Anhidrosis, or the inability to sweat to dissipate heat after exercise, is a condition that develops in horses stabled and trained under hot, humid conditions. It is commonly described as ‘dry coat’ or ‘non-sweating’ disease.

An earlier form of the disease referred to as ‘the puffs’ develops in 50–60% of horses under tropical conditions during peak daytime temperatures in the summer months. Horses that lose the ability to sweat efficiently are unable to maintain their body temperature within normal resting limits, and start to pant to blow off heat.

Horses in hard training programs, particularly those on high grain diets are more prone to develop the condition. However, all ages, breeds, pregnant mares, and idle non-working pleasure horses at pasture can be affected. The condition usually begins in spring or summer, particularly during early unseasonable humid conditions where horses have less time to adapt or acclimatise to the seasonal change.

Cause of anhidrosis

The underlying cause has not been determined. Studies have suggested that anhidrosis syndrome is triggered in the hypothalamus gland by the stress of exercise and chronic dehydration under hot, humid conditions. The condition is not prevalent under conditions of hot, dry heat so high humidity appears to be a trigger factor.

Although traditionally horses introduced from cooler climates to tropical conditions were considered more prone to developing anhidrosis within 1 to 3 months, even horses bred and reared under tropical conditions can be affected. There is evidence that the sweat glands shrink and change in structure in affected horses. This is thought to be linked to exercise and environmental stress, resulting in higher levels of circulating adrenaline hormone in the blood. Some, but not all, affected horses regain the ability to sweat again when returned to cooler climates.

Symptoms of anhidrosis

In the early stages, horses sweat heavily initially, and then over 1 to 3 months develop sites of limited, patchy sweating under the mane and between the front and back legs when exercised. Once a horse starts to lose the ability to sweat readily during and after exercise, they supplement their heat loss by puffing and blowing forcibly for up to 30 minutes after exercise.

Their coat becomes dry, pinches up easily and the hair often thins over the head and upper neck. Often performance is dramatically reduced, as panting expends energy reserves, leading to poor stamina and exercise tolerance. Horses may develop heat overload and collapse during strenuous exercise.

Management and therapy of anhidrosis

The condition is usually ‘managed’ rather than ‘treated’, as anhidrosis is a very difficult condition to treat and full recoveries are very rare if the horse remains in the hot, humid climate. Ideally the horse should be moved to a milder, less humid climate, although this is not always practical.

The following management and feeding measures are often useful in the management of anhidrotic horses:

  • Try to get horses fit before the hotter months of the year. Starting to condition the horse once the temperature is already high may predispose horses to anhidrosis. Some anhidrotic horses may only be able to compete during the winter months and may need to be rested during summer.
  • Feed a daily electrolyte supplement designed for horses in hot, humid conditions e.g. Humidimix. It has been reported that some horses with a history of anhidrosis that improves during the winter do much better if they are supplemented with electrolytes before the onset of hot weather. Some horses have managed to train and race without a relapse. Therefore, do not wait for the problem to return before you start feeding electrolytes.
  • Try to provide a cool environment for the horse. Air conditioned stables are ideal, otherwise high gabled stables with ceiling fans or ridge top roof vents will also help to improve airflow.
  • In paddocked horses, provide adequate shade and plenty of cool water. Alternatively, keep horses indoors during the daytime (if cool stables are available) and allow grazing at night.
  • Body clip horses with a thick hair coat
  • Exercise the horse at the cooler times of the day or night.
  • After exercise, help the horse to cool down more efficiently by hosing with cool water (particularly around the flanks and belly area) and then scrape the horse off immediately (within 30 seconds). This can be repeated several times until panting and blowing is reduced. Once the horse is scraped off it should be walked or stood in front of a fan or in a breezy area to help body cooling. Do not allow wet horses to ‘drip dry’ as excess water in the hair coat retains heat and interferes with body cooling.
  • Fermentation of fibrous feeds and protein in the large intestine leads to the generation of extra heat within the body. Avoid high protein diets during hot conditions. Oats are the most fibrous grain fed to horses, and this can be replaced with lower heat-producing grains such as rolled barley, corn or vegetable oil. Half a litre of corn or ¾ litre of rolled barley or ⅔ cup (175ml) of vegetable oil will provide as much energy as 1 litre of oats.
  • Supplementation with 1000–5000iu Vitamin E (e.g. White-E powder or liquid) has also been recommended by a number of sources.