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Scratch ’n sniff

The number of humans in the developed world suffering from environmental allergies is increasing. The most common form of this condition is hay fever, which is caused by pollen grains in the air and gives rise to irritation of the eyes, airways and skin.

Other causes include house dust mites and furry animals; in fact, allergy to animals is the second most common cause after pollen. Lots of people are allergic to horses. The substances causing such allergies are usually proteins or fatty molecules; irrespective of its structure, if a substance causes allergy, it is called an allergen.

The World Health Organisation database of identified allergens lists five recognised allergens produced by horses. Two of them, Eq c1 and Eq c4, account for the vast majority of allergic reactions in horse-sensitive people. If you itch, sneeze and wheeze and your eyes run when you are around horses, the chances are you are allergic to one of these two allergens.

However, do all horses generate the same amount of allergen? Could it be that some breeds are less of a problem because they produce fewer allergens than others?

Medics in Sweden tested 170 different horses from 10 different breeds for the levels of Eq c4. They measured this protein in the scurf from the horses’ skin and hair, and the saliva and urine, hoping to identify breed differences.

Unfortunately, no single breed showed a consistently lower level of this allergen.

In any breed, however, some individual horses or ponies were genuinely less allergenic — that is, less irritant — to horse- allergic people than others.

Levels of Eq c4 were far higher in skin and hair dust than in saliva, although they were present in saliva at levels that would cause reaction. They were almost undetectable in urine. Most significantly, across all breeds, stallions produced more of this allergen than mares or geldings.

Mist opportunity

Horses can also suffer allergic reactions. Airway irritation giving rise to asthma-like clinical signs is quite common. Severe disease typically causing loud wheezing and heaving is a serious and chronic condition, usually of older horses, but even young racehorses and eventers can suffer poor performance linked to irritation of the lower airways.

In all forms of the disease, environmental allergens like dust and fungal spores may be significant factors. Although dust-free stable management is important, many cases need medical treatment and the use of a nebuliser to deliver a fine mist of medicine deep into the horse’s lungs.

One of the medicines used is dexamethasone, a powerful corticosteroid with excellent anti-in ammatory properties. Because it is potentially performance-enhancing, dexamethasone is a banned substance for many equine competitive sports.

A nebuliser producing inhaled vapour can deliver the medicine direct to the lungs, so the total dose is much lower than would be required by injection or by mouth, but vets must be sure that the drug has been completely excreted before the horse races or competes.

Vets in Australia tested six standardbreds for traces of dexamethasone in their plasma and urine after a single nebulised dose inhaled into the lungs, to see how long it lingered. In five of the horses, the drug was undetectable in plasma and urine after 48 hours. One individual retained the drug for at least 96 hours, however — far longer than expected.

This shows how individual horses can vary in key characteristics, and how important it is to give a wide margin of safety when recommending drug withdrawal times.

Ref: Horse & Hound magazine; 30 May 2019

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