GUELPH – Dripping with sweat and breathing hard through flared nostrils, an agitated horse shakes her head. The horse stumbles, ears pinned back, and kicks out.
During this summer of high temperatures it’s important to recognize and respond to symptoms of heat stress.
If left untreated severe heat stress can be fatal, said University of Guelph Ontario Veterinary College (OVC) assistant professor Amy Lack.
She completed her veterinary training in Missouri, and then completed a residency in equine internal medicine in the southern U.S., so she’s no stranger to handling horses’ health in hot weather.
Severe heat stress is relatively uncommon, Lack said, because most owners are quick to notice and respond to problems.
Recognizing heat stress
“Just like you and I, horses experience heat stress,” Lack said. “The difference is, when we start feeling the impact of the heat, we take a break.”
Because horses work in partnership and enjoy exercise, they often push past their capacity.
So it’s up to people to recognize signs of heat stress and dehydration.
In early stages heat stress can cause depression, lethargy, heavy breathing and dilated nostrils, Lack said.
As heat stress progresses it impacts how cells communicate and proteins function, sometimes leading to brain dysfunction, she added.
An overheating horse may become unsteady or irritated. Horses that are usually easy-going may rear or strike out, becoming dangerous to handle.
A horse experiencing severe heat stress may become “completely unaware of what’s going on around them,” Lack said.
They may collapse, or throw themselves on the ground, and without intervention they “could have a seizure, become comatose, or potentially die.”
Wet-bulb globe temperature
To avoid potential heat stress, Lack recommends being mindful of the wet-bulb globe temperature (WBGT) – a measurement including air temperature, humidity, wind and solar radiation.
Lack said the heat index, measuring air temperature and humidity, is a good representation of conditions in the shade, but WBGT is more accurate for sunny conditions.
If the WBGT exceeds 33, Lack recommends not exercising horses at all.
When the WBGT is between 30 and 33, moderate exercise is safe, but Lack advises taking precautions:
– exercise horses early in the morning when it’s coolest;
– stick to grassy surfaces instead of sandy arenas as grass is cooler;
– take frequent water breaks in the shade; and
– look out for signs of heat stress and dehydration.
Some horses have higher heat tolerance than others; a well conditioned horse will have an easier time exercising in hot weather than one used to spending most of their time grazing in the field.
And a horse unaccustomed to hot weather will overheat more easily.
When Ontario weather warms quickly in March, Lack said it takes horses at least five days to acclimate to the new temperature.
Responding to heat stress
When something doesn’t seem right while exercising a horse, the first step is to stop.
“If you have any concern a horse is not okay, it’s best to touch base with a veterinarian,” Lack said.
Even if the vet doesn’t need to see and treat the horse, they can offer guidance and keep the horse on their radar in case things take a turn for the worse, she added.
“If you suspect a horse is experiencing heat stress, move them to a cool place and start hosing them down with cool water,” Lack said.
Spray the underside of their neck, where the jugular veins are, as this will cool their blood most rapidly.
The horse’s body temperature will quickly warm the water, which should be scraped off with a sweat scraper.
Lack recommends spraying and scraping the horse until they seem less distressed and their skin feels cool.
Hydration
Sweating is the primary way horses control their body temperature.
“Horses are phenomenal sweaters,” Lack said. “Probably the best sweaters in the mammalian world” – they can produce over 10 litres of sweat in less than an hour.
Their impressive ability to sweat is part of what makes them such great athletes, Lack said.
It also makes hydration essential.
Dehydrated horses use water reserves from their gastrointestinal system to make sweat.
So a dehydrated horse will still sweat, but they’ll show symptoms of heat stress sooner than a hydrated horse.
Sweating leads to significant loss of electrolytes like chloride and potassium, Lack said, and losing enough electrolytes can cause a range of issues including:
– muscle cramps;
– kidney injuries;
– neurological signs;
– colic symptoms; and
– colon impaction.
One way to recognize dehydration in horses is by lightly pinching skin on their neck and watching how quickly it goes back.
It helps to practice this when the horse is well hydrated to recognize their baseline, Lack noted, as different horses’ skin has different elasticity.
If the skin stays in the shape of the pinch for longer than usual, that’s a sign the horse is dehydrated.
Other signs include red or dry gums and/or dark, syrupy urine.
If a dehydrated horse seems normal otherwise, Lack recommends just encouraging them to drink.
If they are not acting themselves, call a vet, as they may need intravenous fluids, she added.
‘You can lead a horse to water but you can’t make it drink’
For some horses, hydration is as simple as regularly offering them water, including during trips like trail rides and trailer rides.
Other horses are reluctant drinkers, but there are creative ways to keep them hydrated.
It can help to include more moisture in their food, by making mashes by mixing water with pelleted food or hay cubes, Lack said.
It can also help to sweeten water with molasses or an apple flavoured electrolyte solution.
Horses are more likely to try something new if it’s first offered as part of their usual routine.
Some horses are more willing to try a lower concentration of electrolytes in the water to start, Lack said, but it is important to never exceed the concentration on the label.
Compounding factors
Air pollution increases horses’ risk of heat stress, Lack said.
While 70 per cent of their heat loss occurs through sweating, the other 30% is through their respiratory tract.
“Inhaling particles they don’t usually will impact their ability to cool themselves,” Lack said, which increases their risk of overheating.
Other compounding factors include underlying breathing conditions, muscle problems, or issues with sweating like anhidrosis, a condition in which sweat glands make little or no sweat.
If a horse is exercising in hot weather and not breaking a sweat, “that’s an emergency,” Lack said. “Stop. Call your vet. Something is going wrong.”
More information about dehydration in horses is available at www.equineguelph.ca/pdf/infosheets/Dehydration_Infosheet_July_2014.pdf.
