Anhidrosis in horses is a condition characterized by a persistent reduction or lack of sweat output.1–5 Onset of anhidrosis can be sudden, usually following a period of profuse sweating,6 or gradual with a sweat response slowly decreasing over time.3 Clinical signs include partial or complete loss of sweating ability, hyperthermia, tachypnea, reduced appetite, decreased water intake, alopecia, dull hair coat, and depression.3–6 A quantitative intradermal sweat test (with terbutaline) has been evaluated for identification of freely sweating horses.7
Anhidrosis has been described as a physiologic phenomenon that results as a consequence of prolonged stimulation of the sweat glands by epinephrine secreted as a response to conditions of hot and humid climate.6 The sweat glands accommodate to the high epinephrine content of the blood, thus becoming insensitive to it.6 This phenomenon can be reversed by rest in a cooler and less humid climate.6 More recently, it was found that the water channel aquaporin-5 may play a role in the pathogenesis of anhidrosis in horses.8 An immunohistochemistry analysis revealed a strong aquaporin-5–like activity reaction at the apical membrane of the glandular secretory cells in freely sweating horses.8 In horses with long-term anhidrosis, however, little aquaporin-5–like activity was detected, hence implicating water channel impairment as a possible factor in the development of this condition.8 Horses with anhidrosis require medical management and reduced workload (or removal from physical activity). Anhidrotic horses forced to perform can suffer severe consequences, including multiple organ failure in response to hyperthermia and, in some instances, death.9
Epidemiological aspects of anhidrosis in horses are not well understood. Previous studies have been limited to a low number of horse farms or have not used objective research methods. In a 1982 survey10 of 24 horses affected with anhidrosis in Florida, the frequency of anhidrosis was higher in native horses and Thoroughbreds, compared with imported horses and other breeds, respectively. In that study,10 healthy horses were not included. Thus, it was not possible to assess the origin and breed as risk factors for anhidrosis in horses. In another study11 conducted in Florida 5 years later, the prevalence of anhidrosis in horses was 6% (52/834); the frequency of this condition was higher in training horses, compared with yearlings, lactating mares, barren or maiden mares, and stallions. Anhidrosis was more common in the summer, and it was typically associated with recent transport of horses from geographic regions with a more temperate climate. That study11 was limited to 4 Thoroughbred farms with a history of anhidrosis; farms without a history of this condition were not included. Thus, it was not possible to examine factors associated with anhidrosis at the farm level. The objectives of the study reported here were to estimate the prevalence of anhidrosis in horses in Florida and identify farm- and animal-level factors associated with this condition.
National Agricultural Statistics Service
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