Those of us who have worked as veterinary surgeons in very hot climates – e.g. California, China, etc, are only too aware of the potentially dangerous effects that the extremes of heat can have on a horse. Indeed as our own Irish climate continues to show tendencies to intermittent extremes in heat in recent times, horse owners need to be aware of the potential extra challenges and risks that extreme heat can present to a performance horse. Riders and trainers whose horses travel and perform regularly on the circuit abroad – in racing, show jumping, etc, have acquired considerable experience in coping with this complex area. What are the main problems that can affect a horse’s performance in extreme hot weather?
Like all mammals, horses need to keep their internal body temperature within a narrow temperature range. If they overheat, they risk permanent damage to vital organs and muscles, and may even suffer collapse, heart failure and death.High humidity and heat are a dangerous mix, as this combination impacts severely on the horse’s main cooling mechanism – sweating and evaporation.Horses’ cooling systems depend principally on the ability to sweat freely, the sweat evaporates, and this very effectively sheds excess heat.Horses have much greater muscle mass than other animals such as dogs and cats, particularly heavily muscled dressage horses, so heat dissipation takes longer and may be less efficient than in a small or lighter framed animal.Sweltering summer heat can present considerable health risks for horses – including dehydration, weakness, colic, poor exercise tolerance, or even heat exhaustion and heatstroke.Hot weather, particularly if it is humid, compromises a horse’s ability to shed heat from its body. The animal sweats but this is not always enough to control the heat build up.Young, old, sick or debilitated horses, black/brown, and overweight horses are less able to regulate their body temperature and are especially at risk in extreme heat. These types should be monitored for signs of extreme depression, weakness, drenching sweat or failure to sweat, and even panting. In the case of the exercising horse, with each stride, the muscles flex and strain from the effort—large amounts of heat accumulate from the metabolism of working muscles. Over half of the energy used for muscular activity and movement in a horse is converted to heat. To remove muscular heat, the horse sweats, pulling heat from the interior of its body outwards to the skin in a process known as evaporative cooling. Approximately 60%-70% of the heat of locomotion is normally dissipated from the body using this process. Left unchecked, continued heat build up stimulates a decline towards exhaustion.If increased metabolic and oxygen demands cannot be met, muscle fatigue sets in. While the effects here are much the same as in humans, prevention and treatment are different.When is heat stress or heat over load more likely to occur in horses?
The Heat Index (HI) is a term often used here. It is the sum of the ambient temperature (C) and the Relative Humidity (RH%) and is used as an indicator of the risk of heat stress for horses being worked under hot conditions.From a dressage perspective, dressage horses under certain conditions may need aggressive cooling, similar to the case of horses coming off the cross-country in the eventing competition. Where there is high humidity, it is essential to cool the horse directly after competing to stop the horse’s temperature from rising rapidly on cessation of its performance.Horses’ temperatures rise rapidly if they are asked to work at a high level.Dressage horses work extremely hard in both the warm-up and in their tests. The pivotal time is between 6-6.5 minutes of high level work, so dressage horses and event horses are vulnerable to rapid temperature rises after their strenuous work.However, this is not necessarily so for show jumpers who do not have to work for such a sustained length of time, so are not normally at risk of temperature hikes after competing. Where a horse is worked when the Heat Index is high, it may be advised that a team of four-five people should be working on each individual horse to cool them quickly and effectively – two on each side applying iced water and scraping simultaneously, and one checking the horse’s temperature, until it returns to 101 degrees F.What are the main conditions that can occur in horses performing in very hot humid weather?
Horses worked hard in hot and/or humid weather are susceptible to four serious conditions:Dehydration
Heat Cramps
Heat exhaustion<
Heat stroke
Dehydration:As horses have significant muscle mass, they can generate a tremendous amount of muscle heat and will sweat profusely when worked hard in warm temperatures. As a result, horses are more susceptible to dehydration than most other animals. They will lose more body fluids and the ionized minerals (electrolytes) that they contain, especially one of the main electrolytes sodium chloride or salt.What are the signs indicating dehydration?
The dehydrated horse has lost body fluids and the electrolytes normally found in the fluids. The horse will exhibit muscle fatigue, a lack of will to win, poor recovery from exercise and/or skin which when pinched is slow to return to normal.Skin that remains “tented” and refuses to return to its normal position after pinching, represents serious dehydration of 7-10% of body fluids. There are many levels in between: Mild dehydration of 2-3% might be accompanied by a dry mouth and dry mucous membranes. At about 5% dehydration, eye sockets appear sunken, skin elasticity is markedly reduced, and the horse is weak with a dull, listless attitude and posture. A check on the mucous membrane colour and capillary refill time of the gums provides a good impression of blood perfusion throughout the body. Normal gums should be a healthy pink colour. After pressing or pushing on the gums with a fingertip, this colour should return within two seconds. A normal pink colour with a normal capillary refill time indicates an adequate cardiovascular state, confirming pumping of blood throughout the body tissues. With poor circulatory perfusion, gums appear darker pink, implying stagnant blood due to lack of fluids and capillary refill time slows.The moistness of the gums and a skin pinch test are but crude assessments of hydration status. It is easy to be misled into thinking that all is well simply by measuring capillary refill time, gum colour, or skin pinching or tenting. It may not be possible to obtain a clear picture of mild clinical dehydration using only these physical inspections. Mild dehydration of as little as 2-3% is associated with a decrease in performance. The horse might not be in direct danger of metabolic collapse, but the animal’s ability to compensate for further dehydration, electrolyte losses or heat build up, becomes challenged as exercise continues. Under hot conditions, horses in heavy work can lose up to 11 litres of sweat per hour.