Polysystemic Problems: Fever

54 Polysystemic Problems: Fever



Astrid Nielssen



1. How is the body’s internal temperature determined?

The body’s thermoregulatory center is located within the anterior hypothalamus. Central and peripheral thermoreceptors sense changes in the core and peripheral body temperature and relay this information to the central thermoregulatory center. Deviations in temperature above or below the set point of the thermoregulatory center result in the body using mechanisms to either increase heat loss and decrease heat generation or decrease heat loss and increase heat production, respectively.


2. What is the definition of fever?

True fever is defined as an elevated body temperature resulting from an increased thermoregulatory center temperature set point. This is what differentiates fever from hyperthermia, the condition that arises when the body’s temperature becomes elevated despite a normal thermoregulatory set point. Hyperthermia arises when the normal heat-dissipating mechanisms of the body are overwhelmed, such as in heat stroke or status epilepticus. Additionally it is not uncommon to see hyperthermia in stressed animals. Body temperature should be rechecked in animals with elevated body temperature to make sure that the elevation is persistent.


3. What are some of the causes of an elevated thermoregulatory set point?

Agents that induce fever (pyrogens) can be either exogenous or endogenous. Examples of exogenous pyrogens include lipopolysaccharides from Gram-negative bacteria, components of Gram-positive bacteria, viral particles, and drugs. Exogenous pyrogens induce fever through their stimulation of the release of endogenous pyrogens; these are products released primarily from host macrophages and lymphocytes. A variety of endogenous pyrogens have been identified including interleukin-1, interleukin-6, and tumor necrosis factor. The endogenous pyrogens bind endothelial cells in the thermoregulatory center and are thought to alter the temperature set point through the production of prostaglandins.


4. What are some of the mechanisms through which the body increases heat production and decreases heat loss?

Muscle contractions, shivering, catecholamine release, and thyroxine production are all mechanisms through which the body increases heat production. Peripheral vasoconstriction, piloerection, postural changes to try and decrease body surface area, and heat-seeking behavior are all mechanisms through which the body decreases heat loss.


5. What is a fever of unknown origin (FUO)?

The current definition of FUO in human medicine is that of fever that has persisted for 2 weeks (or 3 days for immunosuppressed patients) and for which no etiology can be identified despite extensive investigation, including in-hospital evaluation. In veterinary medicine FUO is the term frequently used to describe any febrile state of uncertain etiology that has not responded to a course of empirical antibiotic therapy.


6. What are some causes of FUO in the dog?

The various causes of FUO in dogs are often organized into categories including:







































Infectious:
Bacterial: abscesses, discospondylitis, endocarditis, osteomyelitis, peritonitis, prostatitis, pyelonephritis, pyometra, pyothorax, septic arthritis, brucellosis, leptospirosis, Lyme disease
Viral: distemper virus
Mycotic: blastomycosis, coccidiomycosis, cryptococcosis, histoplasmosis
Protozoal: babesiosis, leishmaniasis, neosporosis, toxoplasmosis, trypanosomiasis
Rickettsial: ehrlichiosis, Rocky Mountain spotted fever
Inflammatory: juvenile cellulitis, pancreatitis
Immune-mediated: immune-mediated polyarthritis, steroid-responsive meningitis, systemic lupus erythematosus
Neoplastic: leukemia, lymphoma, multiple myeloma, necrotic tumors
Drug reactions: ketamine, penicillins, phenobarbital, sulfonamides, tetracyclines
Miscellaneous: panosteitis, Shar-Pei fever
True FUO: no known etiology

While the incidence of the various causes of FUO is not well established in canine medicine, immune-mediated and neoplastic diseases are the causes behind a large number of cases.

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  4. Canine Rhinitis

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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Polysystemic Problems: Fever

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