Non-Sweating Disease (Anihydrosis, Dry Coat)
Horses lose around 50% of the heat produced through evaporation of sweat from the body surface during and after exercise, 20% by convection and radiation into air passing over the body during exercise or when standing in a breeze, and up to 30% by transfer of heat from the blood through the lung surface into the expired air during work, as well as by means of 'panting' after exercise in the cool-down period.
Sweat loss is influenced by the type of work, duration of work and the ambient temperature and humidity under which the horse is worked or stabled. A horse can lose up to 11.0 litres of sweat per hour when working under hot conditions, with each litre of sweat that is evaporated from the body surface area removing about 2.5 million watts of heat energy.
Although hot humid weather does not increase the amount of sweat lost, high humidity reduces the exchange of moisture between the body surface and the surrounding air, reducing the efficiency of evaporation and hence body cooling.
Horses that dehydrate during work from sweat and respiratory loss begin to limit their sweat loss to conserve body fluids once about 5% of the body fluid is lost. These horses are then more prone to heat stress and hyperthermia.
Hot and Humid Environments
Non-sweating disease, which results in the progressive loss of the ability to sweat under hot, humid conditions, increases the risk of a horse developing hyperthermia, heat stress and collapse. Although affected horses are not usually dehydrated, they are unable to lose heat efficiently by evaporating sweat from their body surface. These horses therefore have to rely on convection and radiation to control body temperature. They may also develop a more rapid shallow breathing pattern when working or resting under hot conditions. The early symptoms are commonly referred to as "the puffs" as the horse attempts to blow off heat from its large surface by 'panting' in shallow, rapid breaths.
Non-sweating disease usually develops progressively over a period of 2-24 months, most commonly in horses that have been introduced from a cold or even a hot environment, which has a low humidity. Non-sweating disease affects up to 20% of racing horses in training under hot, humid or tropical conditions, and up to 6% of all horses under these conditions. It can affect horses that have been bred and raised in tropical areas, although most commonly horses introduced from cool areas have a higher risk of developing the condition, occasionally seen as a sudden shut down of sweating and high risk of hyperthermia.
Over-Activity of Sweat Glands
The underlying cause has been linked to the continuous stimulation of sweat glands, under humid conditions, which become exhausted and begin to degenerate under hot humid conditions. Affected horses conserve chloride, sodium and calcium by reducing urinary output of these electrolytes.
Most horses that develop the condition begin to blow hard when exercised, lose their appetite, perform poorly and develop a thin hair coat, which in the long term, results in scaling of the skin and patchy hair loss.
Diagnosis can be performed by adrenaline and terbutaline skin sensitivity tests to stimulate sweat secretion. Affected horses do not respond by starting to sweat, even at the highest concentrations of these test drugs.
A number of methods of therapy have been tried and evaluated over the years. These include supplements of iodine in milk protein (casein), high doses of Vitamin E (eg. 1500iu daily of Natural Vitamin E). electrolyte replacers with high potassium and chloride salt mixes (such as the 'Heavy Sweat' Electrolyte Mix) and supplements of the amino acid, tyrosine, as well as anabolic steroids and other hormones. However, none of these alone or in combination appear to have a direct benefit in advanced cases.
Methods of Management of Non-Sweating Disease
The effect on performance and appetite in racing and performance horses and risk of collapse during and after work due to hyperthermia is a major concern. The most widely accepted and successful method is to provide air-conditioned stables or cooling fans to assist heat loss through radiation and convection from the skin surface. It is an expensive method of control, but helps to avoid hyperthermia and maintain comfort and performance.
Other Management hints include:
1. Provide a HEAVY SWEAT ELECTROLYTE as a daily supplement to maintain adequate blood levels of potassium and chloride.
2. High doses of Vitamin E may also be beneficial in some horses.
3. Reduce the intake of roughage to a minimum of 0.75% body weight of hay or about 3kg for a 500kg horse to reduce 'heat waste' from fermentation of large amounts of fibre in working horses.
Replace any raw grain in the ration with extruded (cooked) grain, such as Pryde's Easifeeds, to further reduce 'heat waste' from fermentation in the hindgut, so a horse has less heat from digestion to shed from its body.
4. Under hot conditions, provide cool air movement by cooling fans to remove heat by convection and radiation from the skin.
5. One method that is used in hot, humid conditions, during the hot, humid time of the day, is to soak a hessian (burlap) strip about 2 meters long by 500mm wide in cold water. Place the strip along the back of a horse that is sweating heavily from the withers to the tail butt area. Hessian is preferred because its open weave structure allows heat to escape as the water is evaporated. The hessian acts as a "wick" to absorb and evaporate moisture to cool the skin surface. Direct fans onto the body, or stand the horse in a breezeway to assist the airflow in evaporating the moisture from the hessian bag, and thus more efficiency dissipate heat. Repeat 2-3 times daily under hot conditions.
6. If the horse is still distressed by hyperthermia, apply sponges saturated with iced water to the under belly area and flanks, as these areas hold heat from digestion and after exercise. Heat produced in the muscles during exercise is transferred to a "heat sink" or "heat store" in the hindgut water reserves during the cool-out period. Hosing under the belly area after exercise improves the efficiency of heat loss and cool-out.
7. It is important to remove excess water trapped in the coat by scraping it out within 30 seconds after hosing or swimming. Water held in the coat will rapidly heat up and with the large volume of warm water heat exchange will be reduced under hot, humid conditions, resulting in an insulating affect to reduce evaporation of moisture from the body surface. Do not let a horse 'drip dry' under humid conditions, unless it is being lightly worked to cool-off or placed in a position where an air current can flow over its body, such as in a breezeway or in the airflow from a cooling fan.
8. Consider pre-cooling before exercise when the horse is cool - apply cold water over the body area (not under the saddle area) and then warm the horse up by light trotting exercise. The internal core and muscle temperature will increase, but the evaporation of water in the air flow as the horse works will reduce sweat output and delay heat accumulation in the skin and underlying tissues. Hose down and lightly walk the horse for 5-7 minutes to cool it down using the same principle after exercise.
9. Swimming a horse after exercise will help reduce body surface and belly core temperatures more rapidly, provided the hair coat is scraped off to remove excess water after swimming.
10. Maintaining a short hair coat and ensuring a horse is fit before the start of hot and humid seasonal weather conditions will help to reduce the risk of non-sweating disease.
Some horses regain the ability to sweat after being relocated to a cooler, less humid climate, but often regress to the non-sweating condition once returned to a hot, humid climate.