Aspergillus spp. infections of the airway fall into three basic categories: 1. Destructive lesions of the nasal cavity, paranasal sinuses and guttural pouch 2. Proliferative lesions of the nasal cavity, paranasal sinuses and guttural pouches

Fungal-related lung disease can be the result of direct lung infection (fungal pneumonia/"‘Farmers Lung") but by far the most common implication of fungal spores is the disease known as Recurrent Airway Obstruction (Chronic Obstructive Small Airway Disease/Small Airway Disease/Equine COPD). In this case the affected animal is believed to show a hypersensitivity response to certain species of fungal spores; the fungus is acting as an allergen and not an infectious pathogen. This disease is not considered in this section. Focal fungal granuloma sometimes occur in the lung parenchyma and these may be single or multiple. The clinical consequences vary with location, numbers, severity and, of course, with the pulmonary demand of the individual horse.