Sweet Itch in Horses

By Deidre M. Carson, BVSc, MRCVS & Sidney W. Ricketts, LVO, BSc, BVSc, DESM, DipECEIM, FRCPath, FRCVS.

What is sweet itch?

sweet_itch-1Sweet Itch is a skin disease (sometimes called summer itch or seasonal allergic dermatitis) and is caused by an allergy to the saliva of certain biting flies or midges (e.g., Culicoides spp.).  It is characterized by intense itching and affected animals may spend hours rubbing against trees, fences, doors or anything else available. This results in damage to the skin and hair especially down the upper neck, back and tail base (dock).  In severe cases the entire mane and forelock may be rubbed out and the skin over the ears, neck, withers and tail becomes thickened, dry, rough and hairless. The condition occurs during the warmer months as this is when the flies breed and are active.

How and when does it occur?

Not all horses that are bitten by midges develop sweet itch and a genetic predisposition is suspected. Affected horses have a true hypersensitivity to the saliva of these biting insects. Signs can occur from a very young age and usually become more severe each year.

Signs are seen during the warmer months when the flies are active. These tend to feed in the early morning and late afternoon and can often be seen on the horse's belly and neck.

Control and treatment
It is important to remember that as this is an allergic condition, symptoms can be triggered by the bites of only a very small number of midges. Control, therefore, depends on reducing, if not removing attack by these insects. Affected horses should be stabled between 4 pm and 7 am. The stalls can be made insect-proof by using screens and insect repellent strips. Hoods and rugs reduce the body area exposed to biting but these may be badly damaged by rubbing until the condition is under control.

sweet_itch-2Fly repellent or insecticidal sprays or creams should be used especially on the mane, tail, withers and head but also along the midline and neck. Insect repellent tags such as those used on cattle can also be attached to the horse's head collar.

These measures are not effective in all horses. The environment can be important. The immature stages of these flies live in water so, if possible, affected horses should be moved away from areas where there is mud, wet sand, ponds, streams or rivers nearby.

Topically applied creams and lotions containing antihistamines or corticosteroids can be useful but become very expensive for continual use. Corticosteroids can be given by injection and long acting forms can give relief for several weeks in some cases. It is important to realize that corticosteroid treatment does not cure the condition but merely relieves the itching and that the medication can have serious side effects such as inducing laminitis and increased risk of infection. Nevertheless, in some cases, there is no alternative but to take these risks in order to relieve severe persistent distress and discomfort.

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