Culicoides Hypersensitivity

Also known as Sweet Itch

Description

Culicoides hypersensitivity, commonly referred to as 'sweet itch' is an intensely pruritic dermatitis of horses caused by a hypersensitivity reaction to the saliva of biting insects. The most important causative agent is the Culicoides midge but other insects including Simuliumspp, Stomoxys spp and Haematobia may also have a role. In temperate climates, the condition occurs between spring and autumn (when midges are active) with a period of remission during the winter months.

Signalment

Although sweet itch may occur in horses of all breeds and ages, the disease has a peak age of onset of between 2 and 4 years. Most breeds are affected although sweet itch is rare in Thoroughbreds. It has suggested that genetic factors may have a role in the pathogenesis of the disease as the prevalence of sweet itch in some breeds is higher, such as Icelandic ponies and German Shire horses. Culicoides require marshy areas or still water and horses kept in fields near streams or water may be at higher risk of developing sweet itch.

Pathogenesis

The condition is thought to occur due to both immediate (Type 1) and delayed (Type 4) hypersensitivity reactions to Culicoides salivary proteins. The female Culicoides midge lands on the host and crawls down the hairs to the skin surface, using its mouthparts to pierce the skin. Saliva is injected into the site containing vasodilators, anticoagulants and pro-inflammatory mediators. Allergen-specific IgE antibodies are produced in repsonse by susceptible animals, leading to mast cell degranulation and the release of histamine and many other inflammatory mediators.

Clinical signs

The cardinal sign of Sweet itch is extreme, intense pruritus with characteristic distribution patterns. Commonly affected areas include the head, mane, tail, withers and ventral abdomen. Primary lesions are papules but self-excoration may lead to the characteristic lesions of sweet itch including alopecia, broken hairs, erosions, ulcers. Lesions are frequently painful and In the chronic stages, lichenification, hyperkeratosis and scaling are common features.

Diagnosis

Diagnosis is often based on the animal's history, seasonal clinical signs and the site and nature of the lesions. Other important differential diagnoses that should be ruled out include psoroptic and sarcoptic mange, other insect-related and dermatophilosis

References

• Wilson, A. D., Harwood, L. J., Björnsdottir, S., Marti, E., Day, M. J. (Detection of IgG and IgE serum antibodies to Culicoides

salivary gland antigens in horses with insect dermal hypersensitivity (sweet itch) Equine Veterinary Journal 33 (7)707-713