An evaluation of the molecular epidemiology and transmission of avian mycoplasma genotypes in commercial poultry

Abstract

Mycoplasma gallisepticum (MG) and M. synoviae (MS), the most pathogenic mycoplasmal pathogens of poultry, are both horizontally and vertically transmitted. Targeted sequencing of the 16S-23S rRNA intergenic spacer region (IGSR), mgc2, MGA_0319 and gapA genomic regions was used to develop a database of South African MG sequences, and to elucidate the molecular epidemiology of MG in South Africa. The sequences of the twelve unique MG genotypes identified were highly similar to the available live MG vaccines, necessitating a combination of IGSR and mgc2 sequencing for differentiation. Targeted genetic sequencing was instrumental in identifying previously reported cases of apparent reversion to virulence and vertical transmission of the live ts-11 vaccine in the state of Georgia. The egg transmission and pathogenicity of ts-11 vaccine and ts-11-like isolates from ts-11 vaccinated breeders (K6222B) and their broiler progeny (K6216D) were evaluated in this research. K6216D transmitted via the egg at an average rate of 4.0% in the third and fourth weeks post infection, while egg transmission of K6222B and ts-11 vaccine was not detected. K6216D and K6222B were significantly more virulent and invasive than ts-11 vaccine. These results provide the first conclusive evidence of transovarian transmission of an isolate of the ts-11 genotype. A severe outbreak of MS in the state of Arkansas prompted an investigation of the horizontal transmission of two MS isolates genotyped as S-10 and S-17 based on targeted genetic sequencing. MS transmission from infected seeder chickens was detected by real-time PCR at 6 weeks post infection in both the S-10 and S-17 groups; however, the transmission patterns differed, with S-10 transmitting to adjacent contact chickens and S-17 to fomite contact chickens, but not to fomite contacts medicated with tylosin. A novel infection model incorporating mucin was subsequently developed to evaluate the fomite transmission of MS. Low-level MS transmission from S-17-inoculated cotton fabric, and from S-10-inoculated plastic, cotton fabric and feather fomites to sexually mature broiler breeder chickens was detected by real-time PCR, but not by culture or serology, 3 weeks after fomite placement. These studies represent the first experimental evaluations of the horizontal transmission of MS in sexually mature chickens.