Heat stroke

Horses are a homeothermic species : they maintain a constant body temperature ; independently from the temperature of the environment. When it is too hot, horses can lower their body temperature in two ways : sweating and dilation of blood vessels under the skin (called peripheral vasodilation). These two mechanisms increase elimination of heat towards the outside environment. When these mechanisms are not enough to cool the body temperature, for example during intense exercicse combined with high outside temperature, heat stroke occurs. The body temperature is normally  between 37.5°C and 38.5°C, increases and can exceed 41°C. This leads to dysfunctioning of organs (heart, kidneys, brain…) : due to peripheral vasodilation, blood accumulates in skin vessels and is no longer available in sufficient quantity to irrigate these vital organs properly. Heat stroke is a vital emergency and can be fatal to the horse.

Normally, evaporation of sweat from the body surface is sufficient to cool the horse down. If the air is too humid, natural evaporation is not as efficient. In one hour, a horse can sweat the up to 15 to 20 litres of water in cool and dry conditions, and 30 litres when it’s hot and humid. Only 25 to 30% of sweat evaporates, the remainder runs down the skin.

If the horse does not get enough water, excessive sweating leads to an important loss of water and mineral elements (horse sweat particularly contains a lot of salt), causing dehydration and an electrolyte imbalance. Even if a heat stroke is more often due to an intense and/or long excercise period, it can also occur in a resting horse, especially if he is in hot and  poorly ventilated environments (transport in a trailer, poorly ventilated loose box (be careful when in takedown  boxes where it can get very hot), tied up in the sun…)

• Horses with anhydrosis :  this isa disease that results in the inability of the sweat glands to produce an adequate amount of sweat.
• Overweight horses : subcutaneous fat reduces evacuation of heat produced by muscle contractions.
• Horses with very long and/or very thick hair (horses with hirsutism for example)
• Horses with little or no training subject to physical exercise or to stress : their muscles are poorly vascularized and will not cool down sufficiently.
• Horses transported from  a cool region to a hotter region in order to take part in a trek or an endurance event for example, and whose body doesn’t have time to adapt to heat before the effort.

Precursory clinical signs of heat stroke in horses are :

• A persistant increase in the breathing rate, which persists despite stopping exercise for 10 to 30 minutes, depending on effort – the normal breathing rate of a horse is between 20 to 40 movements per minute.
• An increase in rectal temperature (normally between 37.5°C and 38.5°C). when over 40°C, the horse is in danger.
• An abnormal attitude : horse seems tired and loses interest in its environment, eyes fixed and low head, reticence to move forwards.
• Dry or sticky gum mucous membranes
• Increase in capillary filling time : by pressing with your finger on gum, mucuous membrane becomes white. It normally turns pink again when the pressure is released within one second. If the delay is greater than 1 second it means that there is an increase in capillary filling time.e.
• Other symptoms may be observed: muscle aches, signs of colic, a decrease in digestive sounds.

If no treatment is implemented, the symptoms will get worse : heart rate increases, mucuous membranes are congested (red), the horse staggers, loses consciousness and collapses to the ground. It can then rapidly become fatal to the horse.

• Stop exercise immediately ;
• Bring the horse to the shade in a cool and ventilated zone (use of fans for example) ;
• Cool the horse by all means available, with a shower, bottle or damp cloth. Gently shower the horse with cool water (but not too cold) on the head, the neck, the chest and on the legs. Insist in particular on the back of the neck where the nervous centers responsible for thermal regulation are located. After hosing down always use a sweat scraper to remove excess water so that it does not heat up and reheat the body. By following this procedure, body heat can drop 2 degrees in 10 minutes.
• Give your horse non-iced water. A 500kg horse usually drinks 25 litres of water a day, without exercise. This quantity can triple in hot weather.
• Regularly monitor temperature in order to check that it drops fast enough ;
• If possible, massage the horse gently on all muscular body parts in order to circulate blood towards deeper organs.

If the horse does not recover quickly with these few treatments, the veterinarian must be called immediatly in order to put the horse on a drip. If the horse refuses to drink spontaneously, and after having carefully checked that intestinal transit has not stopped, the vet will be able to administer a rehydrating liquid through a naso-oesophageal tube.

Restoring of all vital functions can take time : depending on severity, the horse must be put to rest for a long period.

During intense heat periods :

• Check that the can have access to fresh water at all times ;
• Give water to the horse regularly during transport, between sports tests or during breaks while out trail-riding ;
• Provide a salt stone and in case of heavy sweating supplement with electrolytes ;
• Avoid intensive exercise and transport during the hottest hours of the day ;
• Make sure to provide a shaded and ventilated shelter ;
• Clip horses with long and thick hair ;
• Shower sweating horses ;
• Allow a period of adaptation to hot climate of 2 or 3 weeks before doing intense exercise ;
• Give refreshing food : mashes, carrots, apples… once or twice a week.