We are now full swing into summertime! With our high temperatures and extreme humidity, we start to see an increased occurrence of anhidrosis in our horses. Anhidrosis or “non-sweating” is when there is a decrease in sweat production even with an increase in body temperature. Studies have shown that horses lose 60-75% of their body heat via sweat evaporation. A humid environment already decreases the efficiency of sweat evaporation and cooling. If a horse is unable to sweat at an appropriate level, he is at risk for hyperthermia or heat stroke.
Physiology of Sweat & Causes of Anhidrosis: Figure 1: In the horse, sweat glands are densely packed in the skin, usually around the hair follicle. These glands have a large amount of blood supply and nerves supply. Sweat is made up of mostly water but also contains proteins and electrolytes. Electrolytes in sweat (sodium, potassium, chloride) are found in much higher concentrations than blood. At this time, there is no known specific cause of anhidrosis. Previous suggestions include hypothyroidism, low chloride concentrations, and elevated epinephrine levels. These have not been consistent findings in all anhidrotic horses. The current thought is that there is over-stimulation of the horse’s sweat glands by stress hormones. The onset of anhidrosis can be gradual or acute. One interesting finding is that in chronic cases of anhidrosis sweat glands atrophy. In horses newly affected, this sweat gland atrophy is not a finding. This suggests that there may be a correlation of long term downregulations on hormone receptors on sweat glands. In addition, when a horse is moved to a northern environment, it typically resumes normal sweat production. Researchers have not identified any age, color, sex or predisposition for developing anhidrosis. Anhidrosis is found to have a similar prevalence in both in locally-raised versus imported horses.
Typical signs of partial anhidrosis:
- Your horse takes longer to cool down and return to a normal respiratory rate after exercise.
- His temperature takes more than 30 minutes to return to normal (98*F-101.5*F) after exercise.
- He only sweats in areas in contact with riding tack.
- He seems to be lethargic or less willing to work during summer months.
- Typical signs of anhidrosis:
- Your horse does not sweat at all
- Increased respiratory rate and effort- even if he’s been standing in his stall all morning.
- He has a dull, dry or long hair coat, which does not flatten during exercise.
- He leaves his pasture buddies to find shade or stands in water/water troughs.
Diagnosis
Anhidrosis is most often a presumptive diagnosis, based on clinical signs and a veterinary exam. The hallmark of anhidrosis is that even in situations that should elicit copious amounts of sweating, anhidrotic horses will have a minimal or inappropriate amount of sweat. It is not unusual for anhidrotic horses to still have sweat in certain locations. This often misleads owners. Partial anhidrosis is the most common presentation of anhidrosis. An owner should consider anhidrosis if your horse’s performance declines as temperatures increase in the summer. Chronic non-sweating horses tend to develop dry flaky skin (especially on forehead), hair loss, easily fatigues, and may have decreased appetite and water consumption. A definitive diagnosis can be made by a series of intradermal injections of terbutaline where each injection has a dilution factor. Terbutaline stimulates sweat glands, which allows the veterinarian to determine the severity of the anhidrosis. If necessary, other diagnostics include blood electrolyte levels, as well as skin biopsies to evaluate the horse’s sweat glands.
Management & Treatment
In the past, the only treatment for anhidrosis was to move the horse to a cooler climate. This helps the horse to manage high body temperatures, and interestingly many horses start to sweat once in a cooler environment. In Florida, it is essential that non-sweaters are carefully managed to prevent hyperthermia (overheating).
Management tools include:
Allowing horses access to water sources and having constant access to clean and cool drinking water.
Feed electrolyte and salt mixture to ensure appropriate electrolyte concentrations.
Exercise when ambient temperatures are lower- early or late in the day.
Turnout limited to evening hours or when the temperature is cooler.
Provide fans and misters.
Treatment: There are a variety of anecdotal tools which have varying levels of success; this includes feeding dark beer, thyroid supplementation or oral supplements (True Sweat, One AC). Unfortunately, there’s no research to support their efficacy. There have been numerous trials of different medications, but those have generally been unsuccessful.
Our practice utilizes acupuncture to stimulate and maintain sweat production. We also use a herbal remedy, New Xiang Ru San, as maintenance therapy in between acupuncture treatments. A recent study at the University of Florida found that acupuncture and herbal therapy had improved sweating in recently anhidrotic horses. Research has shown that acupuncture points contain large numbers of nerve endings, vascular and lymphatic vessels, and mast cells. Stimulating these points can result in pain relief, nervous system stimulation, immunity regulation, and improved thermoregulation.
If you have a horse that you worry that’s partially or fully anhidrotic and you would like a medical consultation, please give our office a call.