Avian Gastric Yeast (AGY)

Date Published: 08/30/2010

What is AGY?

For many years, avian gastric yeast (AGY) was known as Megabacteria, but it is also known by its scientific name: Macrorhabdus ornithogaster. These days it is referred to as AGY. While initially thought to be a large and atypical bacterium, AGY was more recently classified as a type of yeast. The organism most frequently causes gastrointestinal (GI) disease in budgerigars, finches, canaries, and lovebirds, as well as less commonly affecting cockatiels.

What are the Signs of AGY?

AGY predominantly inhabits the junction between the two parts of the avian stomach in an area known as the isthmus. Generally speaking, the yeast leads to a chronic condition in which the bird progressively loses weight. Many patients with AGY appear to continue to eat well; however, typically they are only grinding food and not ingesting enough to maintain their weight. Additionally the organism itself affects digestion and leads to abnormal feces containing undigested food, furthering weight loss. Patients may develop diarrhea or black tarry stool known as melena. Melena occurs after AGY causes bleeding in the GI tract. Over time these birds may develop anemia, worsening their weakness and contributing to the potentially fatal nature of this disease. In addition to these clinical signs, affected birds may demonstrate head bobbing and ruffled feathers; pass undigested seed in the droppings; and/or regurgitate mucus, blood or food.

A separate and much more acute condition has been reported in budgerigars. In this form of the disease, apparently healthy budgerigars will suddenly go off food, regurgitate food or blood, and will often die in as little as one or two days. It is not known with certainty why some birds are affected more acutely than others. However, it has been suggested that genetics may play a role in addition to the amount of infectious organisms to which the bird has been exposed.

How is AGY Diagnosed?

If clinical signs similar to those described above are noticed in a species commonly affected with AGY, then your veterinarian may ask to perform one or more fecal examinations to help identify AGY. Several tests are available and they vary in their ability to detect the yeast. Probably the most sensitive and rapid in-house test is a fecal wet mount examination. Your veterinarian mixes a fresh fecal sample with a saline mixture and examines it under a microscope using certain light settings.

Another test that is variably sensitive is known as fecal cytology. This test is also reasonably rapid to perform and commonly identifies heavy AGY infections. The problem with this test is that a relatively light load of yeast might be missed, or the organism may wash off the slide during preparation.

The final in-house technique sometimes used to identify this organism is known as a Gram stain. This test has additional benefits in that it may allow your veterinarian to identify abnormal populations of fecal bacteria in addition to AGY. However, this test suffers from the same downsides as the fecal cytology test.

Is there a Test for AGY?

AGY on a saline-mount of fresh feces. The arrows indicate Macrorhabdus ornithogaster (aka Megabacteria or AGY). The circle shows bacteria for relative size comparison. Photo courtesy of Dr. Scott Ford.

Yes. The advanced and highly sensitive laboratory test, known as a PCR test, is available to diagnose AGY. This test may be used when a veterinarian is fairly convinced of the diagnosis but is unable to identify AGY on any of the in-house tests described above. This test may also be used after a course of treatment to ensure that the organism is gone. The only downside to this test is that it may require several days to get results, as it has to be shipped to the laboratory and processed, and it often it costs more than the in-house tests. That said, this test is becoming the gold standard for AGY diagnosis, especially in difficult to diagnose cases.

Can AGY be Treated?

Yes. When correctly diagnosed, AGY can be treated with an antifungal known as amphotericin B, which is expensive. Amphotericin B should be given twice a day for a full 30 days. The length of treatment is hard on a sick bird if he doesn’t have AGY, plus the real cause of the bird’s illness is left untreated. That’s why it is extremely important to obtain an accurate diagnosis.

Another antifungal drug called fluconazole is used once in a while, mostly in cockatiels who do not respond to amphotericin B.

What is the Prognosis for a Bird Affected by AGY?

As with any disease, the prognosis will vary on an individual basis depending on factors such the bird’s health before onset and at the time the diagnosis was made; length of time before a diagnosis was made; genetics; and successful administration of medication. Generally speaking the prognosis is good for birds with mild to moderate disease. Unfortunately, in those birds with severe clinical signs the prognosis varies from fair to poor.

Is there a Vaccine to Protect against AGY?

Unfortunately there is no vaccine to prevent AGY. Given the nature of this organism, it is unlikely that a vaccine will be developed in the near future, if at all.

Can AGY be Prevented?

The prevention of AGY is difficult and requires a two-fold approach. First, breeders of affected species should make every effort to clear the infection from the flock. This is likely to be a difficult and time-consuming process, particularly since many budgerigar breeding aviaries have infection rates between 35 and 70%.

The best way for pet owners to prevent AGY infection is to select healthy birds from reputable breeders. If you are bringing new birds into your household, they should be quarantined for 30 days before being placed with your current birds. Ideally any new birds should also be examined by a veterinarian, and birds of a susceptible species, such as budgerigars, may be tested for AGY to further reduce the risk of bringing this organism into your household.

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