Injection site eosinophilic granulomas and collagenolysis in 3 horses

Three horses were presented with a history of having developed raised cutaneous nodules, within 24-48 hours, in areas of previous injections using standard silicone-coated hypodermic needles. Skin biopsies were taken from a selected cutaneous nodule from all horses for histopathologic evaluation. Histologically, the nodules were consistent with a diagnosis of equine eosinophilic granuloma. A hypersensitivity reaction to the silicone, or another component of the coating formulation, was hypothesized to be responsible for these lesions. Two horses were experimentally injected using both coated and noncoated stainless steel hypodermic needles and skin biopsies were obtained 14 days after injection. The sites of the coated needle injections were characterized by severe eosinophilic granulomatous inflammation with and without collagenolysis. The eosinophilic granulomas with and without collagenolysis observed in these horses are proposed to represent a complex immunologic response to the silicone-based coating of most hypodermic needles.