Suddenly, one hot day after a ride, you notice your horse doesn’t seem to be sweating as he normally should or had in the past. You scratch your head and wonder what could be wrong. You think back to the last few weeks and can’t quite remember if this had gradually been happening or if it was a sudden change. It leaves you concerned, and rightly so.

This is called “anhidrosis” and is a condition where a horse loses the ability to sweat. This can actually be quite dangerous to a horse’s well-being and must be carefully managed once diagnosed.

The best piece of advice when suspecting your horse might possibly be suffering from anhidrosis is to contact your veterinarian right away. Only they can diagnose your animal and give you the best options for treatment. However, there are many aspects of anhidrosis owners should be aware of.

Anhidrosis is thought to affect up to 6% of horses worldwide. There are no definitive links to certain breeds, ages, sex, or even disciplines to horses that suffer from anhidrosis. We do know the onset of anhidrosis can either be quite sudden or can develop over time. Not surprisingly, it is most often observed in horses living in hot and/or humid regions of the world. It is also most commonly observed during the hot summer months or during heat waves. This is an obvious time when a horse should be sweating to regulate their body temperature.

We do understand the mechanisms about horses sweating. Under normal conditions, adrenaline is released due to increased body temperatures and it stimulates the sweat glands to function. However, an overstimulation of the sweat glands by an overproduction of adrenaline can lead to anhidrosis. This is thought to be caused by either extreme stress, extreme temperatures, or living in consistently hot/humid conditions. With an overstimulation of adrenaline, the sweat glands become less responsive and do not respond properly, and thus, the horse loses its ability to sweat.

At rest, a horse’s normal body temperature is between 99.5 to 101°F (37.5 to 38.3°C). During exercise, a horse’s body temperature will rise but should return to normal within about 30 minutes once exercise has ceased. They primarily do this via sweating, which is often why horses are seen covered in the rich, lathery sweat after any bout of exercise that increases body temperatures. In horses suffering from anhidrosis, they appear almost completely dry after exercise, or might only have some sweat under their saddle pads or in small patches around their bodies. Because the horse cannot dissipate the buildup of heat caused by high ambient temperatures, humidity or exercise, this can lead to hyperthermia, heat stroke, or death.

In addition to a lack of sweat, other characteristics of a horse suffering from anhidrosis could be:

  • Elevated body temperature past a 30-minute cooling off period. Body temperature remaining above 104°F (40°C).
  • Increased respiration rate past a 30-minute cooling off period. Resting respiration rate for an adult horse is about 14 breaths per minute.
  • Increased heart rate past a 30-minute cooling off period. Resting heart rate for an adult horse is about 30 to 40 beats per minute.
  • Lack of interest in drinking water.
  • Lethargy leading to poor performance and even collapse.

 
There is no known cure for anhidrosis. However, owners can help alleviate symptoms. One key point is to always ensure the horse has free access to either a stall or shade during the daylight hours. Other tips include:

  • Barn safe fans should be used to keep air flow moving in stalls and throughout the barn.
  • The addition of misters in a stall will help horses stay cool.
  • Limit exercise to early morning or late evening hours.
  • Extend the cool down period by wetting the horse’s coat and using fans to help the horse cool down quicker.
  • Keep the hair coat clipped. Regular, more aggressive brushing of the coat helps stimulate blood flow to the skin. 
  • Always monitor body temperature, respiration rate, and heart rate after exercise to ensure the horse is cooling down properly.
  • Always provide fresh, cool and clean water.

Another consideration for horses that suffer from anhidrosis is to ensure they are receiving adequate nutrition. Some suggested treatments are feeding electrolytes or supplementing salt in the diet; this has helped some horses overcome anhidrosis. If a horse cannot sweat properly, their blood chemistry is abnormal and it is thought that added electrolytes helps bring them into balance. Others have even suggested feeding extra vitamin E and C and other nutrients, as there is some antidotal evidence this has also helped horses overcome anhidrosis. It is clear that horses that are stressed need a properly balanced diet to be able to maintain their health.

Thankfully, anhidrosis is not very common in the wider horse population. However, some owners may suddenly find themselves with a horse that either sweats very little or at all. It is critical to always seek advice from your veterinarian when you suspect your horse might be suffering from anhidrosis. Finally, please feel free to contact us for any advice and support that you may need in designing a feeding plan for your horse's specific needs!
 

Chris J. Mortensen, Ph.D.