Equis ISSN 2398-2977

Pyrexia: overview

Synonym(s): Fever

Contributor(s): Rachael Conwell, Alexandra Raftery

Introduction

  • Body temperature is an important parameter routinely measured as part of a clinical examination.  
  • Pyrexia (fever) is defined as an elevation in core body temperature.  
  • In the adult horse the normal temperature range is 37-38.5°C. 
  • In the neonatal foal it is slightly higher (37.5-39°C).
  • Donkeys tend to have a slightly lower body temperature range of 35.6-37.7°C.

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, 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; or ineffective heat dissipation, eg heat stroke   Hyperthermia  .
  • Some pharmacological agents, eg erythromycin in foals, and pathological conditions, such as malignant hyperthermia   Malignant hyperthermia  and central nervous system disorders, can also induce hyperthermia   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.

Mechanisma of pyrexia

  • True pyrexia is induced by the production of endogenous pyrogens or cytokines, eg IL-1±, IL-1², IL-6, TNF±, 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 bacteria, immune complexes and necrotic tissue. 
  • 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.  
  • However once significant elevations in temperature occur for prolonged periods, detrimental effects are evident, including 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 fromPubMed.
  • Taintor J & Schleis S (2011)Equine lymphoma. Equine Vet Educ23(4), 205-213.
  • Cauvin A (2008)Pyrexia of unknown origin in the dog. In Pract30(6), 302-313.
  • Sellon D C, Levine J, Millikin E, Palmer K, Grindem C & Covington P (1996)Thrombocytopenia in horses: 35 cases (1989- 1994). J Vet Intern Med10(3), 127-132PubMed
  • Mair T S, Taylor F G R & Pinsent P J (1989)Fever of unknown origin in the horse: a review of 63 cases. Equine Vet J21(4), 260-265PubMed.

Other sources of information

  • AHT / BEVA / DEFRA (2010)Equine Quarterly Disease Surveillance Report.Website:www.aht.org.uk/skins/Default/pdfs/equine_vol6_2.pdf. Last accessed 20th September 2012.
  • Reed S M, Bayly W M & Sellon D C (2010)Changes in Body Temperature.In: Equine Internal Medicine. 3rd edn. Chapter 3. pp 91-96.


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