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Control of Sweet Itch

Control of Sweet Itch 

Marco Magdalena DVM MRCVS, Molecare Veterinary Services

Sweet itch or Insect-Byte Hypersensitivity is an alergic reaction to the bites of midges (most commonly Culicoides) and other insects. It results in itchiness both due to the insect bite itself and due to an allergic reaction to the antigens in the insect’s saliva or from inhalation of dessicated insects.

The development of the disease depends on multiple factors such as:

Genetic: 35% of Hypersensitivity reactions are inherited and some breeds like welsh ponies, icelandics, arabians, connemaras, Quarter Horses and German shires are more likely to be affected.

Geographic situation: Due to the the geographic distribution of the different insects.

Degree of exposure to the insects: Successive exposures to the insects increase the symptoms.

Age: Normally symptoms start to appear between 2-4 years of age.

Generally sweet itch is a seasonal disease in the UK as cold weather affects the presence of the insects. It is extremely itchy (sometimes unresponsive to steroid treatment), forming primary lesions (wheals and papules) with a distribution along the body depending on the feeding habits of the insect and secondary lesions result from self trauma when the animal tries to scratch itself. When the secondary lesions involve the mane or the tail, we see the typical “buzzed mane” and “rat tail” appearance. Sometimes the disease can get complicated by secondary bacterial infection of the lesions.

There is not a fully effective treatment but treatment can be given to relieve the pain and itchiness (steroids, antihistamines). For this reason, prevention of the disease is important. Reducing or avoiding the contact between horses and insects will be the main objective, in order to achieve this the following controls must be put in place:

Enviromental: Taking the horse out from the current enviroment or avoiding it at the times where the insects are more active (stabling at times of insect activity, physical barriers preventing insect access to the stable, controlling the grazing areas).

Insect Control: Reducing the enviromental load of problematic insects. Measures like draining ponds, cleaning manure from fields and stables, topical insecticides, environmental insecticides, repellents, foggers and fly baits.

Physical Control: Protecting the affected areas of the body most likely to be bitten. Ear nets, face nets, fly rugs, repellents, sprays, nets on the vents of the stable, fans.

Insect Location of lesions Time of activity Environmental Conditions Control
Culicoides spp. Normally mane and tail,but depends on the species Sunrise and sunset Pooled water, manure and decayed vegetation Drainage of standing water, spraying of breeding grounds, ear nets, fans, insecticide screens, rugs with neck and hood.
Black flies Face, ears, abdomen, groin, fore legs and thighs Morning and evening Running water Ear nets, insecticide on ears, vaseline on ears
Stable flies Legs and abdomen Daytime, under shade (early morning and late evening) Manure, decayed organic material and bedding Surface sprays, foggers, fly baits and space sprays
Mosquitoes Sides of the body Dusk immediately after sunset Water Repellants, eliminating standing water and  insecticides
Deer flies Sides of chest, flanks and top part  of legs Daytime Vegetation,Water Surface sprays, foggers, fly baits and space sprays
Horseflies Sides of chest, flanks and top part  of legs Daytime Vegetation,Water Surface sprays, foggers, fly baits and space sprays
Horn flies Around the umbilicus Daytime Cow manure Residual insecticide spray on affected areas

If you feel your horse is showing signs of sweet itch this summer please consult your vet and they can discuss the options for physical, topical and environmental insect control that are available.

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