our expertise: VETERINARIAN ADVICE Tom Hutchins, DVM, DABVP T 86 hat time of year has come…the grass is turning green, the trees are blooming and the heat is on full force! It is also that time when some horses may show signs of a condition known by many as “nonsweaters” or “dry puffers”. This is actually known as anhidrosis. Anhidrosis is the inability to sweat effectively in response to appropriate stimuli. The current theory is that overstimulation of the sweat gland β 2 -receptors causes diminished function or a period of unresponsiveness of the receptors. There are no predilections for a specific coat color, age, sex, or breed of horse. Up to 20 percent of horses may be affected when exercising in hot, humid climates. Welcome to Texas if you aren’t from there and for those of you who are, no big surprise. Some of the clinical signs that may be evident include increased, rapid breathing rate after exercise, later combined with a lack or reduction of sweating. Horses recently introduced into a hot and humid climate may sweat excessively before showing signs of anhidrosis. With acute onset of the condition, horses may demonstrate partial or complete absence of sweating when exposed to the appropriate stimuli. Horses with long-standing anhidrosis may exhibit dry and flaky skin with alopecia or hair loss. They may also be lethargic and decreased water intake may be noted. Body areas that may retain the ability to sweat include under the mane, saddle and halter regions, and the inguinal, perineal, and axillary (where the front leg connects to the chest) regions. Heat-stressed horses may have higher-than-normal levels of circulating catecholamines in their system. Anhidrotic horses have significantly higher levels of epinephrine at rest as well. These catecholamines act as β 2 -agonists (stimulators) and may overstimulate the sweat gland receptors, which results in either desensitization of the receptor or down-regulation of the receptor. Horses maintained in hot, humid climates are at risk, and exercise magnifies the risk. It is important to get an accurate diagnosis because respiratory diseases that cause an increase in respiratory rate can exhibit similar clinical signs. Examples would include chronic obstructive pulmonary disease (COPD) or infectious causes such as pneumonia or lung abscesses. Complete blood counts and serum chemistries may indicate dehydration and urinalysis may show significant increases in concentration. The good news is that there is a diagnostic procedure available to aid in the diagnosis of the disease. Intradermal injections in the neck area below the mane, of a specific β 2 - agonist (terbutaline sulfate or salbutamol sulfate) in different dilutions and a control injection of sterile saline are performed. After 30 minutes has elapsed, the response to the injections is interpreted. Normal horses sweat in response to all of the dilutions, whereas anhidrotic horses show a diminished response to some or all of the dilutions. There is no response to the saline control injection in any horses. Sound environmental management is an important part and one of the truly reliable treatment options. Horses with acute anhidrosis who exhibit signs of heat stress should be immediately taken to a cooler environment, and attempts to reduce the body temperature should be made. Restrict Dr. Tom Hutchins the horse to a stall with adequate air movement (i.e., a fan) during the hot periods of the day. If exercise is necessary, do so during the cooler periods of the day. After exercise, make sure the horse is “cooled off” adequately by hosing it down with water. Grain rations should be fed in reduced amounts especially if high protein, carbohydrate diets are implemented. Allow access to cool, fresh water as well as water with electrolyte supplementation. It should be noted that horses with anhidrosis will be prone to poor performance and will only improve once the capability to sweat effectively has returned. It may not occur again in a horse’s lifetime but is usually a lifelong problem. If other β 2 -agonists such as clenbuterol (Ventipulmin™) for concurrent respiratory problems are being administered, consider this as a probable cause and stop administration. Some other treatment options include supplemental electrolytes, especially potassium salts; can be added to the feed or water. Amino acid supplements, especially those with tyrosine, are commercially available. Tyrosine is necessary for the resensitization of the β 2 -receptors. There are some anecdotal reports of success with iodinated casein and with vitamin E administered daily for at least a month. The take home messages from this are: • don’t expose anhidrotic horses, especially when exercising, to extreme ambient temperatures • exercise during the cooler periods of the day • stall the horse in a cooler environment during the hotter periods of the day • relocate the horse to a more temperate climate • avoid administration of oral β 2 -agonists such as clenbuterol Most horses respond to a change in environment and begin to sweat normally within a few weeks. Horses that have previously suffered from the disease will usually, but not necessarily, become anhidrotic if exposed to hot, humid conditions again.