Horses may develop nodular lesions that may or may not be exudative. Lesions may occur anywhere on the body at any time of the year but most commonly at or around the time when there are biting and flying insects. Lesions may be singular and or multiple. Lesions may or may not be pruritic or painful. Lesions may also be triggered by trauma. Histology reveals multifocal areas of collagen fibers surrounded by granulomatous inflammation containing eosinophils.
Solitary or mineralized lesions are often most easily treated by surgical excision. The lesions are very responsive to glucocorticoids, though risks of glucocorticoids and development of laminitis should be considered. Traditional recommendations are: triamcinolone acetonide (3–5 mg/lesion), with no more than a total of 20 mg administered sublesionally at any one time because of the potential to induce laminitis. Horses with multiple lesions may be treated with oral prednisone or prednisolone at 1 mg/kg/day for 2–3 weeks. In horses with recurrent lesions, intradermal allergy testing, particularly with insect antigens, is recommended. Hyposensitization and insect control can be palliative in some cases.
Also see pet health content regarding eosinophilic diseases in horses.