Airway dysfunction is one of many causes of poor performance in equine athletes. Airway dysfunction can be classified as either acquired or developmental. One acquired dysfunction that is generally overlooked is a fungal infection of the guttural pouch, known as guttural pouch mycosis. Guttural pouch mycosis (GPM) is a potentially life-threatening disease that may initially present as a simple nosebleed (epistaxis) or even poor performance in an equine athlete.

GPM has been documented in various parts of the United States, particularly in the southeast, as well as the United Kingdom. Clinical signs include unilateral (one-sided) or bilateral (bothsided) epistaxis, unilateral nasal discharge, dorsal displacement of the soft palate, dysphagia (difficulty swallowing), and laryngeal hemiplegia (partial or total paralysis of the larynx). Epistaxis occurs due to erosion of the guttural pouch mucosa by a fungal plaque, resulting in hemorrhage from the internal carotid, occipital, or maxillary arteries. Severe hemorrhage may result in rapid death from exsanguination. Dysphagia as well as various forms of pharyngeal dysfunction in GPM result from mucosal penetration and damage by fungal hyphae to the glossopharyngeal, hypoglossal, and/or the pharyngeal branches of the vagus nerves.

A variety of fungi have been isolated from GPM with Aspergillus spp. being common. Aspergillus spp. and other opportunistic fungi are found naturally in the environment and can be found in the upper respiratory tract of normal horses. Wet environmental conditions, prolonged treatment with antimicrobials, an immunocompromised host, or use of corticosteroids have been proposed as factors conducive for fungal overgrowth. It is not fully understood why mycosis occurs in animals that are not immunosuppressed or debilitated. While no age predilection has been reported, the disease is more common in mature horses, but has been documented in foals as young as two months of age.

Dysphagia is the second most common clinical sign associated with GPM, after epistaxis. The cranial nerves within the guttural pouch that innervate the pharynx are the glossopharyngeal, vagus, and hypoglossal. Neurological dysfunction of the pharynx results when the nerves within the guttural pouch are inflamed or fibrosed. Generally, the presence of dysphagia or other neurological deficits at presentation indicates a poor prognosis. The severity of cranial nerve deficits depends on whether nerve injury is restricted to a local neuritis or necrosis of the nerve secondary to fungal infiltration and fibrosis