Macrorhabdus ornithogaster

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Macrorhabdus ornithogaster is a large, Gram-positive, elongate structure (blue arrow). Notice the smaller bacterial flora (white arrows) in the background. Cloacal swab, Gram’s stain.
Several M. ornithogaster (blue arrows) are present and stain Gram-positive (blue-purple). A larger piece of plant fiber (white arrow), often confused with M. ornithogaster, is present at the upper left. Cloacal swab, Gram’s stain.
Shedding of organisms is variable. Organisms may be numerous, as in this patient, or rare. Cloacal swab, Gram’s stain.

Megabacteriosis is an important fungal disease of certain avian species, especially budgerigars, cockatiels, and finches.

Originally thought to be a bacterium, the organism has recently been identified as a comycetous yeast and has recently been named Macrorhabdus ornithogaster. Although the host range and incidence of megabacteriosis are increasing, knowledge of the disease has been inhibited by the difficulty in growing the organism for subsequent study.


Macrorhabdus ornithogaster (previously known as megabacterium) is a unique, large, Gram-positive, bacillus-like structure, measuring 1 to 5 µm in width by 20 to 90 µm in length[1]. Isolation of this organism is extremely difficult which had impaired taxonomic classification. This organism was believed to be a bacterium for many years; however, recent findings indicate that it is a comycetous yeast. M. ornithogaster stains positively by periodic acid-Schiff and silver staining techniques[2]. Electron microscopy has shown that the organism has a 3-layered wall with deep indentations of the inner wall and a translucent outer wall. Nuclei, spores, and pigmented granules have not been observed.

Optical flourescence studies have demonstrated that the organism’s cell wall contains components of cellulose and chitin that are often found in fungi. Electron microscopic studies have revealed the presence of eukaryotic nuclei with a nuclear membrane. Fungi by definition are eukaryotic, heterotrophic organisms. Furthermore, ultrastructural studies have demonstrated a fimbriated compact border within the outer layer of the cell wall that is typical of lower fungi. Special stains and thin sectioning have disclosed that M. ornithogaster has nuclei-like structures and a thicker cell wall (up to 100 µm in width) than bacteria, suggesting that this organism is probably a fungus. Lastly, fluorescence in situ hybridization of the ribosomal RNA also indicates that M. ornithogaster from budgerigars is actually eukaryotic or fungal cells.

Clinical signs

Macrorhabdus ornithogaster is believed to be the etiology of the ‘going light syndrome’ in budgerigars, a fatal disease characterized by progressive weight loss[3]. English exhibition budgerigars seem particularly susceptible to megabacteriosis. However, the prevalence of megabacteriosis is reportedly increasing and the organism may infect a variety of avian species including canaries, finches, cockatiels, lovebirds, chickens, and ostriches.

Research studies suggest that a bird’s innate susceptibility to infection by M. ornithogaster may actually be more significant than the duration of exposure or close contact with other infected birds[4]. The mode of transmisssion is unknown; however, the fecal-oral route appears plausible. Inherent genetic factors also may be involved because the prevalence of megabacteriosis is significantly higher in chicks from M. ornithogaster-positive parents than from M. ornithogaster-negative parents, even when those chicks were raised by M. ornithogaster-negative parents.

Clinical signs generally are nonspecific and indicate chronic disease. The major sign is progressive weight loss over a long period of time (12-18 months). Affected birds continue to lose weight despite an apparently good appetite. In actuality, infected birds grind seeds with their beak, but ingest little feed. This results in an empty crop while ground seed material accumulates in the food dish[5].

Other clinical signs include depression, ruffled plumage, regurgitation manifested by head bobbing, diarrhea, and passage of undigested feed in the feces. In more severe cases, birds may vomit mucus or blood and exhibit melena. Sudden death may occur due to hemorrhage.

An acute manifestation of megabacteriosis has been reported only in budgerigars. With the acute form of disease, apparently healthy birds suddenly exhibit severe depression, ruffled plumage, and die within 12 to 24 hours. These birds also frequently regurgitate blood, possibly due to excessive bleeding in the proventriculus.


M. ornithogaster can be seen in wet mounts of feces or proventricular scrapings or in cytologic specimens stained with Gram’s or Romanowsky (Wright, Giemsa, Leishman, or Diff-Quik) stains. In Romanowsky-stained preparations, M. ornithogaster may stain faintly blue with a clear cell wall. Shedding of organisms is variable. Therefore, M. ornithogaster may not be detected reliably in every infected bird.

The complete blood cell count (CBC) and biochemical profile data are often nonspecific, but may reveal anemia and decreased electrolyte concentrations, respectively. Occasionally, changes in the CBC, such as leukocytosis and heterophilia, may indicate an infectious process. Lymphocytosis, monocytosis, basophilia, and thrombocytosis also may be seen[6].

