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Case History: An adult female 9.95-g Zebra Finch (Taeniopygia guttata) from a California aviary was found dead. It was noted to be thin several days prior to its death.
Gross Findings: On external examination, scant fecal material was present on the tail feathers. On internal examination, there was no subcutaneous, visceral and epicardial fat (emaciation). Pectoral muscle was mildly atrophied and tacky (dehydration).
Histopathological Findings: Within the lumen of the proventriculus (Fig. 1A) and extending into the proventricular glands are numerous 2 x 40 um, basophilic, rod-shaped stacked yeasts (Fig. 1B). Within the lamina propria of the proventriculus, there are variably sized aggregates of mononuclear cells. In the thyroid gland, follicular epithelium was hyperplastic with abundant lightly eosinophilic cytoplasm. Diffusely, follicular lumina were small or indiscernible and contained no to scant colloid (Fig. 1B).
Morphologic Diagnosis/es:
Etiologic Diagnosis: Proventricular macrorhabdiosis
Etiology: Macrorhabdus ornithogaster, a Gram-positive rod-shaped yeast.
Distribution: Worldwide.
Seasonality: Any time of year.
Host range: Primarily captive birds (budgerigars, parrotlets, lovebirds, cockatiels, and finches), but it does infect wild birds. Young birds may be more susceptible than older birds. Often seen in immunosuppressed birds. In this case, the thyroid hyperplasia may have caused immunosuppression and led to the infection.
Transmission: Fecal-oral.
Clinical signs: Weight loss, regurgitation, lethargy, passage of undigested food, and diarrhea.
Gross findings: Emaciation, proventricular dilation and ulceration, thickening of the proventricular wall, mucus production, and softening of the koilin layer of the ventriculus.
Histopathologic findings: Yeast are observed in the lumen of the ulcerated proventriculus and rarely extend into the lumina of proventricular glands. There may be goblet cell hyperplasia with mixed mononuclear infiltrates. Yeasts may extend into the adjacent ventriculus which may be ulcerated.
Diagnosis: A wet mount of feces shows ~2 µm x 60 µm rod-shaped yeasts. Low numbers of yeasts are seen in asymptomatic birds and large numbers in sick birds. A negative fecal examination can occur in infected birds that shed the organism intermittently. Histopathology shows numerous yeasts in the proventriculus and ventriculus. PCR can be positive in asymptomatic birds.
Wildlife population impacts: Mortality can be high in captive populations. Birds that do recover can relapse or shed yeast in their feces.
References:
Ladds, P. 2009. Mycotic and algal-associated diseases in birds. In: Pathology of Australian native wildlife, Ladds P., editor. CSIRO Publishing, Collingwood VIC, Australia, pp. 176–177.
Merck Veterinary Manual. 2016. Avian gastric yeast (Macrorhabdus ornithogaster), https://www.merckvetmanual.com/. Accessed April 2020.
The Necropsy and Pathology services are performed by board-certified veterinary pathologists and necropsy technical staff whose principal role is to determine the cause of death for animals submitted to the USGS National Wildlife Health Center.
The Necropsy and Pathology services are performed by board-certified veterinary pathologists and necropsy technical staff whose principal role is to determine the cause of death for animals submitted to the USGS National Wildlife Health Center.
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