Abstract: Bacteria DY9 strain was isolated in Sichuan province from a dead duck. DY9 strain was investigated by biochemical reaction test, nucleotide sequence of 16S rRNA and cam gene. The results showed that DY9 strain was Riemerella anatipestifer(RA) and the 50% lethal dose (LD50) was more than 2.3×1010 CFU/0.5 mL for 14-day-old ducks. In animal assay, the dead ducks did not characterize fibrinous inflammation and there did not have RA in the livers. The RA did not recover virulence after 10 passages in ducks and not loss virulence after 30 passages in vitro culture in chocolate agar. Morphology of duck embryo fibroblast treated with culture filtrate of DY9 strain was not changed. At vaccinated 14 days, the oil-adjuvant inactivated vaccine and the live bacteria had the relative percentage survival (RPS) with 100% and 79.52% respectively, and at 21 days, they were both 100%. The results indicated that DY9 strain RA was low-virulence field strain, and the virulence was stabile.

Key words: Riemerella anatipestifer; low virulence in field strain; isolation and identification