English
In early August 2023 in Ocala, Florida, a nine-year-old Appaloosa gelding began exhibiting symptoms of anhidrosis, the inability to sweat.
This condition is particularly concerning in horses, as sweating is a primary means for them to regulate their body temperature and cool themselves down in hot weather. [1]
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.
Clinical Presentation
The gelding presented with the following clinical signs:
- A lack of sweat production
- Lethargy
- Elevated respiration rate and effort
- Patches of hair loss
Horse History
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.
Water and salt were also provided free choice.
Intervention
Diagnosis of anhidrosis is typically based on clinical signs and performance. [2] 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.
Outcome
- 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.
Discussion
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. [2]
The exact cause of anhidrosis remains unknown and is likely influenced