Guttural pouch mycosis in a 6-month-old filly

Abstract

A 6-month-old filly was presented with unilateral epistaxis. Based on clinical signs, endoscopic examination, and postmortem examination, guttural pouch mycosis was diagnosed. The young age of the filly and the fact that this was the 2nd diagnosis of guttural pouch mycosis on this farm was unusual.

A 6-month-old quarter horse filly was presented to the Veterinary Teaching Hospital at the Western College of Veterinary Medicine with a history of unilateral epistaxis. The owners had noted intermittent episodes of blood dripping from the horse’s left nostril for 2 d, but no major episodes of bleeding had been observed. The filly had been weaned 1 mo previously and was currently on winter pasture. She had no history of trauma or diagnosed medical problems, had not yet been vaccinated or dewormed, and came from a herd of 9 horses that produced 1 to 2 foals a year. On this farm 45 mo earlier, an 18-month-old quarter horse filly had presented with bilateral epistaxis and had been diagnosed with left guttural pouch mycosis due to an Aspergillus sp, based on endoscopy and postmortem examination.

On arrival at the Veterinary Teaching Hospital, a large amount of fresh blood was noted in the trailer and the filly had dried blood on her muzzle. She was severely depressed, had a heart rate of 56 beats/min, a respiratory rate of 16 breaths/min, and a rectal temperature of 37ºC. Her capillary refill time was 1.5 s; mucous membranes, pink; packed cell volume, 22%; and serum total solids, 5.1 g/L, as measured by refractometry. No cranial nerve abnormalities or dysphagia were noted, and the rest of the physical examination was normal.

On endoscopic examination, a blood clot was seen protruding from the left pharyngeal orifice and aspirated clotted blood was observed in the trachea. The right guttural pouch was explored first and found to be normal in appearance. The left guttural pouch was found to be filled with clotted blood: The lateral wall of the medial compartment and the roof and lateral wall of the lateral compartment were covered by a dark red to black plaque, consisting of blood and necrotic debris, which obscured the normal anatomy. Also, white to tan masses were seen protruding from this plaque (Figure 1). The source of hemorrhage was not identified. At this point, the clinical diagnosis was left guttural pouch mycosis.

Figure 1.

Endoscopic view of the dorsocaudal wall of the left guttural pouch. The lateral compartment is covered by a dark red to black diphtheritic membrane and is filled with blood. A mycotic plaque can be seen overlying the stylohyoid bone and the dorsomedial section of the lateral compartment.<br>(Courtesy of Sameeh Abutarbush, WCVM, now at the Atlantic Veterinary College)

Due to owner constraints, surgery was not performed, and medical therapy was not pursued due to the poor prognosis following the major bleed that had occurred while trailering. The filly was euthanized and submitted for postmortem examination. Histologically, nonseptate, acutely branching fungal hyphae were seen throughout the sections and in association with the wall of a blood vessel (Figure 2); however, the actual site of vascular rupture was not seen. The final pathological diagnosis, which confirmed the clinical diagnosis, was mycotic guttural pouch infection with fungal hyphae compatible with an Aspergillus sp. Culture was not pursued to confirm the suspicion of an Aspergillus sp.

Figure 2.

Histopathological section of the roof of the medial compartment of the left guttural pouch stained with special Grocott’s stain. Note the fungal hyphae evident in all layers of the guttural pouch wall.<br>(Photo courtesy of Dr. Gary Wobeser, WCVM). Bar = 250 μm.

Guttural pouch mycosis (GPM) is a rare, fungal disease of the upper respiratory tract of horses. Classically, it is seen as a unilateral, but occasionally bilateral, epistaxis that begins as minor bleeding episodes but usually progresses to fatal hemorrhage within days to weeks of the initial hemorrhagic episode (1,2). Guttural pouch mycosis shows no predilection for either the right or the left guttural pouch, age, sex, or breed (1). It appears to occur sporadically and has a worldwide distribution (2,3). Presently, the pathogenesis has not been ascertained; however, it has been speculated that the mucous membrane layer of the guttural pouch is disrupted by trauma, local inflammation, a primary bacterial infection, or all 3. This disruption allows opportunistic fungi that are present in the normal equine airway, such as Aspergillus spp, to invade into the deeper tissues, including local arteries and nerves (4).

The clinical signs of GPM can be explained by the fact that fungal growth, and the inflammation associated with it, has a predilection for the roof of the medial and, occasionally, the lateral compartments of the guttural pouch. This area is anatomically associated with the external and internal carotid artery, internal maxillary artery, glossopharyngeal nerve (cranial nerve [CN] IX), vagus nerve (CN X), spinal accessory nerve (CN XI), sympathetic nerves, and stylohyoid bone (1). Due to this close association, a horse with GPM can show signs of epistaxis, dysphagia, parotid pain, abnormal head posture, nasal discharge, head shyness, abnormal respiratory noise, sweating and shivering, Horner’s syndrome, colic, and facial paralysis (1,2). Furthermore, GPM may be asymptomatic and only diagnosed, as an incidental finding, during endoscopic examination of the guttural pouches (2).

