Insect Bite Hypersensitivity (Sweet Itch) in Horses

Biting midges refers to a large class of insect of the Culicoides genus. There are over 1000 species of biting midges world wide, some of which are known to carry parasitic and viral diseases that can be transferred to mammalian hosts, including horses. Biting midges are small flying insects particularly active in the warmer months.

Like many biting insects, midges are blood-suckers. Their bite is relatively painful and it may cause itchy rashes in especially sensitive animals. Biting midges tend to fly around specific areas of the horse’s body including the mane, tail, and abdomen. Bites and rashes might also appear on the legs, face, and ears.

Sweet itch is not a life-threatening condition but it can cause intense annoyance and irritation, to the point that it may alter normal feeding or exercise patterns in affected horses. Horses showing symptoms of sweet itch benefit from prompt veterinary examination. A definitive treatment for this condition is not yet available.

Hypersensibility to biting midges (Culicoides spp.) is the most common allergy in horses.

Horses that become severely itchy often exhibit self-mutilation behaviors. Symptoms of self-mutilation include:

• Aggressively biting at affected areas
• Rubbing or scratching the area on objects
• Open skin wounds that may become ooze pus or bleed

Sometimes biting midges can also serve as a host for onchocerciasis, a parasitic infection that causes a non-seasonal dermatitis similar to sweet itch.

Sweet itch is an allergic reaction to the saliva of biting midges. Allergies are an immune response to a non-threatening trigger called an allergen. As with all allergies, the cause of the inappropriate immune response is unknown, but genetic predisposition is suggested.

The diagnostic process involves a complete physical examination and dermatological examination, focusing on the most affected areas. Biting midges are not found directly on the animal, so diagnosis focuses mostly on ruling out other possible causes. Tests include:

• Skin scrapings
• Tape preparations
• Bacterial or fungal cultures
• Skin biopsy

Treatment of sweet itch focuses on resolving secondary infections and reducing itchiness. Specific treatment is currently under investigation.

Antihistamines or corticosteroids to alleviate symptoms may be prescribed, however their efficacy can be poor. Antibiotics may be required if skin wounds become infected.

Currently, sweet itch cannot be treated definitively. Management involves life-long therapy designed to prevent the condition from worsening and to provide relief of symptoms. Many horses require changes in management strategies over time.

Prevention is the most important tool to fight sweet itch. There are several precautions to minimize the risk of sweet itch:

• Keeping horses in the stable during the insects’ most active hours (dusk and dawn)
• Protecting horses with a fly sheet and other fly apparel items during midge season
• Keep horses away from wetlands or other environments where biting midges thrive
• Use insect repellents specific for midges
• Using topical ointments that soothe insect bites and reduce itching
• When stabled, use fans to create a light breeze that discourages insects or timed misters of fly repellent

Note: always consult a veterinarian before selecting insecticide for use on animals. Make sure any medical practitioners are aware of all species of animals that may be exposed to insecticides. Always use parasite prevention according to veterinary instructions.

Culicoides hypersensitivity is the most common allergy in horses.

• Environmental management
• Insecticides
• Antihistamines
• Steroids
• Antibiotics