Sweet itch: Researchers closer to beating common summer ailment

May 29, 2009

Researchers have succeeded in isolating potential allergens in midge saliva that they believe trigger sweet itch - the bane of thousands of horses worldwide.

Sweet itch, also known as Queensland Itch, Summer Itch, Summer Eczema and Seasonal Dermatitis, is usually caused by an allergic reaction in horses to midge bites.

By isolating potential allergens in midge saliva, researchers open up the possibility of using immunotherapy to treat the disorder, more properly known as Summer Seasonal Recurrent Dermatitis.

The research was undertaken at the University of Bristol and was funded by The Horse Trust in Britain.

Sweet Itch causes horses extreme discomfort, resulting a range of symptoms including severe itching, hair loss, rashes and weeping sores.

Prolonged rubbing and biting by affected horses can result in bleeding open sores. Secondary bacterial infections can occur. Horses can even rub off their mane and upper tail hair.

There is no cure. The only option for horse owners is to ease the symptoms through use of specialised blankets, insect repellents and stabling their horse.

Researchers at the university's School of Veterinary Science has isolated more than 20 potential allergens in the saliva of midges and identified many of the genes that make these proteins.

As there are many species of midge that bite horses, they concentrated on identifying the main salivary proteins in three different groups of midges - Culicoides nubeculosus, C. pulicaris and C. obsoletus.

Work is now starting to manufacture these proteins using recombinant DNA techniques. This involves inserting the midge DNA into an insect virus (a bacculovirus). Insect cells are infected with this modified virus and then produce the protein coded by the midge DNA.

"We are now halfway there," the researchers report. "We know what proteins are in midge saliva and how to manufacture them in sufficient quantities. The next step is finding out which of these proteins allergic horses respond to and then by giving them regular doses of this protein to reduce their immune reaction.

"We hope this can be achieved by 'feeding' the midge proteins to the horse so that its immune system responds to them as it would to a normal food and turns off the allergic response."

The Horse Trust is keen to fund the next stage of this research, but a fall in income from individual donors during the credit crunch has forced it to suspend its grant programme, although it is continuing to fund research and educational grants made in earlier years using reserves.

Other research has indicated a genetic link in Sweet Itch, with the problem being particularly common in Icelandic horses.