Selective brain cooling in the horse during exercise and environmental heat stress.

This research tracks how the brains of horses can selectively cool themselves during periods of high body temperature, either due to environmental heat exposure or intensive exercise. Temperatures were measured across various parts of the horse’s body, showing a marked decrease in brain temperature compared to the rest of the body.

Overview of the Research Procedure

• The study involved five horses, which were subjected to two types of heat stress – physical exercise on a treadmill and exposure to high external temperatures in a climate room.
• To reflect hyperthermia, or excessively high body temperature, the horses’ central blood temperature was raised to approximately 42.5 degrees Celsius during exercise and 41.5 degrees Celsius during the environment-driven heat exposure.
• Temperatures were measured in several key locations – the rectum to denote core temperature, the hypothalamus and the cerebrum for brain temperature, the cavernous sinus for the intracranial temperature, and finally on the skin of the head and midside to reflect external body temperature.

Findings from the Research

• When the central blood temperature of the horse increased above 38.5 degrees Celsius, the temperature in the hypothalamus remained lower by about 0.6 to 1 degree Celsius depending on whether the horse was under heat stress or exercising.
• The temperature in the cavernous sinus was significantly below the central blood’s temperature. During heat exposure, it was 2.2 degrees Celsius lower, and during exercise, it was 5 degrees Celsius lower.
• An experiment to bypass the upper respiratory tract during exercise in one horse resulted in notable reductions in the temperature differences between central blood’s temperature and hypothalamus and cavernous sinus temperatures.

Interpretation of the Research

• The results suggest that horses, even without a specific vascular structure known as a carotid rete (which is commonly found in some animals as a mechanism to cool the brain), can selectively cool their brains during periods of physical exertion or in high external temperatures.
• This selective cooling action may primarily occur through the cooling of venous blood within the upper respiratory tract, as reflected by the marked reduction in the cavernous sinus temperature.

Implication of the Research

• The research has the potential to improve the way horses are managed and cared for during periods of heat stress, particularly in terms of recognizing and averting risks associated with heat-induced conditions like hyperthermia. It further stresses the importance of the respiratory system in thermoregulation.