Written by Dr. David Marlin
For the Southern States, Summer can be a challenging time for horse owners. 
High temperatures, strong sun and high humidity means horses have to use more energy just to maintain their normal body temperature at rest. When exercising their ability to control their body temperature is greatly reduced as evaporation of sweat, the horses’ main strategy for controlling its body temperature, is reduced. Sweating leads to loss of water and electrolytes and dehydration, which can in turn increase the risk of impaction colic, could lead to the worsening of respiratory diseases, and lead to an increased risk of tying up.
Another consequence of hot humid weather, especially for horses that are worked, is anhidrosis. Anhidrosis comes from the Greek word hidrosis, which means “sweating”. The prefix An means “not”. So anhidrosis literally translates as “not sweating”. Anhidrotic horses rarely lose the ability to sweat entirely, but continue to produce sweat on the neck, under the mane, under the tail and between the hind legs. Thoroughbreds and Warmbloods have been shown to be at an increased risk of anhidrosis and it appears there may be a hereditary component. Affected horses usually show a poor tolerance for exercise and reduced performance. The reason why anhidrosis develops is still not clear but prolonged exposure to heat and humidity, especially exercising in hot humid conditions, dehydration, electrolyte imbalance and heat stroke may all play a role in the exhaustion and dysfunction of the sweat glands in the skin. Once this happens it is effectively irreversible although some improvement does occur in Winter. There is no treatment for anhidrosis other than removing affected horses to a cooler and drier climate or keeping them in air-conditioned stables. A recent study in Florida found that around 1 in 50 horses was affected by anhidrosis. Horses used for showing or riding were much more likely to be affected than horses on ranch operations.
For many horses and owners the best solution is to keep horses inside during the hottest parts of the day, which means that they need to be fed hay. However, hot humid weather can also lead to an increased risk of spoilage from mold growth. Molds are always present in hay, even very good and very expensive hay. However, if the hay is relatively dry (has a low moisture content) and is kept in cool and dry or even warm and dry conditions, then the molds only continue to grow very slowly and the hygienic quality of the hay is maintained.
A problem arises when hay is stored in warm and humid conditions; conditions that the moulds love and when they will start to grow rapidly. Feeding hays contaminated with moderate to high levels of moulds presents four potential health problems to horses:
1. horses may refuse to eat mouldy hay as it does not taste nice;
2. if horses eat moldy hay, toxins within molds, known as mycotoxins, can lead to liver damage;
3. horses eating moldy hay are at an increased risk of colic; and, finally,
4. feeding moldy hay leads to inhalation of mold spores and horses are commonly allergic to mold spores, which represent a significant risk for causing or worsening equine asthma (RAO, Heaves).
What steps can you take to manage horses better in hot and humid weather?
⦁ Keeping horses in during the day and turning them out overnight
⦁ Riding during the early morning – air temperature is usually the lowest a dawn.
⦁ Always have plenty of clean fresh water available.
⦁ Horses become easily dehydrated as a result of sweating. Dehydration can increase the risk of impaction colic and worsen respiratory disease. Adding salt and electrolytes to the diet encourages drinking and helps replace lost electrolytes, reducing the risk of tying up.
⦁ Keep feeds and forages in air-conditioned feedrooms to prevent spoilage. (This is not an option for many owners.)
⦁ Steaming hay: 1. increases its water content and can help to reduce the risk of impaction colic. 2. prevents loss of electrolytes that can occur with soaking hay. 3. will kill mold spores reducing the risk of colic and respiratory problems
⦁ If hay or other feeds are contaminated with molds and there is no option but to feed them, then it is worth considering adding a mycotoxin binder such as Mycosorb A+ (Alltech) into the feed (more about mycotoxin ingestion below)
[Footnote: Signs of mycotoxin ingestion tend to occur over several months and can include reduced appetite, colic like symptoms; gastro-intestinal disturbance; liver damage; weight loss; poor coat condition; skin diseases; immune suppression; poor performance; and, general poor health. If you are concerned about anhidrosis or mycotoxin ingestion then speak to your veterinarian.]
