Page 90 - May 2022

EQUINE HEALTH
ANHIDROSIS
DEALING WITH THE INABILITY TO SWEAT
by Heather Smith Thomas
Sweating is an important function of the in tropical regions and some areas of the U.S., WHY AND HOW HORSES SWEAT
horse’s body during hot weather and/or
exertion to keep him from becoming too hot. A small amount of body heat is removed through air exchange via the respiratory sys- tem, but more than 70% of excess body heat is dissipated by sweat evaporating from the skin. Some horses in hot climates lose their ability to sweat (anhidrosis) and are at risk for heat stress and heat stroke.
This problem was first noticed many years ago in British Thoroughbreds taken to tropical countries for cavalry use, racing and polo. A significant percentage of horses in hot, humid regions suffer from some degree of anhidrosis. This dysfunction is most common in horses
. . . more than 70% of excess body heat is dissipated by sweat evaporating from the skin.
especially the Gulf Coast states. Anhidrosis, which literally means without sweat, is most common in hot, humid regions but can occur as far north as Minnesota and Michigan, and in dry climates like Arizona and California.
Matt Randall, DVM of Collier Equine in Waller, Texas, sees cases of anhidrosis every
year in his part of southeast Texas. “Here in our climate, horses are sweating year-round. One
of the problems we run into with constant heat and high humidity in summer is horses that stop sweating. I grew up in southern Montana and we never saw any horses with anhidrosis,” he says. An arid climate cools off at night. Constant heat and humidity put a horse more at risk.
Martha Mallicote, DVM, DACVIM, Clinical Assistant Professor, University of Florida
Large Animal Hospital, says horses are very efficient sweaters, though slightly less so in a humid environment. “It is very important for performance horses to be able to sweat adequately to dissipate heat from exertion,” she says.
The horse can quickly pull fluid from the bloodstream and put it onto the skin through sweat glands, to evaporate and help cool the horse. “In the horse, the sweat glands are already primed--all the time--with electrolytes, proteins, and lipids, and all they have to do is draw water from the blood. It’s ready to go. When they get the stimulus (when the body determines it is too hot and needs cooling), this triggers the glands to draw in water from the blood and then contract and expel it out as sweat,” says Mallicote.
“This is a very efficient mechanism, and it doesn’t take long for sweat glands to recover and be prepared to produce more sweat.” Horses that work hard in hot weather, especially if they are fit and in good condition,
88 SPEEDHORSE May 2022