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 Culicoides spp 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.

Pathogenesis

The condition is thought to occur due to both immediate (Type 1) and delayed (Type 4) hypersensitivity reactions to Culicoides salivary proteins. The proteins are injected during feeding and IgE antibodies are produced in repsonse by susceptible animals.

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

Treatment

• Grossly:s
  • Papules, crusts, alopecia, excoriations, lichenification
• Microscopically:
  • Superficial and dep perivascular dermatitis
  • Many eosinophils
  • Epidermal hyperplasia
  • Hyperkeratosis
  • Dermal fibrosis
  • Possibly eosinophilic folliculitis, intraepidermal pustules and eosinophilic granulomas