Along with the lovely summer weather come insects, including midges. For horses who suffer from sweet itch (Culicoides hypersensitivity), summer can be a miserable time. Affected horses have an allergic reaction and an exaggerated immune response to the saliva of biting midges, which makes them incredibly itchy. Around 2.8% of horses and ponies in the UK are believed to be affected, and they usually start showing signs around the age of 2 to 4 years.
The Culicoides midge is active from March to October but peaks between May and September. Horses are usually affected along their manes, their backs, bellies and tailhead; the areas where the midges bite and feed. Left unmanaged, horses may rub themselves raw, lose their mane and tailhead and body hair, and can suffer from secondary infections.
The condition is believed to have a hereditary component and is more common in certain breeds, for example Icelandic ponies and Friesians. Affected horses may need to be stabled at dusk and dawn and ideally kept away from standing water and marshy land. Symptoms may disappear over the winter, only to recur in the spring.
There is no cure for sweet itch and good management is key to its control. The most important strategy is to create a barrier between the horse and the midge, for example with a specially designed rug. Several brands of rugs specially made to help control sweet itch are available and they can be very effective. Insect repellents are also necessary, and may have to be applied regularly. Those based on oil, which creates a barrier when applied can be useful. Using fine mesh fabric on stable windows (finer than mosquito nets because Culicoides midges can get through these), and fans to stop midges being able to fly can also help.
Trials of a vaccine to inhibit the horse’s immune response to the midge bites are underway, and results have been quite successful. The National Sweet Itch Centre is running the trials. Hyposensitisation – where a small amount of the antigen that the animal is allergic to is injected, which increases tolerance thus reducing the reaction – is sometimes carried out on horses but there is little published information on success rates.
B vitamin supplementation with brewers yeast or specific B-complex supplements containing niacin (vitamin B3) are believed to help reduce itchiness, but they need to be started early, before the midges appear.
Feeding linseed meal is recommended. Researchers in a trial of affected horses showed reduced skin test response to Culicoides midges (a decrease in the allergic reaction) after daily feeding of supplementary milled linseed (450 g per 450 kg bodyweight daily). Linseed’s anti-inflammatory omega-3 fatty acids are likely to be the most important active ingredient, although the researchers mentioned that other phytochemicals may also be involved. Click here for further information.