Sweet Itch

Equine insect hypersensitivity, also known as sweet itch, summer itch or Queensland itch, is askin disease caused by hypersensitivity to the saliva of certain biting midges (e.g. Culicoides), and is the most common cause of summer pruritus. There is a strong seasonal relationship with the occurrence of Culicoides midges. The condition occurs only from late spring to late autumn and usually resolves in colder weather. This allergy takes time to develop; horses are rarely affected at less than age 4-5 years and then show progressive deterioration with advancing age. Pony breeds, including the Welsh pony and the Icelandic pony, as well as the Shire horse, are predisposed. A genetic predisposition seems to play an important rule in the onset of the hypersensitivity since not all the horse that are bitten by the midges develop an allergy to them saliva.

Affected horses and ponies may spend hours rubbing against trees, fences, doors or anything else available. This is noticed more often early morning and late afternoon when the midge feed from them. As result, the skin is damaged and hair are ripped off especially down the upper neck, back and tail base (dock) although often the lower abdomen is affected. 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. Affected areas of the lower abdomen also become crusty and hairless.

Control and treatment:
The only really safe and effective method of management is to reduce, if not preventing, the attack from the midges to the horse. Whatever treatment is employed to control pruritus, the condition will not be completely eliminated unless exposure to biting midges is eliminated.

Regular application of insect repellents, use of fly sheets or rugs and stabling during times of high midge activity (dawn and dusk). Some people have success with the longer lasting pour on insect repellents like Deosect®. DEET is effective but can cause soreness. Switch Lotion seems to help to control the insect biting if it used since the beginning of the Sweet Itch season. It can be repeated as and when needed. Topical application of insect repellents should include treatment of the dorsum and the belly, as pruritus in the dorsum can be triggered by immune-mediated reaction to circulating antigen in some horses, and the belly is a predilection bite-site for some species.
Barrier treatments such as Avon skin-so-soft or benzyl benzoate are also useful.
It is a good idea to move affected horses to open, more exposed fields with a good breeze (midges are weak fliers and get blown away with a wind more than 4mph!) and keep affected horses and ponies well away from woodland and water – especially standing water, such as ponds.
Attempts can be made to make stables insect-proof by using small netting screens at windows and possibly fan at the roof to assist the ventilation. Midges are not stronger flyer!
Many cases can be controlled throughout the summer by using a special body covers, lightweight sweet itch sheet and hood combination with belly band and tail cover. It is extremely important to start to use them before the midge season starts. Therefore preventing the first bite to the horse. Late February – March it is a sensible time to start to put them on the horse.
Stable affected horses and ponies from about mid-afternoon to mid-morning (4pm to 8am; i.e. dusk to dawn) when midges are at their worst.
Many horses or ponies with this condition will also need treatment to control the itch and sometimes to resolve secondary skin infections. Often topical treatment is effective, such as the right soothing shampoo (and has fewer potential side effects) but sometimes tablets and/or injections are needed. In severe cases control of the pruritus can be achieved using systemic steroids per mouth. Once control of the pruritus is achieved, the dose can be reduced. Horses receiving corticosteroids treatment should be monitored closely for laminitis.
Feed supplements designed to support skin health could help, but there a very little research about it.This included things like Cavelesse, Brewer’s Yeast or vitamin B supplements.
For several years groups have been working on the development of a vaccine for the prevention of sweet itch in susceptible individuals. Trials are underway but it may be some time before this is widely available. Currently a European Country started to use a vaccination which was developed as a ringworm vaccine which seems to have as an added benefit a reduction of Sweet Itch signs. It is important to highlights that this is an off licensed drug. A course of 2 injections with 2 weeks in between should be administered before the start of the midge season (February-March). Efficacy seems to be just 50% or less.