Sweet itch hope

Sweet itch hope The possibility of an effective vaccine against sweet itch moved a step nearer recently as researchers succeeded in isolating potential allergens in midge saliva. Work at the University of Bristol has isolated various proteins in midge saliva that could cause Sweet Itch. It is hoped that one or more of these proteins could be used in an immunotherapy programme to reduce the allergic reaction in sweet itch sufferers. Up to 5% of horse and ponies in Britain suffer from the condition. Although it is often thought of as a disease of ponies, horses of any size can be affected.

Up to 5% of horse and ponies in Britain suffer from the condition. Although it is often thought of as a disease of ponies, horses of any size can be affected. Sweet itch causes extreme discomfort with horses experiencing a range of symptoms including severe itching, hair loss, rashes and weeping sores.

Sweet itch is an allergic disease in which the horse or pony over-reacts to bites from certain species of midge. It is not that the midges only bite certain horses. They probably bite all horses, but only affected horses react. Normally, when exposed to foreign proteins, an animal produces antibodies that help to inactivate the foreign material. Horses affected with sweet itch have an abnormal immune response.

Currently there is no effective treatment. Various methods are used to try and prevent the condition, but overall the response is disappointing. 

Now research funded by The Horse Trust offers a potential ray of hope for owners of horses with Sweet Itch.

The research, which is being carried out at the University of Bristol School of Veterinary Science, has isolated over 20 potential allergens in the saliva of midges and has identified many of the genes that make these proteins.

As there are numerous species of midge that bite horses, the researchers have concentrated on identifying the main salivary proteins in three different groups of midges including representatives of the types that are known to bite horses in the UK. Proteins have been identified from three groups of midges - Culicoides nubeculosus, C. pulicaris and C. obsoletus. The latter two groups are wild caught midges that commonly bite UK horses.

The work is now starting to manufacture these proteins using recombinant DNA techniques: the midge DNA is inserted into an insect virus (known as a bacculovirus), insect cells are infected with this modified virus and then produce the protein coded by the midge DNA.

The researchers said "We are now halfway there - 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".

To see the Horse Trust/ British Horse Society information sheet on sweet itch go to:

http://www.horsetrust.org.uk/pdfs/horsehealth/sweetitch.pdf

Reproduced with kind permission of Mark Andrews BVM&S CertEP MRCVS
© Copyright Mark Andrews  - Equine Science Update 2009

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