Sweet itch is a common, seasonal, allergic skin condition resulting from the bites of certain species of fly. Although all horses, ponies and donkeys are bitten and irritated by flies, only some will show the more severe signs characteristic of sweet itch. These unfortunate individuals are allergic to components of the insect’s saliva. More cases are seen amongst pony breeds and donkeys than in horses and an estimated 2 to 3% of British ponies are affected. There seems to be a strong hereditary influence on the development of the condition with the sire and/or dam being affected in many cases.

In the UK, a particular species of ‘midge’, Culicoides pulicaris, seems to be the prime culprit although other species of midge and other flies may be involved in some cases. In other parts of the world different fly species cause most trouble. Culicoides pulicaris prefers to land and bite along the mane, back and tail root which corresponds with the most severely affected areas in most cases. These midges don’t fly far from their wet breeding grounds (e.g. ponds, ditches, troughs, muck heaps etc.) and so there are some local areas and stables where sweet itch is not seen at all and others where it is severe and hard to control. Midge activity is predictable with high activity on warm, still days. Most species (including Culicoides pulicaris) actively bite in the afternoon and early evening and possibly again at dawn.

Most allergic conditions require repeated exposure to the causal factor before symptoms develop. Sweet itch is no different and it usually takes 3-4 summers of exposure to biting flies before sweet itch develops and then it often gets worse year by year. Hence sweet itch is very unusual in foals, and quite unusual in yearlings and 2-year olds and itchiness in these horses is more likely to be the result of another condition such as irritation (rather than allergy) by flies or lice. Most sweet itch cases are first recognised as ‘young adults’ (e.g. 4 to 6 years old) although it may arise for the first time in much older horses and ponies especially after a change in location.

The disease usually presents as a seasonally (April to October) recurring, constant, severe itchiness with broken hairs, hair loss, dandruff and skin thickening. The worst areas tend to be the mane, neck, rump and tail-root. Self-inflicted skin wounds and abrasions are common as the horse or pony rubs and scratches on any available objects. Some cases are seen with a different pattern of affected areas such as the underside of the chest and belly or even all over the body. Sweet itch cases tend to resolve in the autumn as the fly populations subside allowing the affected horse or pony to regrow lost hair although skin thickening may still be evident. White hairs may develop in the mane and tail from irritated areas (look for this when buying a pony in the winter!).

The only reliable way to prevent sweet itch is to avoid your horse or pony being bitten by insects. Although this may prove to be difficult it is well worth while spending considerable time and effort in trying to achieve this end, as this must be a dreadfully uncomfortable problem for the horse or pony and there are no reliable and entirely safe treatment alternatives. Preventative measures include stabling of affected horses and ponies at times of peak insect activity (mid afternoon until the following morning, eg. 4pm-7am), summer sheets and hoods, and repellents applied to the mane, back and tail root each afternoon. It is hard to recommend a universally effective fly repellent for use in horses. Many types are available and seem to work in some cases but not in others. Products range from the ‘cheap and cheerful’ benzyl benzoate lotion to more modern permethrin- or fipronil-containing repellents and products borrowed from the human market containing diethyl toluamide (‘DEET’). More naturally based products containing garlic or citronella oil may help in some cases although there is a general impression that DEET products are often most successful. When trying a new product it is often wise to try it out on a localised area of your horse’s skin first to make sure that you are not going to get a reaction to it. Repellents may also be hung in the stable (e.g. ‘Vapona’) or applied to hoods and sheets. An additional approach which can be successful is to carefully mount a 30-40W electric fan within the stable directed at the doorway so that the midges cannot enter (they cannot fly against even a gentle breeze). If all else fails, moving an affected horse or pony to another area can be effective in some cases as the midges are not found everywhere. An open landscape with minimal tree shelter and absence of water-filled ditches or ponds would carry the least risk.

Electric fencing may be helpful in removing access to fence posts and gates on which the horse may rub. Whilst not addressing the underlying problem this strategy may help break the ‘itch-scratch cycle’ and prevent self-trauma whilst other solutions are instituted. Dietary changes have been proposed by some although there are no universal reports of success. Certain vegetable oils and mineral additives may help some cases. Recent interest in attempts at allergy hyposensitisation may have some promise but this is curerntly unproven. There is no evidence at present to support the use of homeopathy although it is understandable when these methods are tried when all else has failed! Corticosteroids (e.g. prednisolone), which can be given by injection or in feed, can be very effective at reducing the itchiness in the short, and sometimes, long term. However, the rare side effect of laminitis restricts the usage of this type of drug. Certain types of antihistamine can also have some beneficial effect in the short term without the potentially adverse effects of corticosteroids.

In common with many allergic problems, both in people and in animals, sweet itch is a very frustrating condition to have to deal with. It must be dreadfully unpleasant for the affected horse or pony to be suffering almost constant itchiness but there is rarely a ‘quick-fix’, easy solution to the problem. Just one brief period of exposure to biting midges can result in itchiness lasting up to 3 weeks. This has two practical consequences: firstly, preventative measures cannot be relaxed even briefly through the sweet itch season; and secondly, one should be patient for results when assessing treatments or management changes. The satisfactory control of sweet itch is achievable in most cases but requires considerable and constant effort from the owner which is both inconvenient and time consuming.

© The Liphook Equine Hospital November 2005