For many of us, the arrival of spring means that horses can be turned out to graze in mud-free fields and the winter grind of mucking out can be suspended for another year.

But not every owner will share this optimism. For some, the prospect of improved warmth and sunshine also heralds the return of sweet itch and the attendant rigmarole of rugs, stabling and application of insect repellents.

Sweet itch, or insect bite hypersensitivity (IBH), is an allergic reaction of horses and donkeys to the bites of midges: a large family of small insects found in almost every part of the globe. At present, the only effective means of combating this condition is to prevent the horse from being bitten. While this can be successful in reducing the signs of disease, it requires continuing vigilance and is not therefore a cure.

Out for blood

Researchers in several European vet schools have been working for more than a decade to develop a truly effective sweet itch treatment. Before such a treatment can be devised, however, we need to understand how the disease is caused.

To begin with we need to understand why midges bite — and what it is about midge bites that makes them so irritating.

In fact, only female midges bite. They do so in order to feed on blood, which provides many of the nutrients that female midges need to develop and lay their eggs.

But blood is a well-defended resource kept in blood vessels, protected by tough skin. When exposed to the outside environment it forms a solid clot to prevent continued bleeding.

To overcome this, feeding midges chew through the outer skin and produce saliva containing enzymes that break down this barrier. Midge saliva also carries components that expand skin blood vessels causing blood to flow to the surface, as well as several components to prevent the blood clotting.

Rather like an infection or vaccine, the “injection” of midge saliva stimulates a horse’s immune system. While immune responses to vaccines are limited or fade when the infection is cured, however, midges just keep coming and the horse’s response becomes greater.

Building tolerance

Fortunately the immune system has a built-in safety mechanism called tolerance, which usually prevents such hypersensitivity from developing.

For example, tolerance is a normal part of the immune response to parasitic worms. While the initial immune reaction limits the severity of the worm infection, some adult parasites are tolerated as the damage caused by an ongoing immune reaction could be greater than the harm caused by a small number of worms.

Immune tolerance is also the typical reaction to foods, where the “foreign” material the horse eats is not associated with any physical damage.

In sweet itch, the damage of biting combined with the midges’ saliva proteins have misguided the horse’s immune system into continually reacting as if to a threat. So the goal of treatment is to re-educate the immune system of allergic horses, enabling them to become tolerant to midge saliva.

One potential way to do this is to put a few drops of a solution containing midge saliva proteins under the horse’s tongue every day until his immune system reclassifies them as food.

Over the past decade researchers have used modern gene analysis to identify the proteins in midge saliva. Using gene engineering, scientists can now make these proteins artificially in bacterial cultures.

Further research in the coming decade will aim to try out this “oral immunotherapy” on a small sample of horses, to see whether a cure for sweet itch can be developed in the coming years.

Ref: Horse & Hound; 21 April 2016