Skin allergies are in the spotlight as Peter Green MRCVS rounds up the latest research news
Oil change
Sweet itch – now termed insect bite hypersensitivity (IBH) – is a long-term, seasonal skin disease caused by an allergy to the saliva of biting midges and other flies. Severe irritation can make affected horses and ponies scratch and rub themselves to the point of bleeding.
Keeping the midges away with full body rugs is one answer, but these may be problematic in hot climates. Steroid injections, tablets and creams can relieve the signs, but have potentially serious side effects. Other solutions include using industrial fans to blow midges away, grazing affected horses on breezy hillsides to reduce midge biting and using chemical repellents.
We have known for some time that local antibodies in the horse’s skin cause mast cells to break down, which in turn release histamine and other inflammatory chemicals. These cause severe itching and pathological changes in the skin.
In some areas of Australia, IBH can affect 60% of all horses. The University of Queensland dermatology team was interested in the claim that some herbal oils stabilise mast cells and that others repel insects. They investigated the effect of a spray containing plant oils of lemon grass, camphor, may chang, patchouli and peppermint.
The team recruited 27 severely affected horses and ponies, giving the owners either the plant oil spray (or roll-on) or a placebo that looked and smelled the same. The patients were liberally sprayed on the affected areas once daily, for 28 days, then received no treatment for a month. Each was then treated for a further month with the alternative product.
When the placebo was used, there was either no change or a worsening of the irritation. With application of the plant oil lotion, however, clinical signs improved considerably in 19 of the affected horses – with 17 of them recovering completely.
Skin biopsies taken from four horses before, during and after treatment showed improvements in each at a microscopic level. There were no side-effects.
There was no doubt that the plant oils were very effective. Was it just that they repelled the midges or did they actually stabilise the mast cells in the skin and reduce inflammation? Whatever the case, this research gives hope to animals who suffer the summer misery of IBH.
Testing trouble
Some horses suffer chronic or recurring urticaria, an allergic reaction of the skin to a variety of different substances such as pollens, fungal spores, house dust and storage mites. This troublesome condition is termed atopic dermatitis (AD).
To test for substances that an individual AD patient may be reacting to, a vet will use skin sensitivity tests and blood tests. In the skin test, a variety of allergens is injected as a grid pattern on the side of the horse’s neck, to see which ones produce the biggest swelling. Blood tests can detect circulating antibodies to the same allergens.
Vets at Utrecht University investigated whether skin tests and blood tests gave the same results in AD patients. The correlation was very poor. Even more worrying was the finding that if the skin test was performed on both sides of the neck at the same time, the results were different. Very few allergens gave consistent results in double skin tests and blood tests. The Dutch vets emphasise that these tests must be interpreted with great caution.
Further reading
Sweet itch – Australian Veterinary Journal 98. p411-416; allergy testing – Veterinary Dermatology DOI: 10.1111/vde.12871
Ref Horse & Hound; 10 September 2020
