Equis ISSN 2398-2977

Hyperthermia

Synonym(s): Heat stroke

Contributor(s): Rachael Conwell, Janice Sojka, Han van der Kolk

Introduction

  • When a horse exercises, approximately 80% of the energy expended is released as heat. 
  • To prevent over heating, horses have developed a number of adaptive strategies with sweat production being the most important mechanism for dissipation of heat. 
  • A horses ability to dissipate excess heat depends on the ambient environment and the horses degree of adaptation and fitness. 
  • Excess heat and humidity present greater challenges to animals and make maintaining thermoneutrality more difficult. 
  • when a horse is unable to dissipate heat to the point that core body temperatures rises above 41-42°C, hyperthermia results.
  • Cause: hot/humid environment, prolonged strenuous exercise/short, high intensity exercise, dehydrated/exhausted animals.
  • Signs: increased temperature, fatigue, increased heart/respiratory rate.
  • Diagnosis: physical examination.
  • Treatment: immediate/aggressive treatment - cold water therapy, fanning, nasal oxygen administration, IV fluids, NSAIDs/corticosteroids, DMSO.
  • Prognosis: good if caught before neurologic involvement, otherwise poor to grave.

Pathogenesis

Etiology

  • There are 5 distinct mechanisms involved with cooling of a warm blooded animal:
    • Conduction: direct transfer of heat by contact with a cooler object - conduction of heat to the cooler object.
    • Convection: cool air moving across the surface of the body, heat transferred to the cool air, warming it and cooling the body.
    • Radiation: heat radiated outward from the warm body to the cooler environment. 
    • Evaporation: the loss of heat through the process of removing water from the surface of the body through vaporization.
    • Respiration: inspired air raised to body temperature and then exhaled.
  • When these mechanisms are not adequate, either due to excess ambient temperature or because of excess heat production due to vigorous exercise core body temperature rises.
  • High body temperatures result in enzymes and other body processes failing to function; this, in turn, leads to loss of homeostasis and normal physiologic functions.
  • Heat stroke is a combination of hyperthermia and central nervous system dysfunction, characterized by weakness, ataxia, collapse, convulsions, coma and death.

Predisposing factors

General
  • Hot, humid environment.
  • Poor conditioning.
  • Poor adaptation to high ambient temperatures.
  • Anhidrosis.
  • Prolonged strenuous exercise or short, high intensity exercise.
  • Prolonged transport in hot, humid weather.
  • Dehydrated, exhausted horses.
  • Foals treated with erythromycin.

Specific

  • Increase in core body temperature.
  • Clinical signs appear when rectal temperatures increase above 41-42°C.

Pathophysiology

  • Metabolic heat production in the muscle mass of exercising horses.
  • When hypothalamus senses higher brain temperatures thermoregulatory mechanisms begin.
  • Sweat glands are activated and sweating occurs.
  • Cardiac output increases and blood flow to the skin increases.
  • Vascular beds under the skin dilate to increase blood flow to the skin and facilitate heat loss.
  • Core temperature would increase by 0.25°C/min if the heat was not dissipated.
  • Sweat acts to cool skin via evaporation, with approximately 80% of heat lost via this route.
  • The remaining 20% of heat is lost via the respiratory tract.
  • High humidity impairs evaporation; heat loss due to sweat dripping from the body dissipates only approximately 10% of the heat load that can be lost by evaporation.
  • When these mechanisms are outpaced by the amount of heat produced by the muscles hyperthermia ensues.
  • This occurs more rapidly when environmental heat load is high, ie hot, humid conditions.
  • If the horse is also dehydrated, the impaired heat dissipation through ineffective sweating further exacerbates increase in body temperature.
  • High body temperature inhibits motor activity, preventing additional heat generation and resulting in reluctance to move.

Timecourse

  • Hyperthermia typically results after poorly conditioned horses have been exercising for several hours.
  • Once core temperatures exceed 42-43°C, clinical signs can progress extremely rapidly if exercise is continued or appropriate therapy is not implemented.

Epidemiology

  • Performance horses most at risk.
  • Foals receiving erythromycin   Erythromycin   medication.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Hubert J D, Beadle R E & Norwood R E (2002) Equine anhidrosis. Vet Clin North Am Equine Pract 18 (2), 355-369 PubMed.
  • Marlin D J, Schroter R C, White S L et al (2001) Recovery from transport and acclimatization of competition horses in a hot humid environment. Equine Vet J 33 (4), 371-379 PubMed.
  • Guthrie A J & Lund R J (1998) Thermoregulation: base mechanisms and hyperthermia. Vet Clin North Am Equine Pract 14, 45-59 PubMed.
  • Marlin D J, Scott C M, Roberts C A et al (1998) Post exercise changes in compartment body temperature accompanying intermittent cold water cooling in the hyperthermic horse. Equine Vet J 30, 28-34 PubMed.


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