Guttural Pouch Mycosis

Bonnie L. Louque

Dr. H. Ame Walesby

The guttural pouch is a structure unique to the horse. It is an air filled pouch off of the Eustachian tube, with openings into both the middle ear and throat, thus it serves as a communication between these two structures. There is a left and right guttural pouch and each is divided into medial and lateral compartments. The medial compartments oppose each other on the midline of the skull. Major arteries carrying blood to the head and several nerves innervating the face, throat, and head are intimately associated with each guttural pouch, separated by only a thin membrane comparable to wet tissue paper. Problems arise when there is an entrapment of pus (empyema), or air (tympany), or when there is an infection within the guttural pouch. Horses with guttural pouch abnormalities will show signs related to the disruption of nerves and arteries within the pouch.

Fungal colonization of the guttural pouch is called guttural pouch mycosis (GPM). The most common fungus isolated from horses with GPM is Aspergillus spp. This fungus is extremely common in nature and frequently found in hay bales and barns. Researches believe the fungus is inhaled as a small, immature form called a spore, which can be thought of as a tiny package of fungus that can grow up into a new fungus when opened. After inhalation, these spores gain access to the guttural pouch through the slit-like openings located in the throat. Aspergillus spp probably does not cause GPM, but rather is a secondary condition to some unknown disruption within the guttural pouch. It can be compared to a cut you get on your arm that later becomes infected: the bacteria (or fungus) causing the infection did not cause the cut, it just took advantage of the situation by inhabiting the underlying tissue, after the skin barrier was cut open. In other words, without a cut there would be no infection. It is unclear how or why the guttural pouch suddenly becomes susceptible to a fungal infection. Factors that have been associated with cases of GPM are poor ventilation, high humidity and warm temperatures. These factors favor the germination or opening of spores, but would not lead to an infection unless there was a preexisting disturbance within the guttural pouch.

Once the fungal spores become established within the guttural pouch, their presence leads to an intense inflammatory reaction by the body. It results in a buildup of pus and debris that form large plaques within the guttural pouch. These fungal plaques are the accumulation of fungi, pus, and other substances produced by the body. The infection is usually confined to one guttural pouch, but occasionally can be bilateral. Plaques can be large and widespread or small isolated nodules and are a variety of colors, ranging form a mixture of yellow and white, to brown and green. The fungal plaques gradually erode the wall of the large arteries found within the guttural pouch resulting in bleeding of the affected vessel. The most common clinical presentation of a horse with GPM is intermittent bleeding from one nostril. The opening of the guttural pouch into the throat allows the blood to drain from within the guttural pouch out through the throat and into the nasal passage. The internal carotid artery is the most commonly affected vessel. The bleeding can be mild or severe and is controlled early in the course of the disease by clot formation within the affected vessel. Episodes of bleeding will progressively become more severe as the fungal plaques continue to erode the vessel wall. Complete erosion of the vessel wall can result in the animal bleeding to death (exsanguination). Exsanguination usually occurs within 2-3 weeks of the initial episode of bleeding.

GPM can also interfere with the horse’s ability to swallow food and water. Swallowing requires that the base of the tongue and throat move food from the mouth into the esophagus, while the epiglottis guards the opening of the trachea. These structures do not function adequately with GPM. The nerves carrying signals from the brain to this area lie within the guttural pouch and subsequently become inflamed and loose normal function with GPM. The degree of dysfunction varies with the extensiveness of the infection and may or may not be reversible with treatment. Horses with laryngeal dysfunction secondary to GPM frequently develop aspiration pneumonia because of inadvertent passage of food into the trachea and lungs following attempts to swallow. Inspiratory noise and exercise intolerance may also be present. Other clinical signs are abnormal head posture, head shaking and blindness.

A diagnosis of GPM is made through accurate history, clinical signs, and visualization of the characteristic fungal plaques within the guttural pouch using an endoscope. Due to the immediate threat of exsanguination with GPM, surgical occlusion of the affected vessels warrants the best prognosis. Medical therapy, both topical and systemic, has proven to be ineffective in preventing fatal hemorrhage when used alone. There have been cases of spontaneous recovery of GPM with medical therapy alone, but this is rare.

In summary, GPM is condition of horses that demands immediate veterinary attention. Horse owners should be aware that the time between the first episode of epistaxis and exsanguination is usually about three weeks. Prognosis to complete recovery depends on successful surgical ligation of the affected artery and resolution of the concurrent clinical signs, such as the inability to swallow. Aspiration pneumonia carries a worse prognosis because of the difficulty in successfully treat this condition. Following surgery, supportive care is needed until all signs of abnormal nerve function have resolved. Unfortunately, this may take several months and not all horses return to the level of function prior to the infection.