by Bob Doneley BVSc FANZCVS (Avian Medicine) CMAV

In the early 1980’s reports began to emerge around the world about a condition that was causing budgerigars to lose weight and die. Referred to originally as “going light”, this condition was characterised by severe weight loss despite a ravenous appetite. Occasionally an affected bird would vomit or regurgitate a small amount of blood, and many birds passed whole seed in their droppings. A consistent finding on autopsy was an enlarged proventriculus (the first, or true, stomach). When the proventriculus was opened there was often ulceration of the lining, especially where it entered the gizzard. Scrapings from the lining showed large numbers of a huge, rod-shaped organism that was originally thought to be a bacteria hence the name for this condition, Megabacteria (literally, “big bacteria”) Associated Disease.

Since then a lot of work has been done by researchers around the world to better identify this organism. Original work suggested that it was a bacterium with some fungal characteristics. Some European researchers claimed they were able to grow it; other researchers found it was impossible to grow. (It turned out that the European researchers were actually looking at a different organism, not Megabacteria.) In the late 1990’s it was finally determined that it was a fungus, not a bacteria. American researchers christened it AGY Avian Gastric Yeast but this name has never really caught on, and it is still known as Megabacteria. A few years ago David Phalen, a prominent American researcher at Texas A&M University, was able to classify it through DNA profiling and named it Macrorhabdus ornithogaster.

Originally considered a disease of budgies, Megabacteria has since been recovered from a wide range of birds including most parrot species, many finches and canaries, and even ostriches. The clinical signs are virtually identical in all affected birds a ravenous appetite (although birds are often seen to be sitting in their feed dish and eating, many just pick up the seed and drop it), weight loss, increasing weakness and finally death. There is still a lot of work to be done to better understand Macrorhabdus and the disease it causes. What we do know at this stage is the following:

  • It is probably spread through the droppings of infected birds, and possibly through adult birds feeding chicks;
  • It is often found in the droppings of apparently healthy birds finding the organism does not constitute a diagnosis of disease;
  • It flares up when the birds are under stress eg when been transported, showed, or when aviaries and flights are overcrowded. In fact, some affected birds appear to recover once the stress is removed even though they still test positive for the organism;
  • It can be diagnosed through microscopic examination of the droppings but, because it is only intermittently shed, a negative finding does not confirm freedom from infection. There are probably more carrier birds in aviaries than we are aware of.

A lot of different treatments have been trialled, including anti-fungal drugs and even hydrochloric acid in the drinking water. Most treatments have failed, with the exception of an anti-fungal drug, Amphotericin-B. This is given by mouth, either directly or through the drinking water for 10-30 days. This is an expensive treatment and, unfortunately, some drug resistance patterns are starting to emerge. Newer anti-fungal drugs, such as fluconazole, show some promise but it is still early days yet. Treatments such as adding acid or chlorhexidine to the drinking water simply do not work and advice to use them should be ignored.

In all probability there is no guarantee or way of completely eliminating this infection from a bird once it has become established.

Preventing Megabacteria can be difficult. Quarantining and testing bird for Megabacteria is essential to keep this disease out of an aviary. Because it is not shed in the droppings every day, several tests will be necessary to ensure a bird is free of Megabacteria. As expensive as this sounds, it is nothing compared to the devastation of a flock outbreak and the subsequent costs involved in controlling it.