Barium sulfate contrast radiographs may reveal a ‘sandglass-like’ retraction between the proventriculus and ventriculus, which strongly supports a diagnosis of megabacteriosis. The proventriculus may be dilated while the hourglass constriction between the proventriculus and ventriculus is caused by masses of mucus.7 Definitive diagnosis of megabacteriosis is most consistently demonstrated by necropsy and histopathology.

M. ornithogaster also has been found in clinically healthy birds; they may be a component of the normal gastrointestinal flora in captive budgerigars[7]. Research has shown that chronic, subclinical infections also may exist. For example, one study reported that approximately one third of birds colonized with M. ornithogaster failed to show any signs of disease or gross lesions.

Necropsy provides the most consistent diagnosis of megabacteriosis. Birds are usually emaciated at death. The major lesions are found in the proventriculus and ventriculus. The infected proventriculus may have an increased pH. It has been found that Macrorhabdus ornithogaster tends to accumulate in areas of increased pH, such as the transition between the proventriculus and ventriculus. Conversely, organisms are sensitive to the lower pH of stomach acid[8].

It is unclear whether M. ornithogaster reproduces subsequent to an increased pH or actually cause the change in pH. The proventriculus may be inflamed, ulcerated, distended, and may contain whole seeds. The outside diameter of the proventriculus may be increased two- to threefold and it may be separated from the ventriculus by a narrower neck. The mucosal surface of the proventriculus may be covered with a thick, white film that is often coated with blood. Gross lesions include proventriculitis or proventricular ulcers with or without hemorrhages. Lesions are most commonly located in the pars intermedia gastris. Necrotic foci contain dense numbers of M. ornithogaster, which subsequently can be visualized in proventricular scrapings. The largest numbers of organisms tend to occur at the transitional area between the proventriculus and ventriculus. Impression smears or cytologic preparations of the liver and spleen also may reveal encapsulated organisms.

Histopathologic changes include parallel layers of M. ornithogaster on the mucosal surface and occasionally within glands at the proventricular-ventricular interface. M. ornithogaster typically have smooth surfaces with distentions at the ends of the organism. M. ornithogaster are seldom invasive and usually incite little inflammatory reaction. Heterophils, plasma cells, and lymphocytes may be observed infrequently. In chronic disease, submucosal fibrosis may occur accompanied by inflammatory cell infiltrates that extend into the muscular layers.

Other histologic lesions of the proventriculus may include shallow ulcers, disrupted mucosal architecture, an absence of glandular structures, local mucosal necrosis, and small mucosal cysts.3 Histologic changes of the ventriculus may include degeneration of the kaolin characterized by irregular kaolin patterns with reduced dentate matrix material. Degeneration of the kaolin layer may interfere with the birds’ inability to grind ingested seeds.


Typically, birds with megabacteriosis show a very poor response to antibiotics, as would be expected as the organism is a yeast. One effective treatment for many birds has been amphotericin B, an antifungal drug. This drug is best administered orally, either by crop gavage or medicated water. Nystatin also has been reported to be effective in goldfinches. Both amphotericin B and nystatin are polyene macrolide antifungals and bind to ergosterols in the cell membranes of fungi. In light of the fact that bacteria do not contain sterols, the response of "megabacteriosis" to these drugs further indicates that M. ornithogaster are fungi rather than bacteria.

Because M. ornithogaster grows in a more alkaline environment, another treatment option consists of increasing the acidity of the gastric system. This can be done by either acidifying the water with organic acids (apple vinegar, white vinegar, or grapefruit juice) or by using Lactobacillus spp.


  1. Gerlach H (2001) Megabacteriosis. Semin Avian Exotic Pet Med 10:12-19
  2. Camus AC, Heatley J (2001) What is your diagnosis? J Avian Med Surg 15:54-57
  3. Baker JR (1992) Megabacteriosis in exhibition budgerigars. Vet Rec 131:12-14
  4. Filippich LJ, Hendrikz JK (1988) Prevalence of megabacteria in budgerigar colonies. Aust Vet J 76:92-95
  5. Henderson GM, Gulland FMD, Hawkey CM (1988) Haematological findings in budgerigars with megabacterium and trichomonas infections associated with ‘going light’. Vet Rec 123:492-494
  6. Baker JR (1985) Clinical and pathological aspects of ‘going light’ in exhibition budgerigars (Melopsittacus undulatus). Vet Rec 116:406-408
  7. Baker JR (1997) Megabacteria in diseased and healthy budgerigars. Vet Rec 140:627
  8. Van Herck H, Duijser T, Zwart P, et al (194) A bacterial proventriculitis in canaries (Serinus canaria). Avian Pathol 13:561-572