Nodular diseases

The development of a nodule is a fairly common reaction pattern in the skin and subcutaneous tissues of the horse. In many such nodules, the inciting aetiologic agent or antigen is unknown, but in some equine cases there is significant evidence for insect bite origin. The pathogenesis of some equine nodular diseases appears to involve marked collagen degeneration which varies from hyalinization to lysis and can become mineralized. The exact role the lytic collagen has in the pathogenesis of these lesions is still unknown. Vascular changes may be present in some entities. Other nodules may be caused by or complicated by the larva of Habronema spp. The clinical or histological entities outlined in this section include the collagenolytic granuloma, axillary nodular necrosis, unilateral papular dermatosis and cutaneous habronemiasis.