Anhidrosis — the partial or complete inability to sweat normally — is a serious thermoregulatory failure. Evaporative cooling through sweating accounts for approximately 65% of heat loss during exercise. When a horse stops sweating, internal temperature climbs rapidly with any exertion. The condition is most common in horses moved to hot, humid climates.
Causes
The dominant hypothesis involves exhaustion of sweat gland function through chronic overstimulation. Horses moved to hot, humid regions are at highest risk. Johnson et al. (1998) demonstrated reduced sweating response to exogenous epinephrine in anhidrotic horses, supporting beta-adrenergic receptor downregulation as the mechanism.
Clinical Signs
- Absent or markedly reduced sweating despite heat and exercise
- Dry, rough, or dull coat; some horses lose facial hair — coat changes in hot climates warrant ruling out Cushing’s disease and equine metabolic syndrome as contributing factors
- Distressed respiratory pattern: rapid, labored breathing in heat
- Elevated resting temperature: commonly 38.5 to 40 degrees C
- Exercise intolerance: rapid fatigue
- Partial anhidrosis: flank and axilla retain sweating while neck and back remain dry
Management
No treatment is reliably curative. Moving the horse to a cooler climate is the most effective approach; many horses recover spontaneously within 1 to 3 months. Work horses during the coolest part of the day. Maximize ventilation with fans and shade. Water misting over a fan increases evaporative cooling substantially. Active cooling after exercise is essential: sponge with cool water and scrape repeatedly — see our full guide to summer heat management.
Supplementation: One AC (acetyl L-carnitine) is anecdotally reported to restore sweating in some horses. Dark beer has been used in Gulf Coast barns with anecdotal benefit. Neither has strong controlled trial evidence.
Monitoring and Prognosis
Do not exercise if baseline temperature exceeds 38.8 degrees C. Monitor for signs of dehydration, which compound heat stress. Heat stroke (rectal temperature above 41 degrees C, staggering, muscle fasciculations) requires immediate veterinary attention. Horses remaining in hot, humid environments may continue to be anhidrotic indefinitely, but with careful management most can remain in moderate work.
Sources: Johnson PJ et al. Vet Clin North Am Equine Pract 2002; Johnson PJ et al. J Vet Intern Med 1998; AAEP Endocrinology Guidelines (aaep.org).
