Pathophysiology

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.

Clinical findings

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.

Signalment. 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.

History. History can help determine the likely source of elevated body temperature. It should be determined whether the cat is being administered a drug of any type; some can induce fever, in particular, tetracyclines. It should be determined whether vaccines were administered within the previous 1 to 2 months. Vaccine reactions can induce elevated body temperatures via immune-mediated reactions (any vaccine) or if a live, attenuated vaccine is used, via replication of the attenuated agent in the host. This seems to be most common with attenuated calicivirus vaccination of kittens which results in fever and stiffness in some, several days after inoculation. It is also important to determine which vaccines the cats have been administered and the interval between vaccines. Immune responses can wane with time, predisposing the cat to infection. This is unlikely to be a problem with panleukopenia virus; administration of a killed vaccine, twice to cats resulted in 100% immunity at 7.5 years.

Since many causes of fever in cats are transmissible, it should be ascertained whether the cat has had recent exposure to other cats, excrement, or ectoparasites. For example, cats with fever and fleas might be infected with hemoplasmas or Bartonella henselae, infectious agents transmitted by fleas. Cats in housed with other cats are more likely to come in contact with cats carrying infectious agents. It should be determined whether other animals or family members have similar clinical signs of disease. Some infectious agents are regional and so it should be determined whether the cat has traveled recently. For example, in the United States, Cryptococcus neoformans infection is most common in southern California and Vancouver. Determining the prey species for outdoor cats can help rank the infectious disease differential list; ingestion of songbirds (salmonellosis), rabbits (tularemia), or rodents (Yersinia pestis or toxoplasmosis) can transmit infectious diseases associated with fever.

The owner should be questioned concerning clinical signs involving organ systems commonly associated with fever including the oral cavity, central nervous system, cardiopulmonary system, urogenital system, subcutaneous tissues, peritoneal cavity, and gastrointestinal tract.