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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2020) 32 (11) 576-579 doi: 10.1111/eve.13146
Clinical Commentary Diagnosis and treatment of cryptococcal infections in horses
A. J. Stewart* School of Veterinary Science, Equine Specialist Hospital, University of Queensland, Gatton, Queensland, Australia
*Corresponding author email:
allison.stewart@
uq.edu.au Keywords: horse; respiratory; fungal; granuloma; cryptococcus
In this issue of Equine Veterinary Education Daniel et al. present a case of ulcerated nasopharyngeal fungal granulomas in a mare that was ultimately diagnosed as Cryptococcus terrestris (Daniel et al. 2020). Although fungal rhinitis is a relatively rare cause of mucopurulent or serosanguinous nasal discharge, it is not uncommon for these cases to have a protracted history with a failure to respond to inappropriate empirical antimicrobial therapy. Obtaining the correct diagnosis is integral to successful therapy. This horse had a 4-month history of mucopurulent nasal discharge that was unresponsive to multiple courses of antimicrobials and sodium iodide. By the time of referral, there was weight loss, almost complete occlusion of the nasal passage and respiratory stertor (Daniel et al. 2020). Costly treatment failure associated with the inappropriate use of antimicrobials justifies the early referral of such cases to internal medicine specialists, who have the specific knowledge to perform appropriate targeted diagnostics to obtain a definitive diagnosis. There was likely a suspicion of fungal or other granulomatous disease by the referring veterinarian because 7 days of sodium iodide had been administered (Daniel et al. 2020). However, iodides are considered a substandard method to treat fungal disease. Although upper airway endoscopy (Fig 1) and standing
sinus radiographs are ideal to image the extent of the lesions, a tissue sample is required for definitive diagnosis. For nasopharyngeal lesions, specimens for culture, cytology and histopathology can theoretically be obtained by use of an endoscopic biopsy instrument; however, these samples tend to be superficial and are frequently nondiagnostic. Small endoscopic biopsies are often limited to the surface discharge and mucosa, with commensal species frequently cultured. Only mixed bacterial cultures were obtained using a 2.3 mm biopsy instrument from the ulcerated lesions in this case, therefore failing to achieve an accurate diagnosis (Daniel et al. 2020). Fungal granulomas are frequently submucosal, and a deeper sample is often required to obtain a diagnosis by histopathology or culture. A large biopsy sample can be obtained from the nasopharynx by use of a mare uterine
biopsy instrument passed nasally with visual guidance from a flexible endoscope (Stewart et al. 2009; Stewart and Cuming 2015). As the nasopharynx is highly vascularised, profuse haemorrhage is common. As haemorrhage will rapidly obscure the endoscopic field of view, a biopsy should be collected boldly on the first attempt. In anticipation of haemorrhage, a 1:10,000 solution of diluted epinephrine can be prepared and endoscopically delivered directly over the biopsy site by an assistant immediately after biopsy collection to cause vasoconstriction and reduce haemorrhage. In this case, Daniel et al. (2020) ultimately obtained an excisional biopsy by endoscopic laser resection. This allowed
© 2019 EVJ Ltd
Fig 1: Endoscopic image of a cryptococcal granuloma in the nasal passage of a horse.
collection of a large tissue sample for diagnostic purposes with minimal haemorrhage. Surgical debulking can reduce the volume of infected tissue to be treated medically (Cruz et al. 2009; Stewart et al. 2009) but as only two of 25 granulomas were surgically removed, treatment in this case was primarily medical (Daniel et al. 2020). The surgeon should however be commended on resubmission of the tissue to a second laboratory for histopathological identification of fungal organisms. Cryptococci are round to oval, yeast-like fungi (5–
10 lm in diameter), with a large heteropolysaccharide capsule (5–10 lm in diameter), that do not take up common cytologic stains (Figs 2 and 3). The capsule forms a clear halo when stained with India ink and is easily identified with Mayer’s mucicarmine stain (Caswell and Williams 2007). The yeast bodies stain with methenamine silver and periodic-acid Schiff stains. Due to characteristic morphology, cytological or histopathological identification is reliable for diagnosis (Caswell and Williams 2007). Identification of thickly encapsulated budding yeast bodies allows differentiation of Cryptococcus spp from other fungi that cause granulomatous upper respiratory tract lesions in horses such as Conidiobolus coronatus, Coccidioides immitis,
Pseudallescheria boydii and
Rhinosporidium seeberi (Stewart et al. 2008; Stewart and Cuming 2015). Cryptococcal species are generally easy to identify, as they are the only pathogenic fungi that have a capsule. For definitive differentiation between cryptococcus species and varieties, microbiological
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