Hyperthermia is the condition in which the core body temperature rises above the normal physiological range. In horses the normal rectal temperature is 99.0 to 101.5 degrees Fahrenheit (37.2 to 38.6 degrees Celsius); readings above 101.5 degrees Fahrenheit constitute hyperthermia, and readings above 104 degrees Fahrenheit represent a medical emergency. The term encompasses both fever (pyrexia), in which the hypothalamic set-point is raised by pyrogens released during infectious disease or inflammatory stimulus, and non-febrile hyperthermia, in which metabolic heat production or environmental heat load exceeds the body's ability to dissipate warmth.
Heat exhaustion and heat stroke are the clinically most urgent forms of non-febrile hyperthermia in horses. They occur when a horse performing intense work in hot, humid conditions cannot lose heat fast enough through sweating and convective cooling. Horses dissipate approximately 65 percent of exercise-generated heat through evaporative cooling; high ambient humidity reduces evaporative efficiency dramatically, making temperature-humidity index (THI) a better predictor of heat stress risk than air temperature alone. Clinical signs of severe hyperthermia include elevated heart rate, rapid shallow breathing, profuse sweating giving way to a dry hot coat as dehydration progresses, stumbling, and altered mentation. Rectal temperature above 41 degrees Celsius requires immediate cold-water hosing of the large muscle groups and neck vessels, removal of tack, provision of shade and air movement, and veterinary evaluation because sustained temperatures at this level cause protein denaturation, organ damage, and circulatory collapse.
Febrile hyperthermia in horses arises from viral respiratory diseases, bacterial infections, pleuritis, colitis, and other systemic conditions. A fever below 104 degrees Fahrenheit in an otherwise stable horse is generally allowed to persist unless it causes marked discomfort, because the elevated temperature impairs pathogen replication and supports immune function. Antipyretic drugs—chiefly flunixin meglumine and phenylbutazone—reduce fever by blocking prostaglandin synthesis but do not treat the underlying cause. Monitoring rectal temperature twice daily during a fever episode, combined with a current vaccination record and veterinary guidance, forms the basis of a responsible febrile management protocol.
Further Reading
- hyperthermia (Wikipedia)
- For the severe inherited form and its clinical management: malignant hyperthermia in horses — MSD Veterinary Manual