Pathology Case of Month - Zebra Finch

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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).

Photomicrographs from a Zebra Finch from a California

Figure 1: Photomicrographs from a Zebra Finch (Taeniopygia guttata) from a California aviary that was found dead. (A) Numerous yeasts are present in the lumen (*) of the proventriculus. H&E stain. (B)  The yeasts are 2 x 40 um, basophilic, rod-shaped and occur in stacks. H&E stain. (Credit: Susan Knowles, USGS National Wildlife Health Center. Public domain.)

Photomicrographs from Zebra Finch and Herring Gull thyroid glands.

Figure 2: (A) Photomicrograph from a Herring Gull (Larus argentatus) from Wisconsin shows a normal thyroid gland for comparison.  Within the thyroid gland there are variably-sized follicles filled lined by cuboidal epithelium (arrowhead) and filled with colloid (*). H&E stain. (B) Photomicrograph from a Zebra Finch (Taeniopygia guttata) from a California aviary that was found dead.  Follicular epithelium (arrowhead) is hyperplastic with abundant eosinophilic cytoplasm. Follicular lumina are small or indistinct and contain little or no colloid. H&E stain. (Credit: Susan Knowles, USGS National Wildlife Health Center. Public domain.)

Morphologic Diagnosis/es: 

  1. Proventriculitis with intralesional yeast  
  2. Thyroid hyperplasia, diffuse 

Etiologic Diagnosis: Proventricular macrorhabdiosis 

EtiologyMacrorhabdus 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.

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