What is Sweet Itch

Sweet itch (also known as Summer Seasonal Recurrent Dermatitis or SSRD) is caused by an allergic reaction to midge bites. Horses (and donkeys and mules) that suffer from sweet itch react strongly to midge bites, developing painful, itchy skin and weeping sores as a result.

When a midge bites a horse, it injects saliva containing an anti-coagulant (which helps the midge to suck up some blood). The body's immune system recognises the anti-coagulant as a foreign protein and mops it up. All horses are bitten constantly over the summer by midges but in a healthy horse the immune system response to each bite is small and proportionate - just enough to mop up the anti-coagulant. But when a horse suffers from sweet itch, the immune response is disproportionately large (otherwise known as an allergic reaction) and results in significant swelling.

The skin becomes very itchy as a result and the horse generally tries to scratch itself to relieve the itching. The itching can be so severe that the horse rubs itself raw, often removing large amounts of hair and causing large bleeding sores.

Sweet itch is traditionally managed by using insect repellents and anti-fly rugs such as the Boett Blanket. In mild cases this may stop enough of the midge bites to give the horse a good level of relief. But in severe cases, every last midge bite causes major skin irritation and horses often manage to rub themselves raw despite the best efforts of the owner.

Horses with sweet itch may also suffer other symptoms that appear to be related to an over-active immune system such as lethargy.