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Horse Vet Cumbria

Sweet Itch

Sweet Itch is a well-recognised, seasonal skin disease of horses and ponies characterised by itchiness and hair loss, and by oozing, crusting, scaling and thickening of the skin. The base of the tail and the mane are usually the first and worst areas to be affected. Sweet itch is caused by an allergic reaction to the saliva of culicoides midges and up to 5% of the UK horse population is affected to some degree. Although all breeds can be affected the condition is rarely seen in Thoroughbreds. Affected horses typically become symptomatic in their second or third summer and the condition usually worsens from one year to the next.

There are thought to be up to 1,000 different species of midge, 20 to 30 of which will be active in any geographical area. The variety of midge species accounts for some of the variation in lesion distribution from one horse to the next as different midge species prefer feeding at different sites. Some horses only develop lesions in front of the sheath (or udder).

When midges bite the skin of an affected horse of pony, the immune system reacts inappropriately by releasing excessive amounts of the chemical histamine; it is this chemical that causes the itchiness. Some therapies for sweet itch are aimed at reducing the release of histamine.

Sweet Itch is not usually confused with other skin diseases but skin allergies to other insects (flies) or pollens, moulds or grasses could result in similar symptoms, as could a lice infestation or mange due to chorioptes mites.


Sweet Itch cannot be cured but measures can be taken to control it. Unfortunately, control measures can be inconvenient, time-consuming, messy or just an extra expense. Control measures can be divided into those that prevent midges from biting and those that alter the horse’s reaction to any bites he or she still receives.

Preventing midges from biting

  1. Insect repellents. Products you may wish to try include: Cooper’s Fly Repellent Plus® (MSD) (contains permethrin); Deosect® (Zoetis) (contains cypermethrin); Dermoline Sweet Itch Lotion®(Battle, Hayward and Bower Ltd) (contains piperonyl butoxide and pyrethrum extract). Insecticidal ear tags, sold for use on cattle, can be attached to your horse’s rug or head collar, or plaited into the mane.
  2. Physical barriers. Special rugs are available to prevent midges biting, such as a Boett’s rugs. Hood and tail covers are also available. Unfortunately rugs can themselves be uncomfortable.
  3. Other physical barriers. A variety of ointments are available from tack shops. Liquid paraffin is cheap and often effective, if a little messy.
  4. Stabling. To help control sweet itch horses can be turned out only during the middle of the day, between about 10.00am and 4.00pm. Midges are least active at this time of day. Horses are stabled at dusk and dawn and through the night as this is when there are most midges about.
  5. Fans. Fans can be used in the stable block as midges go elsewhere above a certain air speed. (Powerful fans can be purchased through farm supply shops as they are also used on dairy farms to keep flies out of the milking shed, where flies upset cows when they are being milked.)
  6. Relocation. Relocate affected horses to places where there aren’t many midges, either on the same property or on a different yard. Midge populations are highest near woods, near stagnant water and near muck heaps. An exposed field, near the top of a hill with free-draining soil and with no woods nearby would be ideal.

Altering the horse’s reaction to midge bites

These measures should only be used if problems still persist despite having tried or considered measures that prevent midges from biting your horse.

(The first four treatments are all steroids. Steroids are prescription-only medicines and are only available through veterinary surgeons. Oral or injectable steroids can put your horse at risk of developing laminitis and should only be used after you veterinary surgeon has carefully considering the risks and the benefits of treatment and discussed these issues with you.)

  1. Short-acting steroids. One problem faced by the owners is that itchy horses tend to damage (destroy!) rugs very quickly. A short-acting steroid injection can be used to break the itch cycle long enough to allow a rug to be worn. Wearing the rug then prevents further midge bites. Short acting steroids will last up to 10 days depending on the dose and the product used.
  2. Long-acting steroids. Depo-Medrone is licensed for the treatment of musculo-skeletal conditions in horses. It is also known to be useful for controlling allergic skin disease and is licensed for this indication in other species (dogs and cats). Depo-Medrone may last up to a month depending on the severity of the allergy and the dose used.
  3. Oral steroids. Prednisolone can be given by mouth at a dose of about 1mg/kg/day though this dose may need to be doubled to control severe symptoms. Over time the dose should be reduced to the minimum effective dose needed to control the symptoms. This treatment can be quick expensive but you will be able to discuss the costs with your veterinary surgeon.
  4. Topical steroids. Cortavance® (Virbac) is a prescription-only medicine licensed for use in the dog. It is sprayed on to the horse’s skin to prevent itchiness. It has proven useful for controlling sweet itch and its use doesn’t put your horse at risk of developing laminitis.
  5. Cavalesse® (Janssen AH). Cavalesse® is available as a food supplement and as a topical treatment. It contains nicotinamide, a B vitamin, and the manufacturer claims the product reduces the release of histamine after midge bites and improves the barrier function of the skin. The product is relatively new to the market and there are many reports of its use being effective. However, it has yet to be independently trialled to determine its efficacy.

Antihistamines have not been found to be particularly effective for controlling sweet itch.

A clinical trial found Evening Primrose Oil and Fish oil to be of no “great benefit”. (Craig, 1997)

All of the above treatments are available through the surgery. Please contact the surgery if you would like to discuss sweet itch with one of our horse vets.