Anhydrosis (also spelled anhidrosis) is the failure of a horse to sweat normally, either partially or completely, despite heat load and exercise. Because evaporative sweating is a horse’s primary heat dissipation mechanism, anhydrosis is a serious thermoregulatory disorder that can cause dangerous hyperthermia during work in warm weather.
The condition most commonly develops in horses moved from cooler climates to hot, humid environments — Florida, the Gulf Coast, and tropical regions see the highest prevalence. The working theory is that chronic overstimulation of sweat gland beta-adrenergic receptors leads to receptor downregulation, eventually causing the glands to stop responding to epinephrine. Once a horse becomes anhidrotic it may not recover fully even when moved to a cooler climate, though partial recovery occurs in some cases.
Signs include a dry, rough coat after work that should have produced sweat, elevated rectal temperature (above 103°F) following moderate exercise, rapid breathing as the horse compensates by panting, and reduced performance. Diagnosis is clinical, sometimes confirmed by intradermal terbutaline testing. Management focuses on reducing heat load: working only in the early morning or evening, providing access to shade and cool water, and in some cases relocating to a cooler climate. One supplement — One AC (a beer-based supplement) — has anecdotal support and some veterinary backing, though evidence remains limited. Any horse that stops sweating in hot conditions warrants veterinary attention; year-round hydration management is critical for affected horses.
Further Reading
- Anhidrosis — Wikipedia article on the condition of impaired or absent sweating and its physiological basis.
- Anhidrosis in athletic horses — peer-reviewed review of thermoregulatory failure in performance horses in hot climates (PubMed Central).