Sweet Itch – prevention is better than cure

Around 3% of the UK horse population is thought to suffer from sweet itch. The condition affects any breed, age or sex of horse, however it is particularly common in Icelandic and cob type ponies. Environmental factors greatly influence the development of this disease, although it is not contagious, a hereditary predisposition has been suggested. It is generally considered a seasonal condition, extending from spring to autumn. However with current climate conditions, symptoms are increasingly persisting well into the winter months.

Pathophysiology

Sweet itch is an allergic skin disease that occurs as a result of a delayed hypersensitivity reaction to saliva in the bite of Culicoides midges. Only certain horses are allergic to the saliva and their sensitive immune system responds by developing an area of intense localised irritation following being bitten. This causes the horse to begin itching and the self–inflicted trauma that follows leads to the clinical presentation of sweet itch.

Clinical Signs

Characteristic clinical signs are restlessness with constant rubbing against objects and biting at themselves causing hair loss over the mane, tail and hindquarters primarily. Around the head, chest and belly may also be affected depending on the type of midge, as each species has a favoured area of the body. Early lesions appear hairless, however if self trauma continues they become red and inflamed with crusting and, may bleed from open sores. It is not uncommon for horses to rub off the hair from the mane and tail base.

Chronic skin changes begin to develop as the disease advances with the affected skin becoming thickened, wrinkled and darker (black). The hair is often sparse and coarse. During winter months a horse may completely heal, only for the condition to recur in spring when it has contact with midges again. Therefore caution must be exercised when purchasing horses in winter, ask your vet to check for signs of sweet itch.

The Midge

In the UK alone there is up to 1000 different species of Culicoides midge. Their breeding season is dependent on weather conditions and normally extends from March to October. Though sweet itch is generally considered a seasonal disease, recent milder and damper winters has allowed midge breeding to start earlier. Culicoides larvae (immature form of the adult midge) are able to survive harsh frosts but are killed in extended drought conditions.

Prevention

Preventing exposure of horses to Culicoides midges remains the best cure for sweet itch.

Using a Boett blanket before the fly season starts, which is usually from February onwards as this is when they begin to bite. Boett rugs are made of a unique fabric that prevents flies and midges penetrating it, therefore the rug should extend to cover from the mane to the tail base. A hood is also available, see www.sweet-itch.co.uk for further information.

All affected horses should be turned out in a paddock surrounded by electric fence, away from barbed wire, to prevent self trauma and the development of sores from rubbing. Ideally it should be an exposed, hilltop field away from woodland and standing water (ponds or marshes) as these are ideal breeding sites for Culicoides flies. Stabling at peak insect feeding times during dusk and dawn (from 4pm to 8am) has been advocated, but this may depend on the severity of the condition. Fine-mesh screens can be used to help insect-proof stables, but in some cases stabling will only worsen the condition through the provision of stables walls to rub on.

There are many fly repellents commercially available however their effectiveness is questionable and they require liberal applications up to three times daily. Benzyl benzoate mixed with liquid paraffin applied topically to the mane and tail appears to be a good fly deterrent, but similarly needs to be reapplied often to be effective. Applying baby oil can also stop midges being able to feed.

Anti-allergy treatments consist of corticosteroids and anti-histamines. Steriods are most effective at preventing itching and allowing sores to heal due to their powerful anti-inflammatory action on the immune system. However steroids do have important side-effects in horses, which should be discussed with your vet. Anti-histamines are often not very effective but can provide some relief from itching in certain horses.

Horse owners who participated in the sweet itch “vaccine” trials from 2006 to 2008 reported encouraging results, with an improvement in sweet itch symptoms, but it did not achieve total success. These trials have now ended and are subject to approval by the Veterinary Medicines Directorate before manufacturers can launch the product.

A new product was introduced in 2009 called Cavalesse which contains the active ingredient Nicotinamide, a type of vitamin B3, which helps reduce skin inflammation by decreasing the production of histamine from the immune system; a trigger factor responsible for causing itchy skin. It is not a cure but can be a very useful aid in the management of this condition, in both active cases and when given before the season starts. A pack of Cavalesse consists of an oral solution given daily and a topical gel. One pack will provide treatment to a single horse for 3 months. Treatment should continue throughout the risk period.
Although all possible precautionary measures may be taken to protect your horse from Culicoides midges, it can be very difficult to completely prevent the symptoms of sweet itch. Without treatment this disease tends to worsen each year.

For further information visit www.sweet-itch.co.uk or phone the National Sweet Itch Helpline on 01352 771718.