By Vet Bethan Harper
Sweet itch is the colloquial term for insect bite hypersensitivity in horses.
This common skin condition occurs when affected horses have an extreme reaction to the saliva of the Culicoides midge. When the midges bite the skin a small amount of saliva is deposited, which sets off an allergic reaction causing the horse to feel itchy.
This response is variable in individual horses, which is why some horses are extremely affected, and others not all. Other insects can also cause hypersensitivity reactions, but the Culicoides midge is the most common.
Clinical signs of sweet itch are usually a result of the affected horse trying to scratch the itch. This most commonly materialises as a rubbed mane and tail, but more severe cases can end up with patches of raw skin all over the body and secondary skin infections.
Regularly grooming your horse and being vigilant for patches of hair loss, scaling, inflammation, or dullness is the best way to catch signs of sweet itch early.
Unfortunately, the signs can be quite vague so your vet may need to rule out other causes of dermatitis.
There is no cure for sweet itch, and the most important thing you can do is catch the symptoms early and start appropriate management. Preventing midges from biting the horse in the first place is the best approach, so management strategies focus on reducing exposure to midges. This can be achieved by reducing turnout at high-risk times such as dusk and dawn when midges are most active, and avoiding areas where midges are prevalent such as water logged fields and those with a natural water source, like a river or beck, nearby.
You should also invest in a good quality full body fly rug and mask for your horse when turning out, and use a suitable insect repellent. In extreme cases where horses are bitten even while stabled, you can place a mesh over the stable door/window to prevent midges getting in. However, this should be introduced slowly as it may cause anxiety or stress particularly if it blocks their ability to see other horses.
Once clinical signs appear there is no easy way to alleviate the symptoms, and you may need to try different things to see what works for your horse. In some cases the use of corticosteroids may be indicated to reduce inflammation and stop the itch cycle, but this won’t be suitable for all horses due to the side effects associated with steroid use. This should always be discussed with your vet. Anecdotally some people have found antihistamines to be beneficial in managing itchy horses, but very high doses need to be used so this is something you should discuss with your vet.
Certain shampoos can be used, particularly ones containing oatmeal, to soothe the skin and help to reduce inflammation and itching. There are also some supplements that can help to improve the skin barrier.
There is a vaccination called Insol that can be used off license to help reduce the clinical signs associated with sweet itch. This vaccine was initially developed to target ringworm in cattle, but has been found to downregulate the biological pathway that causes a hypersensitivity response in sweet itch.
Two doses are given two weeks apart, at the end of winter/start of spring before peak midge season starts. It is important to note that Insol doesn’t replace good management strategies, but rather is used alongside them to reduce the effects of any midge bites your horse receives. Not all horses respond to the vaccine, but many owners have reported reduced clinical signs when using it compared to previous years.