The guttural pouches are predisposed to fungal infections (mycosis). Infection can occur in one or both pouches. Each pouch is divided into two compartments (lateral and medial) by the stylohyoid bone and occipitohyoideus muscle. The medial compartment has a prominent fold that houses critical structures, including the internal carotid artery, Cranial nerves IX, X, XI, and XII, the cranial cervical ganglion, and the post-ganglionic sympathetic nerve. It is this location in the top of the medial compartment that is most susceptible to fungal infection, although in approximately one third of cases the fungal lesions may develop in the smaller lateral compartment. In the lateral compartment the fungi may attach to the external carotid artery or Cranial nerve VII (facial nerve).
There is no apparent age, sex, breed or geographic predisposition. Infection has been seen in very foals (from 2 months of age). The most common fungi that are involved are Aspergillus fumigatus and Emericella nidulans (previously Aspergillus nidulans). Lesions are composed of necrotic tissue, fungi and bacteria. They are typically described as ‘diphtheritic’ membranes, meaning the formation of a false adherent membrane to damaged underlying epithelium.
Signs of infection
The disease typically starts out with a nasal discharge (typically one-sided) and head shaking. This may be followed by epistaxis (blood emerging from the nostril). The blood is caused by erosion of fungi through the wall of the internal, or less commonly, the external carotid arteries. Being arterial the blood is typically bright red, can be high volume, and as such can be life-threatening. There may be a number of neurologic signs that are caused by fungal damage of nerves. These include dysphagia (inability to prehend and swallow feed) with feed particles and saliva appearing at the nostril, facial paralysis, paralysis and atrophy of the tongue, stridor, and change in vocalization.
Diagnosis and Treatment
The diagnosis is based on clinical signs and endoscopy.
Guttural pouch mycosis is considered to be a life threatening disease, due to the risk of sudden haemorrhage from major arteries of the head. The treatment of choice is artery occlusion on both sides of the lesion. This can be achieve using embolization coils or placement of nitinol (nickel and titanium alloy) plugs. This reduces the risk of fatal haemorrhage and typically reduces the size of the fungal lesion, although this does not always happen. Rarely occlusion of the artery may cause blindness on the same side.
Lesions can also be treated with topical antifungals, including povidone-iodine (betadine), enilconazole, voriconazole or itraconazole. Itraconazole can also be administered orally. Nystatin, natamycin, fluconazole, or miconazole are not considered to be effective against the more common fungal isolates.
Unfortunately neurologic signs, if present, may not be reversible, or take months to improve. This is problematic for horses that cannot effectively swallow. Feed aspiration into the airway is common.
Image credits: Prof David Freeman, UF
Tags: Respiratory; Neurology