Anhidrosis in Horses

Veterinarians in the southeastern U S. are familiar with anhidrosis and will likely diagnose the condition based on the owners description and a physical examination of the horse.
Specific testing can be performed to confirm anhidrosis although it is rarely necessary. This involves injecting terbutaline a bronchodilator into the skin to cause sweating around the injection
site. Absorbent pads collect the horses sweat for 30 minutes then the weight of the pads is compared with the weight of normal horses pads to show the degree of difference in perspiration volume 1
Treatment 

The treatment of anhidrosis involves symptomatic relief. Keep horses as cool as possible during hot humid weather 2 Employ cool mist fans shade or stall rest to lower the ambient temperature and maximize the horses comfort.
If you must work your horse restrict activity to cool mornings or evenings. Schedule shows and events for the times of the year when the weather is less likely to be
hot.
Electrolytes acupuncture experimental medications and dietary supplements have been used to improve anhidrosis but there is little scientific research proving the efficacy of these treatments.

Prognosis for Horses with Anhidrosis
As long as a horse with anhidrosis is kept calm and cool during the hottest times of the year it can live fairly comfortably. Overwork or extreme stress and heat can be dangerous though causing a horse to collapse convulse and even die.
How to Prevent Anhidrosis
There is no known protection against developing anhidrosis even horses that are born and raised in hot environments can develop the condition.
Horses that already have anhidrosis will be more comfortable if they are moved north where long bouts of hot humid weather are less common 2