Chronic small airway disease in the horse: immunohistochemical evaluation of lungs with mild, moderate and severe lesions.

The peroxidase anti-peroxidase technique was used to demonstrate free and intracellular immunoglobulin (Ig) within the lungs of 23 horses with chronic small airway disease. Histologically, all the horses had chronic bronchiolitis; however, the lesions varied in degree from mild in eight horses, to moderate in nine horses and severe in six horses. Lungs from three horses which had no gross or histopathological lesions were used as controls. In comparison with control horses, horses with mild chronic bronchiolitis had increased numbers of Ig A-containing and non-immunoglobulin staining cells around the vasculature and bronchioles. As the severity of the lesions increased so did the number of IgA-, IgG(Fc)- and in several cases non-immunoglobulin staining cells around the vasculature, bronchioles and in the alveolar septa. In severely affected horses, large amounts of free IgG(Fc) were observed interstitially and in alveoli. In areas of mucosal epithelial hyperplasia and metaplasia large amounts of free IgA and IgG(Fc) were sometimes observed interepithelially in a pattern which differed from that in control horses.