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You bring your horse in from the field to find that skin on his shoulders, flanks or underside has erupted in a mass of small, soft swellings. The weather has been hot — could it be heat rash?

“This problem has very little to do with heat,” says Kieran O’Brien MRCVS of Penbode Equine Vets, Devon. “It’s a totally different condition to the heat rash or prickly heat that we sometimes suffer as humans, and is better termed urticaria.”

Kieran explains that urticaria describes an immediate reaction to an “antigen”, a toxin or a substance the body considers foreign. This may have been inhaled, ingested or applied to the skin. While not directly associated with heat, urticaria does tend to be seasonal — with cases most commonly occurring in late spring or summer.

“The lumps are typically the size of a £1 coin, but may be smaller, larger or even joined together,” says Kieran, adding that these hives or wheals may ooze a clear fluid called serum.

“Occasionally they may be itchy, but the horse is not usually bothered. They tend to be more alarming for the owner.”

Spot the difference

“There is a form of heat-induced urticaria, but this is rare,” says Kieran. “The most likely cause, if the horse has just come in from the field, is a horse fly attack. If this happens frequently, it’s time for a serious review of your horse’s fly protection measures.

“But there are other possible triggers,” he adds. “The most obvious is something applied to the skin, such as anti-fly gel. Another classic cause is medication, including bute, penicillin, certain wormers or other medications, such as those used for colic.

“These lumps are sometimes called protein bumps, a reaction not necessarily to excess protein but to individual proteins in the diet. There is very little good evidence to implicate dietary issues, however, and lots of assumptions are made. Possible triggers include alfalfa and garlic, but it’s a myth that sugars in the diet cause these reactions.

“Contact with certain substances can trigger urticaria,” adds Kieran. “Lying in nettles can cause clusters of swellings on the abdomen and the sides of the hindlegs, while paper bedding is another possibility. In these cases, the low-down location of the rash may offer a clue as to its cause.”

Bacteria, viruses and airborne agents such as fungal spores, pollens and chemicals can also result in an outbreak of raised skin patches.

Soothing solutions

The majority of cases should self-cure, clearing up overnight.

“If the horse is in distress, however, especially with nettle rash, then seek veterinary advice,” says Kieran. “A horse with lumps all over his body will probably need treatment in the form of a short-acting corticosteroid. While the short-acting nature of this means that the risk of laminitis is very low, it is never zero.”

Kieran advises against giving your horse antihistamines without veterinary supervision.

“Antihistamines aren’t always effective and need to be given in quite large doses according to the severity and location of the outbreak and the horse’s response to them,” he explains.

“You can apply cooling lotions, such as calamine, but it’s best not to ride your horse if the lumps are in any area that comes in contact with his tack. If a significant part of his body is affected, he should be given time off.”

With luck, the swellings should disappear as quickly as they arrived — and stay away.

“If they fail to resolve within a few days, or keep reappearing, further investigation will be necessary,” adds Kieran. “This entails taking a good look at your horse’s diet and environment, before progressing to allergy skin testing if triggers cannot be identified.”