A report on clinical aspects and histopathology of sweet itch.

This study explored the characteristics and histopathology of sweet itch, a recurring skin condition in horses in Ireland, which appears similar to hypersensitivity reactions to the saliva of certain insects. Histology of the reactions to intradermal injections of Culicoides extract revealed notably enlarged blood vessels and movement of eosinophil cells.

Understanding Sweet Itch

• Sweet itch is identified as a chronic skin inflammation in horses that typically occurs annually from April to November in Ireland. It is heavily linked to allergic reactions in the skin, mainly resulting from the saliva of specific insects that feed on blood.
• The study finds key similarities between the tissue changes observed in sweet itch and those seen in immediate-type hypersensitivity reactions, which are immune responses to allergens.

Observations and Findings

• In the study, the skin condition was identified by subepidermal oedema (fluid retention underneath the skin), prominent eosinophilia (an increase in eosinophil cells typically as a reaction to allergies or parasitic infections), and enlarged, twisted blood vessels.
• However, microfilaria—larvae of parasitic worms—were not found within the examined sections obtained from 5 horses experiencing sweet itch. This is counter to the usual expectation of finding these parasites in skin irritations related to insect bites.

Challenge Tests and Responses

• The histopathology (study of changes in tissues caused by disease) of the immediate skin reaction following an intradermal injection of Culicoides extract (insect saliva) demonstrated dilation of dermal blood vessels and the migration of eosinophil cells.
• Seven horses were tested with an intradermal injection of whole extracts from Culicoides, Stomoxys calcitrans, Tabanidae along with saline. Of these, all seven showed immediate reactions to the Culicoides extract, while 3 had similar reactions to Stomoxys calcitrans.
• Immediate as well as delayed reactions were only noted at the site where Culicoides extract was used, suggesting the unique allergenic potential of this specific insect.
• Conversely, three normal ponies, when tested with the same insect extracts, demonstrated just minimal skin reactions. This could imply a special sensitivity or predisposition in certain horses to react to these insect stimuli.