A Hot Topic - Heat Stress!

This article is from the February 2021 Horse Deals magazine.

What is heat stress and how does it occur?

In the hot and humid climatic conditions of Australia - heat stress is a serious risk to the horse and can occur in all performance athletes from the pony club horse to the elite athlete. It is, however, more common in racehorses, endurance and eventing horses.

Heat is produced by working muscles as a by-product (metabolic heat). This needs to be “lost” by the horse. The core temperature can increase by 1˚ Celsius a minute when being exercised. If exercise continues in an environment where heat loss is not efficient, the body temperature can increase up to 42˚C. This is heat stress and results in
serious consequences.

How do horses dissipate or lose heat?

To dissipate this metabolic heat there are several cooling mechanisms:

1. VASODILATION
This is widening of the blood vessels in the skin and allows heat to be lost from the blood into the air.

2. SWEATING
This removes approximately 65% of increased heat via evaporation of water from the body surface. Horses working in hot humid climates can produce up to 30 litres of sweat an hour.

3.VIA RESPIRATORY TRACT
25% of metabolic heat lost via respiratory tract on exhalation.

When there is an imbalance in the amount of metabolic heat generated and a reduction in the amount lost to the environment the heat stress occurs.

FUN FACT! An endurance horse in a 160km race produces enough heat to boil 770 litres of water!

What are the predisposing factors?

1. Climate
High ambient temperatures, low wind speed and high humidity result in the cooling mechanism being less effective due to a reduction in the temperature gradient between the horse and the surrounding air.

2. Lack of fitness/early in training programme

3. Poor conditioning/acclimatisation to local weather conditions

4. Travelling long distances

5. Lack of access to water/not drinking adequately

6. Excitable horses

7. Older horses

8. Overweight horses

9. Thick coat e.g. horses with Cushing’s Disease

10. Horses that sweat excessively or do not sweat at all (anhidrosis)

What are the signs of heat stress?

Mild signs may not be obvious so be aware of signs for early detection of a potential problem – these include:

1. Dilated superficial skin blood vessels

2. Loss of performance

3. Increased heart rates

4. Rapid shallow breathing (panting)
• Mouth breathing may be seen

5. Flared nostrils

6. The skin is hot to touch

7. High rectal temperature

8. Horse is agitated and distressed    

9. Impulsive kicking with hindlimbs or striking with front limbs

10. Staggering/uncontrollable gait

11. Agitated/distressed
• Collapse

Normal Vital Signs
Temperature: 37.7-38.5˚C
Respiratory Rate: 8-20 breaths per minute
Heart Rate: 30-44 beats per minute

Clinical effects of heat stress:

The effects of heat stress affect many of the body systems and can have very serious consequences. They include:
• Dehydration
• Electrolyte disturbances
• Muscle damage
• Kidney damage
• Colic – this occurs as a result of reduced blood flow to the gastrointestinal tract
• Endotoxaemia - bacteria may “leak” from the intestine into the bloodstream and causes endotoxemia and multi-organ failure
• Neurological issues – this occurs as a result of reduced blood flow to the brain causing swelling. This manifests as a change in behaviour such as depression and irritability.

What is the treatment of heat stress?

The horse should first of all be taken out of the hot conditions, so moved to a shaded area. A cooling breeze should be added if possible, such as a fan or an air-conditioned stable (if that luxury is available). The horse can be walked very lightly but not excessively and tight turns should be avoided.

Apply large amounts of cold water using a hose, watering can or sponge. Having “chiller” bins (large bins with cold water) ready at events or clinics can be very useful. Excess water should be scraped and then the process repeated.

Small amounts of clean water should be provided to rehydrate the horse – offer both plain water and water with electrolytes.

It may take an hour or so to recover. If the horse needs to travel, then sufficient recovery time should be considered. After an episode of heat stress then give the horse a break for 10–14 days with a gradual return to work.

Key Point
“Aggressive cooling is the most important factor in reducing heat stress and greatly reduces the risk of collapse and potential injury.” -Dr David Marlin and Dr Martha Misheff

When should I call the veterinarian?

Veterinary care should be sought if there is no response to initial management. If the following signs are seen then veterinary management should be sought immediately:
• Severe breathing difficulty/mouth breathing
• Distressed/agitated
• Rectal temperature above 40˚C

• Stops sweating
• Neurological signs
• Collapsed
• Signs of colic

Veterinary treatment may involve intravenous fluids and anti-inflammatory medications.

Prevention of heat stress

Ensure there is adequate water intake - an average 500kg horse drinks 25–35 litres a day which may double in warmer weather (50–70 litres a day). The use of electrolytes should be considered.

Ensure there is shade when turned out and avoid prolonged or intense periods of exercise in hot, humid conditions. Avoid exercise at the hottest time of the day and cool down after exercise.

After exercise - remove tack and wet the whole horse with copious quantities of cold water. If the horse has a thick coat then consider clipping.

Horses lose significant amounts of water when travelling so allow recovery from travel. Allow 24–48 hours to recover from a long journey before exercising strenuously.

Summary of heat stress

The muscles of the horse produce heat when worked and normally this heat is dissipated by dilation of skin blood vessels, sweating and via the respiratory tract. In hot and humid conditions these processes are less efficient and can result in heat stress.

Aggressive cooling (cold hosing) is the most important factor in reducing heat stress and greatly reduces the risk of collapse and potential injury.

Seek veterinary advice in severe cases.

ANHIDROSIS

What is Anhidrosis?
• A condition where horses sweat lightly or not at all
• Comes on gradually
• Hereditary component
• The sweat glands do not function properly so the cooling mechanism is reduced

Signs include:
• Inadequate or no sweat
• Puffing/panting during exercise
• Increased body temperature
• Unwillingness to work
• Dry coat

Diagnosis:
• Clinical signs
• Sweating test with injections of beta 2 agonists

Anhidrotic horses are a higher risk for heat stress and therefore require careful management.

Article by with Dr Paula Williams
BSc (Hons) BVSc MRCVS MANZCVS of WestVETS

Paula qualified in the UK in 1993 and has been an equine veterinarian since then. She completed an internship at Rossdale and Partners in Newmarket, UK and has subsequently worked in equine hospitals both in the UK and Australia. Paula is currently an equine practitioner at WestVETS Animal Hospitals in Queensland.

Her clinical interests include diagnostic imaging, the investigation and management of musculoskeletal issues in the equine athlete, the equine foot, neonatology and internal medicine.

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