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Infection & disease

Atypical presentation of Cryptococcus neoformans in a koala (Phascolarctos cinereus): a magnetic resonance imaging and computed tomography study

Martínez-Nevado, E, Alonso-Alegre, EG, Martínez, MÁ, Rodríguez-Álvaro, A, de Merlo, EM, García, JG & Real, IG 2017, Journal of Zoo and Wildlife Medicine, vol. 48, no. 1, pp. 250-254.

Magnetic resonance imaging (MRI) and computed tomography (CT) are valuable tools for diagnosing cryptococcosis in koalas as the techniques, used together, can assist in identifying the precise location and extent of a granuloma.

  Cryptococcosis is a fungal infestation, or mycosis, that can occur in koalas with fatal effects. Caused by the fungus Cryptococcus neoformans, cryptococcosis is most commonly subclinical but can result in epistaxis (nasal bleeding), mucopurulent nasal discharge, dyspnea (shortness of breath), and facial distortion due to growths when the fungus infects the nasal cavity. The disease can also present neurologically, with symptoms including blindness, nystagmus (involuntary eye movement), limb paresis, opisthotonus (muscular tension causing the spine to arch backwards) and seizures. To confirm a diagnosis of cryptococcosis, it is necessary to isolate C. neoformans or histologically demonstrate its invasion of tissue. Clinical sign observation, advanced imaging techniques and mycologic evidence can further assist in a more accurate diagnosis of clinical cryptococcosis.

  The subject of this report is a 10-year-old male koala with a four-month history of progressive unilateral exophthalmos (protrusion of the eyeball from the orbit) and no other symptoms. The orbit was evaluated via radiograph and MRI. While radiographs were initially suggestive of frontal sinus disease, the MRI revealed a large mass in the caudolateral area of the left nasal cavity and ipsilateral paranasal sinuses. Examiners used post-contrast enhancement of the mass identified in the MRI study to generate a more accurate picture of its extent. The most likely differential diagnoses based on these MRI results were osteochondroma, nasal adenocarcinoma, lymphoma, and cryptococcal granuloma. The koala declined in condition after developing neurologic symptoms and died soon after. A post-mortem CT study revealed the same tissue mass observed in the MRI in lower definition, though in poorer definition. The CT study did, however, expose in greater detail the osteolysis of the boundaries of the nasal cavity next to the mass. Complete post-mortem examinations confirmed that the mass observed was a locally extensive, infiltrative granuloma with multiple yeasts of Cryptococcus species. The diagnosis of cryptococcosis was also confirmed using serum cryptococcal antigen testing.

  While techniques including serologic antigen testing, radiography and biopsy are instrumental in diagnosing cryptococcosis, this study demonstrates that MRI and CT should also be considered as standard protocols for evaluating the koala nasal cavity. MRI and CT are complementary to the former techniques as they generate more precise information about the location and extent of the mass. Also, the authors recommend that practitioners consider the possibility of nasal cryptococcal infection in koalas exhibiting exophthalmos, as this symptom was the only clinical sign of the disease exhibited by the koala described in this case study.

 

Summarised by Joanna Horsfall

 

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