What Causes Sweet Itch in Horses?

Sweet Itch or "Queensland Itch", as we call it in Australia, is one of the most irritating and frustrating conditions for horses and their owners. Left uncontrolled, the constant itching can result in hair loss and over time thickening and wrinkling of the horse or pony's skin.

Sweet itch is a skin condition caused by an allergy, or hypersensitivity, to the bites of sand flies or midges (Culicoides species). It may be found in any horses and ponies, especially those that live in warmer regions, including areas in Canada, Australia, the US and many other parts of the world.




Source:Wikipedia

Although the disease has been known for centuries, the first scientific description describing the link between the itch and midge bites was made in Australia in the 1950s by a vet called Riek. He noticed that the disease seemed to occur only in areas where midges were abundant. Riek took an extract of midges and injected it into the skin of affected horses. He found that a small swelling appeared within a few minutes. This type of “acute” reaction is typical of certain cases of immune “hypersensitivity” or allergies so Reik concluded that sweet itch was an allergic reaction to the midge bites.

The bites from these insects lead to intense itching and rubbing of the body, usually around the head, mane. withers and tail but also along the back. Other regions of the body that are less susceptible but can be affected are the horse's chest, belly and legs.

The Culicoides midge is a parasitic insect which feeds on the blood of other animals, depositing a small amount of saliva at the site of the wound. Sweet itch is an allergic reaction to the chemicals in the saliva (which actually contains a harmless protein) and is therefore an immune system problem. In the process of repelling invading insect saliva the horse attacks some of its own skin cells 'by mistake' and the resulting cell damage causes the symptoms described as Sweet Itch. There are some 20 different species of the Culicoides midge, and each favours a particular part of the horse's body to bite.

Other insects and flies may bite and exacerbate the already present symptoms of Sweet Itch, including horse flies, stable flies, black flies and mosquitoes.

Source: Horseweb-UK

The Culicoides midge breeds during the Spring and Summer months and its level of activity is dependent upon weather conditions. Culicoides adults mainly rest among plants and shrubs and are most active in twilight, dawn and dusk, under calm conditions with little wind. The midges like to breed in wetland areas as they need moisture to complete their life cycle, so they will be more prevalent around areas with dams, swamps or creeks. As with any other cold blooded creature, the midge's activity increases with the temperature.

Horse dung can attract flies and midges and some horse owners claim it is helpful to remove the dung from your horse's yard on a daily basis. The presence of cattle can also encourage an influx of midges and flies so, if practicable, keep your horses well separated from other livestock.

Midges generally do not like windy conditions as they are not strong fliers and the wind may hinder their ability to land on target animals. Consequently, horses kept in less exposed or very sheltered yards fields tend to suffer more from the condition.

Not all horses will develop Sweet Itch or Queensland Itch. It is thought that some breeds possess a genetic predisposition to the condition and a hypersensitivity to the saliva of the midge. Draught horses and ponies are two such breeds. Sweet Itch is generally accepted to be rarer in breeds like the English thoroughbred. A horse can develop Sweet Itch at any age, but most start to display symptoms between the ages of 1 and 4. However, it is not uncommon for an equine of 10 years of age or more to start to display the classic signs of Sweet Itch for the first time, depending on the environment in which it lives.

The symptoms of Sweet Itch are usually easy to identify. Horses with Sweet Itch might display behaviours of continual shifting and agitation, swishing of the tail and tossing of the head. They will rub or bite at the effected part of the body. They will rub against fence posts and often roll in the dirt, twisting and turning with all four legs off the ground.

If left untended, what starts as a simple insect bite can result in on-going health problems including hair loss, broken manes and tails, crusted or open skin wounds, dermatitis and ulcerations. This may lead to secondary infections, general weight loss and an irritable temperament.