February newsletter: Heat stress and electrolyte balance in horses



With the unusually high temperatures and drought conditions that we are experiencing this summer, it is imperative that the approach to our horses’ increased water and electrolyte needs is appropriate. Heat stress can dramatically affect the horse’s health and performance if not managed correctly.

During exercise, there is a significant increase in the amount of heat produced by working muscles. Muscles cannot transform energy into movement with 100% efficiency. As a result, some of the energy is lost in the form of heat. The rate of heat production by working muscles is proportional to how hard the muscles work. Therefore the faster a horse goes the more heat it produces. However, long periods of low intensity work (e.g. long trail rides) and transportation in hot weather also increase the rate of heat production significantly. An efficient cooling mechanism

The single most important means the horse has for getting rid of the enormous heat load generated during exercise is via evaporation. Sweat is evaporated off the skin surface and cools the horse, as it increases its sweating rate and moves more blood to the capillaries under the skin. This accounts for approximately 65% of the heat dissipation. The horse also increases its rate of breathing to aid in the effort to release this build-up of heat, and the lungs account for approximately another 25% of heat dissipation. The capacity of the respiratory tract to dissipate heat from the body becomes very important under conditions of high humidity and high temperature when evaporative conditions are not favourable. High humidity makes evaporative cooling less efficient. Horses sweat at much higher rates than any other animal and although this provides for a very efficient cooling mechanism it has its shortfalls. Equine sweat is hypertonic, meaning that it is more salty than body fluids. This causes the horse to lose higher quantities of electrolytes through sweat than we do. The horse also loses copious amounts of body water compared to what we lose, leaving the horse in danger of suffering from both dehydration and electrolyte imbalance/loss.

Signs of electrolyte deficiency or imbalance can include poor performance, slow recovery after exercise, muscle problems (such as tying-up), reduced sweating, and increased risk of fracture and “thumps” or synchronous diaphragmatic flutter (SDF). Thumps is most common in endurance horses but can occur in any horse, especially large horses and horses working hard in warmer weather e.g. racehorses, dressage horses, showjumpers and eventers. The importance of electrolytes Electrolytes regulate the transfer of water through cell membranes into or out of the cells. They are responsible for getting nerves to fire and muscles to contract, as well as pumping of the heart, movement of food and water through the gut and the filtering of waste products through the kidney and liver. Essentially all of the physiological actions in the body require electrolytes but most importantly, they need to be present in the fluids in the appropriate amounts for these biochemical reactions to proceed in the right manner. The fine balance between water and electrolyte concentration is essential for appropriate muscle contraction and also in cooling the horse.

Each electrolyte has its own particular function:

•Sodium helps balance the body water levels and maintain blood pressure.

•Chloride is required to maintain the balance of acids and bases (alkalis).

•Potassium helps balance the fluid inside the cells and is vital for optimum muscle, heart and kidney function. Potassium loss decreases muscle strength, tone and ability to contract.

•Calcium is essential for maintaining normal, controlled skeletal and heart muscle contractions and excessive losses can result in a condition called ‘heaves’. Calcium also builds bones and teeth and contributes to normal function of heart, nerves, muscle and blood clotting.

•Magnesium is important for normal muscle function, bone formation and reducing nervous behaviour.

An electrolyte imbalance/loss can occur when horses sweat at more than 10 litres/hour or during extended periods of profuse sweating. Replacing lost electrolytes post exercise is vital for optimum health and performance, delayed onset to fatigue and quicker post exercise recovery. It is important however, that the electrolyte replacement product used contains electrolytes in similar proportions to those lost in sweat and that adequate amounts are fed so as to match amounts lost in sweat. Rate of supplementation will vary with the degree of fitness of the horse, work levels required and environmental conditions. Free access to water should always be provided. When administering electrolytes to replace the losses that occur with sweating it is vital that they be administered with water.

Administering large single doses of electrolytes in the form of an oral paste for example, can cause the gut to absorb water from the surrounding blood vessels to dilute the concentration within the gut, further worsening the dehydration within the muscle cells, for the short term at least. Likewise, trying to rehydrate a horse with water alone will also delay the hydration process. The body monitors and controls fluid volume by responding to the sodium concentration of body fluids. Giving water to a horse that has just undergone a lot of sweating will dilute the sodium concentrations further, signal that the horse is not thirsty and cause the horse to stop drinking. If a horse drinks water alone, this cannot be held in the body and the kidneys will try and remove as much of the extra water as possible. If water is taken in with electrolytes, either in feed, given by syringe or dissolved in the water, the water can then be held in the body and the horse will rehydrate. Although horses will drink water with electrolytes in, it is strongly advised to give electrolytes in feed as it is difficult to meet requirements by giving electrolytes in water, and if the horse is not used to the taste it may not drink at all. Balance of water and electrolytes is crucial for effective rehydration and prevention of dehydration.

A simple pinch test can basically determine whether a horse is dehydrated as a result of heat stress. When you pinch the horse’s skin on the neck, it should resume its original position immediately. If the skin takes a while to resume to its normal position it could be assumed that the horse is somewhat dehydrated. When choosing an electrolyte supplement, look for the sodium or sodium chloride (salt) content. The higher the sodium chloride content, generally the better the product. This is usually expressed either as g per dose or %. For an electrolyte powder, the sugar (sometimes shown as dextrose or glucose), should be low (less than 5% or 5g per 100g or 50g per kg), otherwise you will be feeding a lot of sugar and not enough electrolyte. For electrolyte syringes, a good syringe should be delivering at least 20-30g of electrolyte per dose.

Points to bear in mind

• Supplementing only around the time of competition or changing how you supplement around the time of competition would probably be considered undesirable for two main reasons.

1. Firstly, if your horse is not used to the taste of salt in his feed, then supplementing before competition could put your horse off eating.

2. Secondly, you are highly unlikely to have much impact on whole body electrolyte status by starting to feed electrolytes around the time of or during competition, instead of over a sustained period. There is also the risk that a sudden increase or change in electrolyte supplementation around the time of a competition could cause disturbance to the hindgut.

• What an individual horse needs in terms of electrolytes will be determined by a combination of diet, work, breed, fitness, weather and also factors peculiar to your individual horse’s metabolism. It is very unusual for horses to be fed too much electrolyte, provided that you stick to manufacturers’ recommendations. Signs that you are feeding too much electrolyte could include feed refusal, excessive drinking (more than 4 buckets per day), a very wet bed and/or loose droppings.

Common sense and good stable management, especially with regard to fresh, clean water, shelter and exercise time should keep your horses happy and healthy in the warmer weather but if you are concerned that your horse may have possible heat stroke or dehydration, then it is important to seek veterinary advice immediately.

References: Dr David Marlin, Suffolk UK Tania Cubitt PhD, Australia As always the veterinary team at Fourways Equine Clinic is available at any time to assist you and your horses.

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