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Pyrexia: overview

icanis

Synonym(s): Fever


Introduction

  • Body temperature is an important parameter routinely measured as part of a clinical examination.  
  • Pyrexia (fever) is defined as an increase in core body temperature.  
  • In the adult dog the normal temperature range is 38.3-39.2°C. 
  • In the neonatal puppy it is slightly lower (35.6-36.1°C) increasing to 37.8°C by 4 weeks Normal vital signs.
Print off the owner factsheet on Fever - is it serious? Fever - is it serious? to give to your client.

Temperature homeostasis

  • The thermoregulatory center in the hypothalamus is responsible for regulation of core body temperature, ensuring the core temperature is tightly controlled within a narrow range of the set point. This is based on information received from peripheral and central thermoreceptors and allows normal body homeostasis to occur.  
  • The central temperature receptors are found in the anterior hypothalamic-preoptic area. The anterior hypothalamic-preoptic area and the peripheral receptors transmit signals into the posterior hypothalamic area, which activates autonomic and behavioral responses to regulate body temperature.  
  • The body temperature may be elevated due either to pyrexia or hyperthermia.  
  • In cases of true pyrexia the set-point is reset to a higher level and this is recorded as a higher rectal temperature.  
  • In hyperthermia Hyperthermia, the set point of the hypothalamus has not been altered. Increased body temperature occurs due to either excessive heat production, eg intense exercise/seizure activity, hypocalcemic tetany; or ineffective heat dissipation, eg heat stroke Heat stroke.
  • Some pathological conditions, such as malignant hyperthermia Malignant hyperthermia and central nervous system disorders, can also induce hyperthermia. Hyperthermic conditions do not respond well to antipyretic medication. Often, the underlying cause will be evident from the history, helping to distinguish between pyrexia and hyperthermia.

Mechanisms of pyrexia

  • True pyrexia is induced by the production of endogenous pyrogens or cytokines, eg eg IL-1α, IL-1β IL-6, TNFα, from activated immune cells, especially macrophages and monocytes, in response to exogenous pyrogens in the body.  
  • Exogenous pyrogens are numerous and include the products and components of infectious agents (bacteria, viruses, fungi, protozoa), bacterial products (eg enterotoxins, lipopolysaccharides, etc), immune complexes, tissue inflammation or necrosis, and several pharmacologic agens. Some neoplastic cells may also produce cytokines that result in a febrile response.
  • The endogenous pyrogens are responsible for resetting the thermoregulatory centers set point by causing prostaglandin release. This is why COX inhibitors are effective in reducing fevers through their inhibitory effects on the arachidonic acid pathway. 

Systemic effects of pyrexia

  • As a physiological response, elevations in body temperature can have both advantageous and adverse effects.
  • Improved host defenses in pyrexic animals with bacterial infections have been associated with better survival rates, reduced duration of disease. Although this is controversial.
  • Fever is commonly considered to kill bacteria, but this is likely untrue and bacteria continue to replicate at temperatures that would otherwise prove fatal to animals and people.
  • However once significant elevations in temperature occur for prolonged periods, detrimental effects are evident, including increased number and severity of clinical signs, increased metabolic rate, increased oxygen demand and muscle wastage and weakness.

History and clinical examination

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Pyrexia of unknown origin (PUO)

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Diagnostics

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Treatment

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Special risks

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Chervier C, Chabanne L, Godde M et al (2012) Causes, diagnostic signs, and utility of investigations of fever in dogs: 50 cases. Can Vet J 53 (5), 525-530 PubMed.
  • Cauvin A (2008) Pyrexia of unknown origin in the dog. In Pract 30 (6), 302-313 VetMedResource.
  • Battersby I A, Murphy K F, Tasker S et al (2006) Retrospective study of fever in dogs: laboratory testing, diagnoses and influence of treatment. JSAP 47 (7), 370-376 PubMed.
  • Dunn K J & Dunn J K (1998) Diagnostic investigations in 101 dogs with pyrexia of unknown origin. JSAP 39 (12), 574-580 PubMed.
  • Dunn J K & Gorman N T (1987) Fever of unknown origin in dogs and cats. JSAP 28 (3), 167-181 VetMedResource.

 

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