Immunogenicity of Riemerella anatipestifer broth culture bacterin and cell-free culture filtrate in ducks.

Strains belonging to Riemerella anatipestifer serotype 1 are responsible for most of the major disease outbreaks caused by this organism in ducks in Thailand. Therefore, the immunogenicity induced by a broth culture bacterin prepared from a local strain (1081) of this serotype was studied in ducks and a protective index (PI) was derived. The inactivated vaccine contained the equivalent of 5 X 10(9) colony-forming units of bacteria per 0.5-ml dose. A single subcutaneous inoculation of the broth culture bacterin in 2-week-old Khaki Campbell ducklings gave adequate protection (PI = 88) against challenge with the homologous strain 7 days after vaccination, while the highest protection (PI = 95.6) was obtained 2 weeks after vaccination. The duration of immunity was at least 6 months. Inoculation of the vaccine at both 1 and 5 weeks of age gave the greatest protection. The vaccine was found to be safe in one-week-old ducklings and it also gave significant protection (PI values from 66.7 to 100) against challenge with virulent strains of the homologous serotype. The immunogenicity of broth culture bacterin prepared from local strains of various serotypes was also studied. Strains of serotypes 1, 6, 11, 14 and 19 gave no significant protection against challenge with strains of heterologous serotypes. Concentrated cell-free culture filtrates prepared from strain 1081 of serotype 1 and strain 328 of serotype 19 induced highly significant protection against homologous challenge (P < 0.01), but cross-protection between the two strains was not significant.