Anhidrosis – A Hidden Danger

”I often trail ride with a group of friends. This spring I’ve noticed that while their horses sweat on warm days, my horse hardly feels damp even under his saddle pad. Also, it seems like any amount of exertion causes him to breathe harder or more quickly than he should. Sometimes I will even see his nostrils flaring just standing in the field or in his stall on a warm day, even when we haven’t ridden at all. I haven’t heard him cough and he doesn’t seem sick in any way. Should I be concerned?” –Jessica L.

Jessica should definitely be concerned, as it sounds as if her horse may be affected by anhidrosis.  Anhidrosis is defined as a complete or decreased ability to sweat in response to increased body temperature.  This condition can affect any horse, although it seems to be more common in hot, humid climates.  It can come on gradually or appear all at once.  Horses affected by anhidrosis are often called “non-sweaters” and are affected in varying degrees ranging from mild performance limitations to life threatening hyperthermia or heat stroke.  Sweating is a key component in a complex system that allows your horse to regulate his body temperature.  The results can be deadly if that system fails. The normal body temperature of a horse ranges from approximately 99.0-100.9 degrees F.  As a horse exercises, his muscles generate heat as a byproduct of energy metabolism.  Circulating blood absorbs heat from the muscles and carries it to the lungs where some of the heat dissipates during exhalation, and to the skin where heat can radiate from the horse’s body.  If the horse produces more heat than he is able to unload through breathing and radiant cooling, his core temperature begins to rise.   The horse’s internal thermostat, a part of the brain called the hypothalamus, senses the increase and sends signals to the sweat glands distributed in his skin.  The sweat glands begin to pump out sweat.  Sweat is mostly composed of water but also contains dissolved minerals called electrolytes.  The electrolyte concentration in a horse’s sweat is higher than a human’s.  Heat is carried away from this skin as sweat evaporates, reducing the horse’s body temperature.

Sweat Gland in the Skin

The harder a horse works or the hotter the weather, the more he should sweat.  During intense exercise he can lose 10-15 liters of fluid in an hour through sweat and exhaled water vapor.  Between 65-75% of heat is dissipated through sweating, while only 15-25% of heat is dissipated by the respiratory tract.  On a hot and dry day, sweat may evaporate almost as quickly as it forms and your horse may lose a large amount of fluid before you are aware of it.  (He may be slow to replace that fluid too, because of the high concentration of electrolytes in his sweat.  When we sweat, we mainly lose water leaving us with an electrolyte imbalance which triggers a thirst response.  Because horses lose a larger amount of electrolytes in their sweat, they are slower to develop an electrolyte imbalance and slower to feel thirsty).  While excessive sweating can leave a horse dehydrated, a partial or complete lack of sweating may pose a greater danger as the horse has no effective way to unload the heat that is building up in his body.   Without intervention from his owner, his body temperature will remain elevated and can reach dangerous levels (106-110 degrees F after exercise) putting him at risk of heat stroke. The cause of anhidrosis is not well defined, but is generally considered to be associated with “over stimulation” of the horse’s sweat glands.   Originally it was thought that anhidrosis was due to difficulty acclimating to hot weather.  However a survey of horses in Florida found that more native horses were affected than imported animals.  In normal horses sweating is initiated by stimulation of the sweat glands, which are activated by neurotransmitters (substances released by the brain).  Based on recent studies, experts believe that there is either something wrong on the stimulation end (i.e. with the signals from the brain) or on the receptor end (i.e. there are decreased numbers of sweat gland signal receptors or the sweat glands are desensitized to the signal). Regardless of the cause of anhidrosis, the important thing is to recognize and address it.  The degree to which a horse suffers from anhidrosis may vary and the onset of clinical signs may be gradual or acute.   A horse owner should consider anhidrosis if their horse’s performance declines as the ambient temperatures rise during the spring and summer months.  The most common initial clinical findings are poor performance or lethargy (quickly becoming exhausted), a dry coat after exercise (the severity of anhidrosis varies, so a horse may be completely dry or just a little damp under tack or between his hind legs), increased respiratory rate with rapid shallow breaths, flared nostrils, elevated pulse and an elevated rectal temperature that requires an extended period of time (greater than 30 minutes) to return to normal range after exercise ceases.  The clinical signs of anhidrosis may be more subtle in the non-exercised horse, but these horses will have minimal or no sweat production in situations that should elicit copious sweating, poor appetites, lethargy and fevers which may be misconstrued as infection.  In chronic cases, horses may develop dry flakey skin (especially on the forehead), hair loss or thinning, fatigue, anorexia and/or decreased water consumption. The diagnosis of anhidrosis is usually straightforward as the clinical signs alone are confirmatory.  If the diagnosis is unclear in very subtle cases, a “sweat test” may be performed in which your veterinarian injects small dilutions of a drug, typically either epinephrine or terbutaline, under the skin of the horse’s neck.  These injections produce local sweating in a normal horse but not in the suspected anhidrotic horse.  Blood work which includes an electrolyte analysis may be helpful in formulating a treatment plan.

Cooling Out

The only cure for anhidrosis is relocation to a cooler climate.  Anecdotally, anhidrotic horses traveling north from Florida have been observed to begin sweating in the trailer upon crossing the North Carolina border.  Veterinarians joke that the best cure for anhidrosis is October 15th (when the temperature in most parts of the country drops consistently below 75 degrees F.) Since relocation is not an option for most horse owners and many sport horses need to compete through the spring and summer, the focus remains on treatment and management.  There are many different treatment and management options for anhidrosis, and what works on one horse may not work on another.  Exercise him early in the morning or in the evening when the heat index is low.  Cool him out aggressively with cold water and fans after exercise.  Monitor his vital signs and do not stop cooling efforts until they return to normal.  Be sure that he has shade during turnout or ideally turnout at night.  Provide a fan and ample ventilation in his stall.  Provide constant access to cool, clean drinking water.   See…all things you would do anyway, right?!  Continuing on…adjunctive feeding of electrolytes or “lite salt” may help some horses maintain appropriate total body electrolyte concentration.  Supplements, such as One AC or Platinum Refresh have proven helpful for some anhidrotic horses (and are best started in the spring before clinical signs appear).  Others respond well to acupuncture or a daily bottle of dark beer!  Work together with your veterinarian to formulate a treatment and management plan for your anhidrotic horse so that he may enjoy peak health and performance during the spring and summer months.