Avian Gastric Yeast

Avian gastric yeast (AGY), Macrorhabdus sp., formerly known as megabacteria, is an ascomycetous yeast that grows in a narrow zone of the stomach (actually the isthmus that is the junction between the proventriculus and ventriculus) and can infect wild birds, ratites, poultry and companion birds.

This organism can prove very frustrating to diagnose in a bird suspected of harboring it. In our pet birds, clinical signs can be very vague indications of illness, or the bird may show evidence of chronic disease, characterized by regurgitation, weight loss and death. This is a common presentation in the budgerigar. In some species, such as color mutations of the green-rumped parrotlet, the disease may present as an acute or sub-acute disease. However, in many companion bird species, infection may not result in clinical signs. It is suspected that there are different degrees of pathogenicity of AGY.

AGY is thought to be widespread in budgerigars, and has been described in lovebirds, parrotlets, canaries, various species of finches, ostrich and domestic chicken. I have diagnosed this condition in other species, as well, including cockatoos and cockatiels.

Birds with AGY don't consistently shed the organism in their droppings, so screening fecals isn't always diagnostic. There can be false-negatives, meaning that the bird has AGY, but the tests are negative. Gram's staining of any regurgitated (from the crop) or vomited (from the stomach) material is more likely to result in diagnosis, but again, false-negative test results still occur.

Biopsy of the isthmus portion of the GI tract (or scrapings from that area) will most likely result in an accurate diagnosis, but this is a rather drastic procedure to perform and can be quite difficult and expensive in the smaller species. Droppings collected over five days and examined as a wet mount may prove to be the most sensitive and convenient means of detecting sub-clinically infected birds. Someday, a DNA PCR test may become available to aid in diagnosis.

Histology of the isthmus portion of the GI tract may prove diagnostic when dealing with a necropsy specimen.

The drug of choice for treating AGY is currently Amphotericin B, administered orally. Birds being treated with this medication should be closely monitored. Experimentally, Amphotericin B dosed at 100 mg/kg PO BID for 14 days resulted in 8 out of 10 birds testing negative. When dosed at 100 mg/kg PO BID for 28 days, the organism was eliminated in 100% of the test birds. Fluconizole, another antifungal medication, proved too toxic to eliminate AGY in test subjects, but when given at lower (safer) doses, it failed to eliminate the organism in many cases. This study was performed by Dr. David Phalen and his group at the Schubot Exotic Bird Health Center in 2002.

Any bird that has clinical signs of regurgitation, vomiting or weight loss should be screened for AGY. Although diagnosis and treatment can prove difficult, suspect birds should be tested, none-the-less.

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