The challenges of treating guttural pouch mycosis in horses have been described by Nathan Slovis, who is director of the McGee Center at the Kentucky-based Hagyard Equine Medical Institute.
Airway dysfunction, which can be acquired or developmental, is one of many causes of poor performance in equine athletes, Slovis writes in the latest issue of Equine Disease Quarterly.
One acquired dysfunction that is generally overlooked is a fungal infection of the guttural pouch, known as guttural pouch mycosis.
Guttural pouch mycosis is a potentially life-threatening disease that may initially present as a simple nosebleed or even poor performance in an equine athlete.
Clinical signs include noses bleeds, affecting either one side or both sides; a one-sided nasal discharge, dorsal displacement of the soft palate, difficulty swallowing, and partial or total paralysis of the larynx.
The nose bleeds occur 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 blood loss.
Difficulty swallowing, as well as various forms of pharyngeal dysfunction, arising from guttural pouch mycosis result from mucosal penetration and damage by the fungus to nearby nerves.
A variety of fungi have been isolated from guttural pouch mycosis, with Aspergillus species being common. Aspergillus species 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 old.
Difficulty in swallowing is the second most common clinical sign associated with the condition, after nose bleeds.
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 difficulty in swallowing or other neurological deficits at presentation indicates a poor prognosis.
The severity of cranial nerve deficits depends on whether nerve injury is restricted to local neuritis or necrosis of the nerve secondary to fungal infiltration and fibrosis.
Treatments include both medical and surgical options.
While there are reports of successful medical treatment with systemic and topical antifungal medications, medical treatment is generally considered to be less efficacious than surgical treatment.
Topical treatments with enilconazole and miconazole have been used with success in some cases.
The presence of a diphtheritic membrane (necrotic material over the surface of the fungal plaque) may prevent penetration of systemic antifungals.
Additionally, there is an increased risk of fatal hemorrhage due to the longer course of medical treatment, Slovis writes.
The preferred method to treat severe guttural pouch mycosis is to surgically insert a coil or balloon into the affected blood vessel to quickly cut off the blood supply (see the x-ray image above).
Typically, once the blood supply has been removed, the fungus regresses.
Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.