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 Simulium spp, 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 & risk factors

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. Clinical signs may develop in older horses if moved to pasture area with a heavier insect burden. 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 inhabit marshy areas or still water and horses kept in fields near streams or water may be at higher risk of developing sweet itch. Additionally, in order to breed successfully the midges require decaying manure and bedding, therefore poor hygiene increases the risk of 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 usually increase in severity with age. 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 skin conditions and dermatophilosis. Allergy testing including serology for IgE and intradermal allergy testing and may be used in combination with the horse's history and clinical presentation to aid diagnosis. However the sensitivity and specificity of these tests is relatively low and they should not be used as a sole method of diagnosis.

Treatment

The aims of treatment are to control pruritus, treat any secondary bacterial infections and reduce the number of midges coming into contact with the affected horse. In many cases, appropriate environmental changes are sufficient to ease clinical signs. Anti-midge rugs are available and prevent the insects from landing and feeding and are most effective when used in conjunction with an insect repellant containing citrus extract or DEET. Access to marshy pasture or fields near open water should be prevented and horses should be stabled at night when midges are most active. Fans and anti-fly mesh may be provided in the stable to provide additional protection from midges.

In severe cases, further treatment using corticosteroids to reduce inflammation and pruritus may be required. These should be used with caution in order to avoid possible side-effects including laminitis and iatrogenic hyperadrenocorticism. Antihistamines may be beneficial as a more long-term method of control but are only useful if administered before the onset of pruritus. Some authors have reported successful treatment using hyposensitisation in which a vaccine is formulated using antigens selected on the basis of intradermal allergy testing. Omega 3 and 6 fatty acids incorporated in the diet may aid in reducing pruritus and controlling inflammation.

References

• Higgins, A., Snyder, J. R. (2006) The Equine Manual Elsevier Health Sciences
• Lavoie, J. P., Hinchcliff, K. W. (2009) Blackwell's Five Minute Veterinary Consult: Equine John Wiley and Sons
• Wilson, A. D., Harwood, L. J., Björnsdottir, S., Marti, E., Day, M. J. Detection of IgG and IgE serum antibodies to Culicoide salivary gland antigens in horses with insect dermal hypersensitivity (sweet itch) Equine Veterinary Journal 33 (7)707-713