Macrorhabdus ornithogaster is a well-known cause of proventriculitis in birds. Although this organism was originally termed Megabacterium due to its large, rod-like appearance, the organism has since been classified as an anamorphic ascomycetous yeast. Clinical signs may be variable and include sudden death or chronic wasting. Diarrhea or enteritis has also been reported in birds colonized by Macrorhabdus; however, these birds can have concurrent enteric parasites, bacterial infections, or other diseases that could cause diarrhea.

Gross lesions may include proventricular edema, hyperemia, or hemorrhage, but a characteristic feature of this disease is overproduction of mucus and the proventricular lumen often contains abundant mucoid material. The proventriculus may also be dilated, and in severe cases there can be proventricular rupture and peritonitis. Culture of the organism is difficult. However, the organism is easily recognizable in histologic sections, providing that appropriate sections are available for examination. The proventricular/ventricular junction, or isthmus, is most useful in identifying these organisms. Although gross changes are most prominent in the proventriculus, histologic changes are more prominent in the ventriculus. Histologically, affected birds typically have marked disruption of the koilin layer with disorganization and degeneration, and there are large numbers of yeasts which have a matchstick or logjam appearance. Lymphoplasmacytic to heterophilic inflammation may be present, and there may be foci of hemorrhage. In the proventriculus, there is mucous cell hyperplasia with a thick layer of mucus covering the mucosal surface.

The exact prevalence of this organism is unknown, and not all infected birds have clinical disease or histologic lesions. Macrorhabdus is considered common in budgerigars, canaries, finches, and parrotlets. One study identified organisms retrospectively in approximately one-fourth of canaries and budgerigars. Infection has also been reported in chickens, partridges, and ostriches. Chickens tend to be mildly affected, partridges were reported to have rare, fatal disease, and severe clinical disease may be present in ostrich chicks.

An important differential in pet birds with proventricular dilation is proventricular dilatation disease (PDD). PDD is characterized by chronic wasting disease with or without neurologic signs. These two diseases are easily distinguishable histologically with PDD characterized by lymphoplasmacytic ganglioneuritis of the alimentary tract and absence of koilin disruption. However, sampling of tissues is critical. In birds being submitted as mail in necropsies, it is recommended that a complete set of formalin fixed tissues be submitted and should include (but not be limited to) crop, proventriculus, ventriculus including isthmus, and brain.