Scientific and Equine Consultant

Essential advice to help your horse cope with hot weather and how to recognise the signs of heat illness

Essential advice to help your horse cope with hot weather and how to recognise the signs of heat illness

Dr David Marlin


Hot versus Cold – Big versus Small

In general terms, when it’s warm it’s better to be small and when it’s cold it’s better to be large. It’s better to be a Polar bear in Winter than a small Mouse and in Summer it’s better to be a Mouse. The horse is a little different in that it can cope well in the Siberian Winter and the heat of the Desert. It’s large so that gives it the advantage in Winter as the ratio between skin surface area and weight is low (around 1 square metre for every 100kg compared with 1 square metre for every 40kg for a human) so that means heat is lost slowly. When it comes to hot weather the horse should be at a disadvantage because of its size (large animals lose heat slowly) but the horse has two adaptations that allow it to cope. Firstly, it can actually tolerate much higher body temperature than we can. After exercising a body (rectal) temperature of 41°C for a horse, whilst elevated, does not present much of a health risk, but for a human this would be a serious cause for concern. The other advantage the horse has is being able to sweat faster than any other animal. A square cm of horse skin can produce sweat around 3 times as fast as a square cm of human skin. The only risk in relying on sweating to keep cool is that it becomes less effective the higher the humidity. Sweat cools the skin down, and in turn the blood flowing through it, by evaporation. In hot dry air the sweat evaporates very quickly but as the humidity increases the speed at sweat evaporates becomes less and less. When the air is saturated with moisture (100% humidity) sweat does not evaporate. Fortunately in the UK we rarely if ever experience such conditions.


Sweating & Dehydration

One of the risks of being able to sweat to much is that horses are at risk of dehydration. Dehydration in turn can increase the risk of certain health problems, for example, colic and respiratory disease. If there is less water in the body, then food material in the GI tract becomes firmer and moves more slowly through the intestines. With dehydration the mucus in the airways of the lungs become thicker and moves more slowly leading to greater accumulation of allergens and even bacteria or viruses. This may lead to inflammation or infection. And with increased sweating there is increased loss of electrolytes; horse sweat contains 11g of electrolytes per litre and is much more concentrated than human sweat. Over a period of weeks and months this can lead to electrolyte depletion or imbalance (depending on what is being provided by the diet as horses cannot make electrolytes but must get them from food) and an increased risk of problems such as reduced performance, tying-up (exertional rhabdomyolysis) and “thumps” (synchronous diaphragmatic flutter); the latter is most common in endurance horses but does occur in racehorses and eventers.


Getting Acclimatised to Heat

Long periods of warm weather present less of a health risk to horses than sudden changes. For example, a sudden increase in temperature the week before Badminton has caused some problems for horses on the cross-country in past years. The reason for this is that the horses are simply not used to it or “acclimatised” to the heat. Horses, like people, can acclimatise to heat either by living in a warmer climate or living and exercising. The benefit of living in a warm climate is however perhaps only 10-20% of the benefit that comes from living AND training (exercising) in the heat. The process of heat acclimatisation (if a horse is suddenly taken from a cool climate to a warm one) takes place in around 2-3 weeks if exercise is carried out each day. One of the risks for horses to fail to cope with warm weather is where training is done in the Summer in the cooler parts of the day (i.e. early morning and evening) but that the horse is still competed at the weekend in the hottest part of the day. If you want to compete in the heat then you do need to train in the heat to at least maintain performance and at best reduce the risk of any heat related illness.


Problems with Hot Weather

We have already mentioned that hot weather carries a risk of horses becoming dehydrated. Horses are also likely to sweat more, and of all the electrolytes its sodium (from ordinary salt) that is likely to be limiting as forages and feeds are naturally low in sodium but high potassium. Providing a salt block is a good thing to do but controlled studies show that the majority of horses are not able to correctly balance their sodium needs from access to salt blocks. A better way is to add some table salt to the diet. A general guide would be ½ to 1 25ml scoop per day for horses not in work, 1-2 scoops per day for horses in medium work and 2-3 scoops per day for horses in hard work.

