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Case report: Fungal Encephalitis in a Domestic Shorthair Cat

History

A 10-year-old male, neutered, domestic short-haired cat had undergone investigations for a six month history of progressive weight loss and lethargy. The referring veterinarian’s blood tests had revealed basophilia; 0.45 (0.01-0.26) x109/L initially, which on repeat testing two weeks later, had resolved. However, at that time neutrophilia; 21.55 (1.48-10.29)x109/L and monocytosis; 0.85 (0.05-0.67)x109/L was noted. Survey, thoracic, abdominal radiology and abdominal ultrasound were unremarkable. Retrovirus testing was negative.

Examination

On presentation, the patient was noted to be non-visual, with bilaterally dilated, non-responsive pupils. There was exophthalmos and congestion of the sclera of the right eye. General anaesthesia was induced for advanced imaging, which facilitated thorough, intraoral examination; a large firm, fixed mass was found protruding into the right ptergopalatine fossa (fig.2.1).

fig 2.1 – Appearance of oral mass on induction of anaesthesia, prior to biopsy.

MR-scans (figs. 2.2-2.4) revealed caudal extension of the intraoral mass into the right, retrobulbar space, with infiltration into the brain along the third ventricle, to envelope the right, dorsolateral aspect of the thalamus. There was evidence of mass expansion and marked gadolinium enhancement.

fig 2.2 – MRI T1W post-contrast sagittal slice of the brain shows an enhancing band of tissue extending from the hypothalamus along the third ventricle.

 

fig 2.3 – T1W post-contrast transverse slice, at the level of the thalamus, showing extension of the oral mass dorsally, into the cranial vault and right frontal sinus.

 

fig 2.4 – T1W post-contrast dorsal slice, showing right, retrobulbar extension and diffuse infiltration of the midbrain.

Clinical and imaging findings were consistent with a provisional diagnosis of infiltrative neoplasia. However, other differentials included abscessation/granuloma.

Cytology of needle aspirates from the mass (fig. 2.5) revealed pyogranulomatous inflammation and frequent fungal hyphae were present within the cytoplasm of macrophages and multinucleated giant cells.

fig. 2.5 – photomicrograph showing macrophage phagocytosing fragments of fungal hyphae (arrow). 400x magnification

A tentative diagnosis of a nasopharyngeal fungal granuloma with extension into the cranial vault and resulting fungal encephalitis was made. A guarded prognosis was offered and euthanasia was discussed with the owner. However, the owner elected to try medical therapy.

Treatment

The mass was de-bulked to reduce the risk of ingestion/ ventilation complications and obtain tissue for histopathology, culture and sensitivity testing. Prednisolone was started at 0.5 mg/kg q24h P.O. to reduce secondary brain parenchymal oedema.

Histopathology confirmed fungal granuloma with hyphal forms only seen (therefore excluding coccidioides, blastomyces and histoplasma), and suggesting an opportunistic saprophytic species (figs. 2.6 & 2.7).

fig 2.6 – photomicrograph showing degenerate fungal hyphae in colonies within pyogranulomatous inflammation. 200x magnification

 

fig.2.7 – variably degenerate fungal hyphae in a colony amongst pyogranulomatous inflammation and haemorrhage. 400x magnification

Fluconazole at 12mg/kg q24h P.O. was started as it has superior CNS penetration, compared with itraconazole (though it was likely the blood brain barrier was not intact in this case anyway).

Culture results returned as Aspergillus versicolor complex. As aspergillus species can have inherent resistance to fluconazole, the medication was then changed to itraconazole 5mg/kg q12h P.O.

Once available, results of sensitivity testing confirmed that the fungus was sensitive to itraconazole.

Follow up

Unfortunately, the patient continued to deteriorate neurologically and became progressively anorectic so the owner elected for euthanasia.

Discussion

This was an unusual case as CNS mycoses are uncommon in cats in the UK. Commercial veterinary laboratories do not routinely perform fungal culture, so samples were forwarded to a human mycology laboratory. Aspergillus versicolor is a slow-growing, filamentous fungus found on food products and in damp indoor environments. Looking back through the patient’s history there was an incident noted one year prior to presentation where the cat had exhibited distress whilst eating. He subsequently sneezed out blood and food particles. Given that aspergillus is a common food contaminant, a piece of food getting lodged in the nasopharynx seemed a plausible initiating source for infection.

Unfortunately mycoses involving the CNS usually respond poorly to treatment. In this case, disease was very advanced at the time of presentation. Often, nasal signs predominate, in which case infection may be picked up at an earlier stage.

References

  • ABCD guidelines on aspergillosis (2017) available at www.abcdcatsvets.org/aspergillosis
  • Bentley RT et al (2018) Fungal infections of the central nervous system in small animals, clinical features, diagnosis and management. Vet Clin North Am Small Anim Pract 48, 63-83
  • Estey C (2017) Fungal meningoencephalitis. In: Textbook of Veterinary Internal Medicine. 8th Edn. Eds S.J. Ettinger and E.C. Feldman. Elsevier, Missouri. pp 1407-1408

Photomicroscopy reproduced with kind permission of Dr Tim Cushing DVM, Dip ACVP, MRCVS Cytopath Ltd