In early August 2023 in Ocala, Florida, a nine-year-old Appaloosa gelding began exhibiting symptoms of anhidrosis, the inability to sweat.
Without the ability to sweat, horses are at risk of overheating, especially when exercising in hot temperatures. This can lead to serious health complications, including heat stress and heat stroke.
The gelding presented with the following clinical signs:
- A lack of sweat production
- Elevated respiration rate and effort
- Patches of hair loss
The horse had been purchased 8 months prior and relocated from Texas. The gelding had no known health issues, and there was no documented history of previous anhidrosis episodes.
During the period of the horse’s clinical presentation, average daytime temperatures were approximately 90°F (32°C) with relative humidity levels nearing 99%. This resulted in a heat index of 130.6°F (54.78°C).
The horse was lightly exercised, including groundwork, for five to six days a week at a walk and trot. The horse was housed in a 2-acre paddock with unrestricted access to a covered stall.
The horse’s daily diet was divided into two meals and consisted of:
- 1 lb (0.45 kg) of soaked timothy pellets
- 200 grams of Omneity Pellets
- 15 grams of table salt
- 1 gram of Optimum Probiotics
The horse’s forage included 6 lbs (2.7 kg) of coastal bermudagrass hay, as well as free-choice access to pasture.
Diagnosis of anhidrosis is typically based on clinical signs and performance.  In this case, the horse’s lack of sweat and elevated respiratory rate and effort were highly suggestive of anhidrosis.
Upon diagnosis, the horse was stalled during daylight hours and all exercise was halted.
Over a span of four days, salt intake was gradually increased to a total feeding rate of 120 grams per day. This amount was divided into three feedings:
- 45 grams of salt in the morning
- 30 grams of salt at lunch
- 45 grams of salt in the evening
Due to restricted pasture access, the horse’s forage intake was increased to 12 lbs (5.4 kg) of coastal hay per day. The rest of the diet remained unchanged.
- Within three days of being supplemented with 120 grams of salt per day, the horse resumed normal sweating patterns, and its respiration rate stabilized to a normal rate.
- When reintroduced to an exercise routine and daytime turnout, the horse continued to sweat adequately.
- As temperatures dropped to 85°F and humidity levels decreased to 68% (resulting in a heat index of 92°F) by late September, the horse’s daily salt intake was adjusted to 90 grams. Despite this reduction, the horse continued to exhibit normal sweating patterns.
Anhidrosis poses a significant health risk to horses, impacting their ability to thermoregulate, or maintain a stable internal body temperature in varying environmental conditions. If left unaddressed, anhidrosis can potentially lead to severe conditions, such as heat stress and heat stroke. 
The exact cause of anhidrosis remains unknown and is likely influenced by multiple factors. Researchers believe it occurs because the secretory cells in sweat glands gradually stop working properly, especially in hot or humid conditions. 
Prolonged exposure to extreme heat, with record-breaking high temperatures in Central Florida through July and August, likely contributed to this case. Hot and humid conditions require the horse to sweat more, potentially overwhelming the body’s sweating mechanism and increasing dietary needs for sodium and other electrolytes. 
In this case study, the significant increase in salt intake was directly correlated with an improvement in the horse’s sweating patterns. This suggests that the horse’s initial salt intake was likely insufficient given the extreme environmental conditions.
The rapid recovery observed in this gelding after increasing salt intake underscores the the importance of regular monitoring of horses exposed to high temperatures and humidity. Early intervention and a well-balanced diet played a key role in restoring this gelding to optimal health.
While salt supplementation was instrumental in resolving this horse’s clinical signs, it’s important to note that individual needs for salt can vary. At a minimum, we recommend adding 1-2 tablespoons of salt to their daily ration and providing free-choice salt to all horses.
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- McCutcheon, L.J. and Geor, R.J.Thermoregulation and exercise-associated heat stress. In Equine Exercise Physiology. 2008.
- Hubert, J.D. et al. Equine Anhidrosis. Vet Clin North Am Equine Pract. 2002.
- Jenkinson, D.W. et al. Equine sweating and anhidrosis Part 2: anhidrosis. Review Vet Dermatol. 2007.
- McCutcheon, L.J. et al. Sweating rate and sweat composition during exercise and recovery in ambient heat and humidity. Equine Vet J. 1995.
- National Research Council Nutrient Requirements of Horses. 2007.
- Jansson, A. and Dahlborn, K. Effects of feeding frequency and voluntary salt intake on fluid and electrolyte regulation in athletic horses. J Appl Physiol. 1999.