Water intake may increase significantly in hot weather so it’s important to supply at least two 15 litre buckets and check them at least twice a day. If it’s very hot during the day your horse may be better off stabled for all or the hottest part of the day and turned out morning and evening or overnight. Brick stables are likely to be cooler than wooden stables. In some cases it can be easier to reduce irritation from flies inside (e.g. spraying the wood around doors and windows with insect repellent).

Hot weather can lead to food going off quicker than normal. This is especially true for feeds with oil in. Heat causes oils to degrade (oxidise) more quickly. This can lead to horse refusing food.  Heat will also degrade the vitamin content of feeds and supplements. Another problem with hot weather is that it increases how much energy horses use even at rest to try and control its body temperature (thermoregulation) and so horses may lose some weight in hot weather.

A horses’ capacity for exercise may also be reduced in hot weather and they may tire earlier than expected when training or competing. Large horses (e.g. dressage horse, show-jumpers), heavier breeds and overweight horses are at greater risk of heat related problems in hot weather, especially if they are training or competing.  Hot weather and calm days are also often associated with a decrease in air quality and levels of pollutants may rise. This can present a challenge to horses with chronic respiratory disease, particularly RAO.

Horses with pink areas of skin, especially on the face, can be prone to sunburn so use a good factor 50 block and or a fly screen to reduce the risk of sunburn. Remember also that anything black absorbs heat and heats up more than anything white.


What to Look Out For In Hot Weather

Signs that your horse may be suffering from the heat include:

  • Lethargy
  • Panting (faster shallow breathing)
  • Nostril flaring
  • Increased rectal temperature
  • Decreased appetite and thirst
  • Dark urine
  • Reduced urination
  • Reduced performance
  • Dark mucous membranes
  • Muscle spasms
  • “Thumps” (synchronous diaphragmatic flutter)
  • Abnormal (irregular) heart rhythm
  • Slow recovery after exercise

This is often referred to as heat exhaustion but if not managed properly and quickly can progress to heat stroke. This may include ataxia (being unsteady on the feet) and or collapse.


Severe heat stroke/heat exhaustion can lead to renal failure, myopathy (muscle damage), laminitis, liver failure and can be fatal if not treated promptly. If you think your horse may be suffering heat related illness, move your horse into the shade and start to cool by pouring large amounts of water all over the body. If a hose is available then use that. If ice is available then use that to cool the water. Do not worry about scraping the water off, just apply more water. If your horse has developed heat exhaustion/heat stroke you may need to cool continuously for 10-15 minutes before you start to see an effect. You are unlikely o do any harm and your horse is at much greater risk from not being cooled. If shade is available nearby and the horse is steady on its feet then move into the shade whilst continuing to cool.


How to Help Your Horse in Hot Weather

Clipping you horse is an obvious step. Keeping your horse stabled in the hottest part of the day and turning out overnight may be an option if your stables are brick and well ventilated. If you don’t need to compete in the heat then riding early morning or late evening will reduce the risks of heat illness. If you are travelling horses then leaving very early or very late not only avoids the heat of the day but also the traffic. Whilst you are moving the ventilation will be better; the last thing you want is to be stuck in traffic on a hot day. When training or competing offer water immediately after exercising as this is the time when a horses thirst is strongest. Try to avoid ice cold water but don’t restrict intake. It does not cause colic in healthy horses. If you are competing, then leave water in the stable right up until the time you are going to tack-up. If you have warmed-up, then there is no harm in washing your horse down and allowing him a drink before you compete. Feeding electrolytes daily will help keep your horse hydrated and reduce the risk of tying-up, colic and respiratory disease. If you have to compete in the heat of the day then train at least 3-4 days a week in the heat. Remember that even if your horse is “acclimatised” to the heat then he will not be able to perform at the same level as in cooler weather. Horses with exposed pink areas, especially on the face can easily get sunburnt so use a UV mask and or sun-block.



Hot weather can present a challenge to horses, especially if they are competing, are old or overweight or have existing health problems. Sensible management in hot weather can help reduce the risk of heat related problems. Horses can acclimatise to heat but only if they are exercised in the heat. When acclimatised horses will be at les risk of heat related illness but exercise capacity will still be reduced compared with capacity for exercise in cooler weather. Learning to identify the signs of heat related illness and knowing when to call for veterinary help can save lives.

Date: June 30, 2015