Sweet itch is a skin condition caused by an allergic reaction to midge bites (primarily Culicoides spp). It is also known as insect bite hypersensitivity or summer seasonal recurrent dermatitis and can affect horses, ponies and donkeys all over the world according to the distribution of the midges.
In the United Kingdom, there appears to be an increased incidence in pony breeds. In one survey between 2 to 8% ponies were said to be affected. However, it can affect any breed and sex and has been shown to be inherited in some cases.
All animals are bitten by midges but only those that are allergic to the bites show clinical signs. There are different species of Culicoides which feed at different sites, some at the mane and withers, others at the tail and/or belly and legs. The animal may be allergic to one or more species and therefore they may show signs on one area only, such as the tail or all over the body in severe cases.
Repeated exposure to the midges is to some degree protective. Icelandic ponies born in Iceland where there are no midges develop much more severe disease when they are moved to Britain than those Icelandic ponies born in this country.
Clinical signs
Affected animals show varying degrees of pruritis (itchiness) which leads to self trauma due to rubbing. The most common sites affected are the mane and tail but sometimes only the belly is affected and in severe cases the animal may show signs all over the body, including the legs. Rubbing leads to alopecia (hair loss), ulcers and bleeding with secondary crusts (scabs) and infection. Many of these changes are reversible out of season when there are far fewer midges. However, in more chronic cases the skin can become hyper pigmented (blackened) and thickened with ridges, especially along the mane. Severely affected animals may lose weight due to chronic irritation and show behavioural changes from tail swishing, rolling, and rubbing the belly on the ground, to being unrideable at the peak midge feeding times of day (dusk and dawn).
Buying a pony with sweet itch or suspected sweet itch
If you are buying a pony that is said to be managed successfully by one of the different treatment options, you should be aware that a change in location may either improve or exacerbate the condition to a point that it is no longer manageable. Sweet itch sufferers may also deteriorate with age.
When buying a pony in the winter out of the midge season, in severe cases the thickening on the neck and tail head may still be evident. However, it is often difficult to detect animals which show milder signs or those that have been managed effectively through the summer. Often there is evidence of different hair lengths, particularly at the tail head, but it is important to ask the owners to declare if the pony suffers from the condition.
At present there is no reliable blood test or other allergy test to detect sweet itch sufferers out of season but research is ongoing.
Management
Currently there is no cure for sweet itch and lifelong treatment is necessary. Management is directed in four main areas; reducing midge exposure, reducing secondary skin trauma/infection, symptomatic treatment and modulation of the immune response.
Reducing midge exposure
The life cycle of the midge is important in understanding how to reduce exposure. The adult midge can only fly a few hundred metres and the females lay their eggs in damp moist soil or slow moving water, so try to avoid putting your animal near these areas if possible. The life cycle can only be completed at temperatures above 5C which is why the condition is not seen in colder countries and is seasonal in Britain. The midges preferred feeding times are dusk and dawn and so stabling between 4pm and 8am with a fine mesh screen away from the muck heap is effective in some cases. Stable fans are often used in other countries both for cooling and to reduce the midge exposure as they are not strong flyers.
One of the most popular and most successful methods of avoiding midge bites is the use of specially designed rugs such as the Boett blanket, Snug rug or Rambo sweet itch rug. Some are impregnated with an insecticidal product (permethrin). These are designed with a very small mesh that doesn’t allow the midges to penetrate and can be made to measure and adapted to fit almost anywhere on the body. However, they need to be applied well before the sweet itch season (February/March) to be most effective as they are often damaged once the animal is itchy.
Insecticides and fly repellents including permethrin, cypermethrin, Deet, light oils, citronella for example may help in mild cases but need to be applied frequently. There are also herbal remedies on the market both in feed and topical which are reported to have repellent properties
Reducing secondary skin trauma/infection
The use of electric fencing to stop scratching is often employed but can be distressing to the animal that will often find other ways to relieve the discomfort and is ideally used alongside other methods. The use of scratch pads (which are specially designed rubber mats) allows a place for the animal to get relief whilst minimising secondary trauma.
Symptomatic treatment
This includes the use of steroids and antihistamines which have limited value and in the case of steroids has been associated with laminitis. However, shampoos containing aloe vera, oatmeal for example may provide some relief and emollients such as benzyl benzoate can act as a barrier and have a soothing action.
Modulation of the immune response
Signs of sweet itch are in fact due to a more complicated reaction of the immune response than just allergy which means that desensitising the animal to the midge allergens is currently not available. However, there is much research in this area.
Immunomodulation
Trial work using non pathogenic bacilli (bacteria) Bio Eos BE-T-101 has been ongoing since 2005. This modulates the immune response from a dysregulation (Th2 mode) that leads to the classic signs of sweet itch to a more normal response to midge bites (Th1mode). The trial now using weekly oral dosing has shown encouraging results and the final blinded randomised, placebo controlled trials conducted in 2010 are currently being analysed. More information on this treatment can be obtained from the National Sweet Itch Centre.
Selective breeding and the possibility of genetic tests in the future may add important information for horse owners.
Lifelong management is necessary in all cases of sweet itch and some animals may need to be removed to a different location. Severely affected cases respond less well and euthanasia may be the only option for some owners.