Although unilateral epistaxis was the main presenting complaint in this case, epistaxis is not pathognomic for GPM. Differential diagnoses for a horse with epistaxis include exercise-induced pulmonary hemorrhage, ethmoid hematoma, guttural pouch or pharyngeal neoplasia, and tracheobronchial foreign body. Differentiating these diseases is best achieved through endoscopic examination. On endoscopic examination, classically, the mycotic lesion involves the roof of the medial compartment and may extend into the pharyngeal recess and laterally to the roof of the lateral compartment (1,2). Blood at the pharyngeal orifice of the guttural pouch indicates that guttural pouch hemorrhage has occurred within the previous 3 to 5 d (3), which, in this case, was consistent with the history provided. Although not possible in this case, identification of the artery from which any hemorrhage is originating is key, before proceeding with surgical therapy, to ensuring occlusion of the appropriate vessel (5). Radiographs and clinical pathologic analyses have been determined to be of little value, as radiographic change with GPM is minimal and clinical pathologic analyses typically will show an anemia, only if a recent significant bleeding episode has occurred (3).

Gross pathologic examination of the mycotic guttural pouch characteristically reveals a yellow-brown to black mottled dry diphtheritic membrane with dry, dull white fungal plaques growing on it (1). This membrane and fungal plaques are typically adhered to the tissues of the roof of the medial pouch and found in association with the internal carotid artery, with possible extension onto the roof of the lateral pouch and ventrally to the stylohyoid bone (1,4). Infrequently, as in this filly, the diphtheritic membrane and fungal plaques are principally associated with the lateral pouch and the external carotid artery or the maxillary artery (5). On histopathologic examination, interwoven septate hyphae should be seen infiltrating the wall of the guttural pouch. These hyphae can be seen invading, with reducing density, into the deeper tissues, including local arteries and nerve fibers. An area of major vessel thromboarteritis, erosion, aneurysm, and hemorrhage may be seen along with inflammation and fungal infiltration into surrounding nerves (4).

The filly in this case was euthanized, as opposed to leaving her untreated, due to the poor prognosis following her major bleeding episode, as over 50% of untreated horses die due to a fatal hemorrhage (6). However, rare occurrences of spontaneous recovery have been reported (1,2). Medical and surgical treatment options have been attempted in cases of GPM with variable results. Medical treatment consists of infusing topical antifungal agents through a catheter into the guttural pouch; this may be combined with systemic antifungal agents. Antifungal agents employed in the treatment of GPM have included topical aqueous iodine solutions, intravenous iodides, oral and topical thiabendazole, topical nystatin powder, and topical irrigation solutions containing 1% gentian violet and 6% neomycin, with or without iodine (7). Results with antifungal therapy have been variable to poor, and medical treatment alone carries a poor prognosis for prevention of a fatal hemorrhagic episode (8).

Surgical treatment is currently the preferred therapy and consists of occluding the major vessel, often the internal carotid artery, that is feeding the mycotic plaque, leading to clot formation at the vascular lesion and prevention of a major hemorrhagic episode. Originally, occlusion was accomplished by ligating both the cardiac and cerebral sides of the lesion; however, ligating the cerebral side of the lesion proved difficult in many cases, and in the 1980s, placement of a balloon-tipped catheter into each of the affected arteries was developed (9). This procedure has resulted in a good to excellent prognosis for preventing fatal hemorrhage with relatively infrequent complications, such as recurrence of moderate guttural pouch hemorrhage, iatrogenic Horner’s syndrome, retrograde infection, blindness, and incisional complications (5,8). Recently, the use of detachable, self-sealing latex balloons and coil embolization have been proposed as new procedures for occluding the arteries, with a lower reported complication rate (10). In general, the prognosis after surgery is good to excellent, with regression of mycotic lesions within 2 to 4 mo; however, if dysphagia and other neurologic signs are present prior to surgery, the prognosis for resumption of normal neurological function is guarded (7,8).

This case presented an uncomplicated diagnosis of guttural pouch mycosis; however, it was unusual in that it occurred in a horse only 6 mo old, although the literature includes reports of a 2-month-old colt, a 3-monthold colt, and 2, 6-month-old fillies being affected by GPM (2,11), and that it was the 2nd case of GPM on this farm.

Two cases of GPM on the same farm has not been reported previously. According to the owners, there was no genetic relationship between the 2 horses and the 1st case of GPM was euthanized nearly 3 y before the filly described in this report was born. Furthermore, the filly reported in this case was raised on a summer pasture that the owners had not owned, when the 1st horse acquired its infection, and had spent only its last month on the winter pasture that both horses had grazed on. It is difficult to speculate how both these 2 horses might have acquired the GPM, as there is little information available on the pathogenesis of GPM. It has been suggested that dusty barns may add to the environmental load of Aspergillus spp. that infected horses have been exposed to (4); however, individual susceptibility to GPM must exist for GPM to maintain its sporadic nature. On this farm, the 2 cases of GPM were likely coincidental; however, further investigation into the pathogenesis of GPM should be pursued.