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Background: Macrorhabdus ornithogaster, a yeast-like fungus, has the potential to infect various bird species, including companion birds. Although birds infected with M. ornithogaster may often remain asymptomatic, the infection can develop into chronic wasting gastritis and even progress to gastric cancer, highlighting the importance of early detection of M. ornithogaster infection. Despite direct fecal examination being a commonly used diagnostic method, the polymerase chain reaction (PCR) is anticipated to offer a higher detection rate. However, the actual diagnostic accuracy of the PCR for M. ornithogaster remains unknown. Case Description: Ninety fecal samples collected from companion birds that visited or were admitted to a hospital, regardless of their stage of Macrorhabdus diagnosis or treatment, were subjected to PCR testing. An accuracy analysis was then performed, considering symptomatology, direct fecal testing, and sequencing. The PCR test had a sensitivity of 83.33 %, specificity of 95.00 %, false negative rate of 16.67%, false positive rate of 5.00%, positive predictive value of 89.29%, negative predictive value of 91.94%, prevalence of 33.33 %, positive likelihood ratio of 16.67, negative likelihood ratio of 0.18, and diagnostic odds ratio of 95.00. Conclusion: The findings suggest that the PCR for Macrorhabdus possesses high diagnostic accuracy, with the ability to accurately identify uninfected individuals as negative. While the direct fecal examination is appropriate for routine primary screening, in cases where M. ornithogaster is not detected by fecal testing, the PCR may provide a more accurate and definitive diagnosis due to its high specificity. [Open Vet J 2023; 13(12.000): 1769-1775]

Infection by Macrorhabdus ornithogaster (MO) formerly called Megabacteriosis has been diagnosed as the cause of depression, wasting, regurgitation, fluffed feathers, lethargy, and soft watery bulky feces and also the cause of mortality in a flock of breeding canaries over a long period of time (3 months) in Mashhad, Iran. Large numbers of Macrorhabdus ornithogaster were detected in wetmount fecal preparations of the feces and direct smears from mucosal scrapings of proventrilculus of one bird under light microscopy. The diagnosis was confirmed by demonstrating a chronic proventriculitis histologically which was associated with MO organisms along with molecular tests.

Since 2000, Macrorhabdus ornithogaster "megabacteriosis" has been diagnosed in the avian diseases laboratory in a diversity of avian species and varied spectrum of disease. The disease in some species (chickens, turkeys, guinea fowls) was clinically characterized by emaciation, prostration, loss of appetite, cachexia and death, with a typically chronic course. A more acute disease was observed in finches (canary-Serinus and zebra-Taeniopygia) and budgerigars (Melopsittacus undulatus). The large rod shaped organism, visible from 100 times magnification, with and without staining, could be detected in sick and also in reasonably normal individuals of some species, such as chickens, turkeys, quails and pigeons. In rheas (Rhea americana), ostriches (Struthio camelus), canaries, zebra-finches, guinea-fowl (Numida meleagris) and budgerigars. The disease was severe, causing to up to 100% mortality. The infection could be detected in some species along with other infectious or disease problems, such as endoparasites (helminths, coccidia) and ectoparasitism (order Mallophaga or/and order Acarina). The cultivation of M. ornithogaster was successfully achieved in solid and liquid media, originated from chickens (four isolates), guinea fowl (1 isolate), chuckar partridge (1 isolate) and canary (1 isolate). A very interesting finding at microscopy was motility of M. ornithogaster, as detected both in cultures obtained on agar for pathogenic fungi and passaged into thioglycolate broth, as well as on samples observed in wet preparations from in vivo. Differences in colony aspects were noted among the isolates. Experimental infections were attempted in chicken and japanese quail, using a chicken isolate, allowing the detection of the organism in the proventriculus and liver in apparently normal birds. One chicken isolate was injected intraperitoneally in Balb/c mice and resulted in 100% mortality.Desde 2000, diversos casos de infecção e doença por Macrorhabdus ornithogaster (megabacteria) foram diagnosticados no Setor de Doenças das Aves (Escola de Veterinária da UFMG). A doença clínica foi caracterizada por emagrecimento, prostração, perda do apetite, caquexia e morte, em curso crônico, embora com forma mais aguda em canários e periquitos. O microrganismo grande, em forma de bastão, visível a partir de 100 aumentos sem e com coloração, pode também ser detectado em aves de aspecto clínico normal, principalmente galinhas, perus, codornas e pombos. Em emas (Rhea), avestruzes (Struthio camelus), canários, mandarins, galinhas da Angola (Numida meleagris) e periquitos Australianos (Melopsittacus undulatus), a severidade da doença foi sempre maior, ocasionando até 100% de mortalidade em alguns plantéis. Na maioria das espécies a doença foi detectada em aves com endo e/ou ectoparasitismo. O cultivo de M. ornithogaster foi obtido em meio sólido (ágar para fungos patogênicos) e subcultivado em meio líquido (thioglicolato), do proventriculo de galinha, galinha da Angola, perdiz de chuckar e canário. O resultado mais surpreendente na microscopia de M. ornithogaster foi a presença de motilidade, detectada tanto de cultivos in vitro quanto de preparações úmidas de in vivo. Diferenças nos aspectos das colônias foram notadas entre os isolados. Infecções experimentais em galinha (SPF) e codorna japonesa permitiram a detecção do organismo nos proventrículos das aves de aspecto normal. Nas codornas, à necropsia notaram-se hemorragias hepáticas. A infecção experimental em camundongos via intraperitoneal resultou em 100% de mortalidade, também com lesões hepáticas. Aspectos do cultivo, a importância da doença, as espécies de aves susceptíveis e seu papel na epidemiologia são discutidos.