukin 1 and tumor necrosis
factor, which enter the central nervous system and change the thermoregulatory set point. The thermoregulatory set point
may also be altered by intracranial disease including trauma and neoplasia, or drugs like tetracyclines. Shivering and vasoconstriction
are 2 of the most important physiologic responses to a thermoregulatory set point change that result in generation and conservation
of heat, respectively.
Fever < 105°F may be beneficial for the management of infectious diseases due to potentiation of phagocytosis, interferon
release, and lymphocyte transformation. During chronic inflammatory conditions resulting in fever, activated mononuclear
cells also sequester serum iron, decreasing bacterial replication. Body temperatures > 106 F can be detrimental due to effects
on cellular metabolism. Disseminated intravascular coagulation can result from extreme increase in body temperature. Compared
to dogs, cats are less likely to develop the detrimental effects of fever.
The differential list for fever in cats is long. In cats, infectious causes of fever are much more common than primary immune
diseases or neoplasia. Initially, the clinician should use the signalment, history and physical examination to identify the
initial differential list. Diagnostic tests or therapeutic trials are then used to confirm the primary differential.
The age, breed, and sex of the cat can help rank the differential list for fever. For example, young cats often have infectious
diseases; old cats often develop neoplasia. Inbreeding can result in predisposition for infectious diseases. For example,
feline infectious peritonitis is most common in pure-breed cats. Male cats are more likely to fight, partially explaining
the increased incidence of feline immunodeficiency virus in this sex.