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==Introduction==

 

Culicoides hypersensitivity, commonly referred to as 'sweet itch' is an intensely pruritic dermatitis of horses caused by a [[Hypersensitivity - Introduction|hypersensitivity]] reaction to the saliva of biting insects. The most important causative agent is the ''[[Culicoides]]'' midge but other insects including [[Simuliidae|''Simulium'' spp.]], [[Stomoxys calcitrans|''Stomoxys'' spp.]] and [[Haematobia irritans|''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.  

==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==

==Signalment & risk factors==

==Pathogenesis==

==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.

The condition is thought to occur due to both [[Type I Hypersensitivity|immediate (Type 1)]] and [[Type IV Hypersensitivity|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 response by susceptible animals, leading to mast cell degranulation and the release of histamine and many other inflammatory mediators.

==Clinical signs==

==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.   

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==

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.  

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 Mange|psoroptic]] and [[Sarcoptic Mange|sarcoptic mange]], other insect-related skin conditions and [[Dermatophilosis - Horse|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.

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.

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.

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==

==References==

*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''

*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''

[[Category:Allergic Skin Diseases]][[Category:Dermatological Disorders - Horse]][[Category:To Do - Clinical]]

 

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[[Category:To Do - Review]]

 

[[Category:Allergic Skin Diseases]][[Category:Dermatological Diseases - Horse]]

[[Category:Type IV Hypersensitivity Diseases]][[Category:Type I Hypersensitivity Diseases]]

[[Category:Integumentary System - Hypersensitivity Reactions]]

[[Category:Expert Review]]