Guttural pouch mycosis in a Quarter Horse filly
Emily Rätsep, Jessica Richards
Animal Health Laboratory, University of Guelph, Kemptville, ON (Rätsep); MacDonald-Richards Equine Service (Richards).
AHL Newsletter 2024;28(2):24.
Early in April, a Quarter Horse filly was presented for pyrexia, purulent nasal discharge, right-sided swelling of the head, and a cough of 1 week duration. At that time, the right ear and upper eyelid had a notable droop. Testing of the nasal discharge was both culture and PCR positive for Streptococcus equi subsp. zooepidemicus. Treatment was initiated, and the pyrexia and lethargy resolved within 5 days. As part of follow up, an endoscopic exam of the guttural pouches was performed, during which large plaques of purulent debris were visualized in the right guttural pouch near the opening. As there was minimal improvement with treatment, referral was initiated. However, prior to transfer, there was acute severe hemorrhage from the nostrils and mouth, resulting in sudden death of the filly.
At necropsy, there was marked hemorrhage present within the nasal passages and guttural pouches bilaterally, as well as within the tracheal lumen and the stomach. The guttural pouches were dissected, and an extensive tan-yellow plaque was present, effacing approximately 80-85% of the mucosal surface of the right guttural pouch (Fig. 1). The plaque both effaced and obscured the right internal carotid artery, right external carotid artery and the cranial nerves in the right guttural pouch (hypoglossal, glossopharyngeal, vagus and accessory nerves). Necrosis was extensive, particularly within the right guttural pouch, but also extended through the septum with a plaque visible within the left guttural pouch on its medial aspect. On sectioning into tissue beneath the plaques, areas of dark red to black discoloration extended into the underlying muscle, connective tissue and the right stylohyoid bone subdividing the lateral and medial portions of the right guttural pouch.
Histologically, hemorrhage frequently dissected necrotic areas. In a section taken from the septum between the right and left guttural pouches, there was full-thickness necrosis of the tissues underlying the plaques. The plaques were composed of necrotic cellular debris, a mixed inflammatory cell infiltrate, and dense colonies of coccobacilli (Fig. 2A). These areas were overlaid by thick mats of fungal hyphae that appeared histologically consistent with Aspergillus spp. (Fig. 2B).
The postmortem findings confirmed a diagnosis of guttural pouch necrosis with severe hemorrhage as the cause of death in this horse. Based on the presence of the large necrotic areas of mucosa combined with dense plaques of friable, dark yellow-green material, a fungal cause was suspected grossly and was further confirmed histologically. Guttural pouch mycosis is a relatively rare disease of the upper respiratory tract in horses. Although there is no apparent predisposition to development of guttural pouch mycosis, it is most commonly reported during the warmer months in stabled horses in temperate climates.
While guttural pouch mycosis is usually unilateral, extension from one guttural pouch to another is possible, as occurred in this case. The underlying cause is uncertain, although it is suspected that the fungal infection occurs opportunistically following damage that occurs due to trauma, local inflammation and/or a primary bacterial infection. Following injury, damage to the mucosal barrier results in an environment hospitable to colonization by a secondary fungal pathogen, resulting in invasion into deeper tissues, including the underlying vasculature and nerves. A predilection towards forming on the roof of the medial compartment of the guttural pouch in close anatomic association with the carotid arteries and various cranial nerves explains the various reported clinical signs that include: facial paralysis, Horner’s syndrome, sweating, shivering, nasal discharge, dysphagia, abnormal swelling of the head and epistaxis. Invasion of the carotid arteries by fungal hyphae can result in fatal hemorrhage.
In this case, it is suspected that an initial bacterial infection (i.e., S. equi subsp. zooepidemicus) may have resulted in localized trauma and secondary fungal colonization. Unfortunately, this pathogenesis is challenging to confirm as chronic changes will often obscure the initial inciting causes. As there are multiple potential causes of nasal discharge in horses, including Streptococcus equi subsp. equi (which is now an immediately notifiable disease in Ontario), this case highlights the importance of using multiple cooperative methods of investigation, such as endoscopy, bacterial culture, PCR, and histology when working up a case. AHL
Figure 1. Opened guttural pouch, equine. The guttural pouch contains clotted blood and a friable tan-yellow plaque overlies the mucosa.
Figure 2. Histologic sections of affected guttural pouch. A. Area of necrosis underlying fungal mat (*), with colonies of bacteria (X). H&E stain, 20X. B. Area of necrosis overlaid by PAS-staining fungal mat with histologic features suggestive of Aspergillus spp. There is some invasion of the underlying necrotic tissue by fungi (<). PAS stain, 40x.
References
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