ColicInHorses

August 2000

Serving Horse Owners Across the State

Vol 1


Articles

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Anhidrosis: Loss of the Ability to Sweat
Babetta Breuhaus, DVM PhD
Associate Professor, Equine Medicine
North Carolina State University

Thermo regulation is an important component of exercise performance that is sometimes overlooked. Muscles convert approximately 20% of the chemical energy they burn into mechanical energy for work; the rest is released as heat. This heat must be dissipated from the body (primarily by skin vasodilation and evaporation from the respiratory tract or sweat) in order to maintain normal body temperature. In exercising horses, approximately 15-25% of heat loss occurs through the respiratory tract, while as much as 65% of heat loss is accomplished by sweating.
    Anhidrosis is a condition of adult horses characterized as a decreased ability or inability to sweat in response to appropriate stimuli. It tends to occur in hot, humid environments, and, in the United States, is most prevalent in the Gulf Coast states. Inadequate sweat production, particularly in a hot environment, interferes with body temperature regulation both at rest and during exercise. Thus, anhidrosis potentially has a profound detrimental affect on a horse’s ability to work and compete athletically.
    Clinical signs of anhidrosis include exercise intolerance, rapid respiratory rate, elevated body temperature, a slower rate of recovery of respiratory rate following exercise, and absence or a decreased amount of sweat. Some anhidrotic or hypohidrotic horses have areas of the body that retain the ability to sweat to varying degrees (eg under the mane or halter, or in the inguinal, perineal or axillary regions). Anhidrosis is suspected based on clinical signs, and can be confirmed by observing failure of a horse to sweat in response to injection of a ₂-adrenergic agonist into the skin.
    Immediate treatment of an overheated horse employs application of cold water to the body surface and movement of the horse into a cooler environment (shade, air conditioning, fans, etc). Long term treatment of these horses is more difficult and often is unsuccessful. Once a horse has developed clinical signs, it is usually necessary for the horse to undergo a period of rest. It may even be necessary to move the horse to a cooler geographic area. Various supplements have been tried, including electrolytes, thyroid hormone, and even beer. A recently developed nutritional supplement that contains precursors to the synthetic pathway for dopamine has been reported to be successful in some horses.
    The cause of anhidrosis is unknown. Presumably, there is an abnormality in stimulation or production of sweat. Histologic examination of sweat glands from anhidrotic horses has revealed them to be atrophied, with plugged ducts. However, it is unknown whether this histologic appearance is the cause of anhidosis or merely a result of disuse.
    Equine sweat glands are stimulated to secrete by activation of $₂-adrenergic receptors. It has been proposed that anhidrosis results from desensitization or downregulation of $₂-adrenergic receptors, but to date this has not been demonstrated and the mechanism is unknown. Recently, it has been suggested that hypothyoidism may play a role in $-adrenergic receptor dysfunction in anhidrotic horses. Hypothyroidism has long been associated with anhidrosis, and treatment with iodinated casein has been reported to be successful in some individuals. Thyroid hormones modulate adrenergic receptor function, such that tissues from hypothyroid individuals are less responsive to -adrenergic agonists. Both desensitization and downregulation of $-adrenergic receptors has been shown in hypothyroid animals. No detailed studies of thyroid function have been performed in anhidrotic horses to date.
    The American Horse Shows Association has funded a study at North Carolina State University to compare thyroid function in anhidrotic horses to thyroid function in horses that sweat normally. Specifically, serum concentrations of thyroid hormones and thyroid stimulating hormone (TSH), both at rest and in response to intravascular injection of thyrotropin releasing hormone (TRH), will be compared between anhidrotic an normal adult horses. These studies will be performed once during the summer when the affected horses are not sweating, and once again in the winter when they are normal. Anhidrosis will be confirmed by an exercise test (response to lunging) and an intradermal terbutaline sweat test. A normal horse from the same stable will be chosen and will go through the same procedures as the anhidrotic horse. Neither horse can be on any medications that might interfere with thyroid function (see below). The horses then will undergo a TRH stimulation test to measure thyroid function.
     The physiology or rationale behind this proposal is as follows. Thyroid hormones normally are synthesized and released from the thyroid gland in response to TSH from the anterior pituitary gland, which in turn is stimulated by TRH from the hypothalamus in the brain. Diagnosis of thyroid disease in the horse traditionally has been difficult because single or isolated measurements of thyroid hormones are difficult to interpret. TRH and TSH stimulation tests are more useful, but are not routinely performed in practice. In addition, thyroid testing must be performed in drug free horses fed a proper diet because thyroid metabolism and/or transport can be affected by a variety of drugs and physiologic or pathophysiologic states. Of particular relevance to the horse, glucocorticoids, phenylbutazone, dietary imbalances, and fasting decrease serum thyroid hormone concentrations without affecting thyroid function.
    If you have an anhidrotic horse and are interested in participating in the study, please contact Dr. Breuhaus at 919-513-6247. Results of this study are expected to aid our understanding of anhidrosis. If thyroid function is altered in anhidrotic horses, normalization of thyroid function may ameliorate clinical signs. If thyroid function is not altered in horses suffering from anhidrosis, supplementation with exogenous thyroid hormone is not only unnecessary, but potentially damaging to a horse’s health. Understanding the pathophysiology of this disease is essential in order to develop effective treatments and to prevent development of the condition.

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