Extreme bouts of hot weather are particularly stressful for horses and for some, even more stressful than extreme bouts of cold weather.
Thus it becomes particularly important to identify the individuals at a greater risk for heat stress and what factors can provide extra care for them when necessary.
Those particularly prone to heat stress include young foals; overweight, ‘out of shape,’ and older ones; horses with metabolic disturbances such as insulin resistance, equine metabolic syndrome and PPID (also known as Cushings disease); and those in paddocks with black dirt surfaces and little or no shade. Vigilance is also needed when trailering or transporting, and avoiding the heat of the day altogether if possible.
The earliest warning signs are generally behavioural. The animal will appear dull, listless, lethargic, droopy eared, lose impulsion, lack appetite and thus with time may begin to lose weight.
As the heat stress progresses into heat exhaustion, the core body temperature begins to rise and physiological symptoms of distress begin to appear. A horse’s normal body temperature is close to 101.5 F (38.6 C). The rectal temperature of the exercising horse can rise to around 103 F (39.4 C).
Body temperatures greater than 105 F (40.5 C) become worrisome and body temperatures as high as 106 to 107 F (41.1 to 41.6 C) are indicative of heat stroke and may require emergency veterinary intervention and fluid therapy to attend to the seriousness of the condition.
Clinical symptoms of advancing heat exhaustion may include: prolonged skin tenting and dull eyes indicative of dehydration; depressed posture with head down; rapid breathing (even panting); a quickened pulse; and a paradoxical cessation of sweating or a thickening of the sweat despite the skin remaining hot to the touch.
A skin pinch test along the mid-neck can be used to check the hydration status on a horse during spells of hot weather. Skin tenting which remains longer than several seconds is an indication of dehydration. The clinical signs of heat stroke vary depending upon the nature and extent of brain damage, but include a lack of co-ordination, disorientated behaviour, coma and death.
An environmental predictor for assessing the risk of heat stress can be factored by adding the ambient temperature in degrees Fahrenheit and the percentage of relative humidity. When this value is greater than 150, the risk of heat stress is high as the ability of the horse to lose body heat to the ambient temperature, especially while being exercised, is greatly reduced.
Strenuous exercise on a hot and humid day can become problematic in a short period of time for even the best-conditioned horse.
Horses physically overextended in these environments may develop a condition called ‘thumps’ or synchronous diaphragmatic flutter or exertional rhabdomyolysis also known as ‘tying up’ as a result of severe electrolyte disturbances.
On a hot day a 450-kilogram horse may drink upwards of 10 gallons of water, maybe more. Water buckets, tanks and waterers will need particular attention in hot weather to prevent algae and bacterial growth and ensure plenty of clean water is available.
Pastured horses with access to stagnant water sources such as ponds, dugouts or sloughs may be susceptible to contracting Potomac horse fever especially in August and September. So these water sources are best monitored every day to detect problems such as fever, dehydration, and/or diarrhea. Early detection and quick intervention are key to a successful outcome should problems arise on a hot day.
Reliable sources of free-flowing salt and minerals are eagerly sought after by horses whenever they are subjected to heat stress. Ensuring their availability gives the horse/s the resources they need to balance their electrolytes.
Natural areas with shade trees are an ideal situation and buildings can also bring relief, depending on their design and orientation. Whenever a breeze or wind presents itself, the horses will typically position themselves to take advantage of such airflow. It may be necessary to provide stabled horses with turnout times during the cooler early-morning hours, late at night or overnight and place fans strategically within the stable to offset poor ventilation. Misting fans are even more effective at cooling. Blankets or sheets during hours of strong sun retain heat and quickly become uncomfortable.
If a horse is struggling from heat stress, immediate measures are moving the horse into a shaded area, preferably one with fans or airflow. Encourage the horse to drink water. Splashing, sponging or spraying cool and/or cold water onto the horse is an effective and safe way to lower the body’s temperature by aiding evaporative cooling. Remove the water with a sweat scraper and repeat continuously until the horse’s skin feels cool. If necessary, the major blood vessels of the neck and inner thighs can be iced.
Keeping horses comfortable during periods of hot weather include identifying high-risk individuals, assuring free access to clean water, adequate salt and mineralization and utilizing husbandry means to attenuate the heat stress as much as possible.