Molecular epidemiological investigation and identification of Mycoplasma gallisepticum in large-scale duck farms.

Mycoplasma gallisepticum (MG) can infect a wide range of birds, seriously jeopardizing the development of the poultry industry. A nested-PCR method was constructed based on mgc2 gene for evaluating the prevalence of MG in duck flocks. A total of 1002 samples were detected from Shandong and Inner Mongolia by this method. The total positive rate of MG was 22.75 % (228/1002), with the highest prevalence in commercial laying ducks (34.83 %), the lowest in dead duck embryos (6.67 %), and a proportionate prevalence of 17.33 % in commercial meat ducks. Genetic evolutionary analysis by mgc2 gene showed that the homology among the wild strains of duck derived MG was 73.3 %-99.8 %, which revealed genetic diversity, and the homology with the international chicken-derived MG reference strain was ranged 68.4 %-98.4 %, indicating that some duck-derived MG strains had an independent origin. Duck embryo pathogenicity tests showed only the mortality rate of duck embryos infected by strain 1-9 reached 60 %. To clarify the genetic background of strain 1-9, the data of whole genome sequencing showed that the full length of 1-9 was 962,503 bp, and the GC content was 31 %. Among the predicted 1576 genes, a total of 599 genes were functionally classified, and the virulence genes were distributed in 21 virulence factors. Phylogenetic tree based on 16S rRNA and single copy gene set were confirmed that there was a far genetic evolutionary distance between the isolate 1-9 and the chicken-derived reference strains. Our findings provided a scientific basis for the research on the pathogenicity and exploring functional genes of duck-derived MG.