Failure of a low virulence Streptococcus gallolyticus serotype 1 strain to immunize pigeons against streptococcosis

Ten pigeons were inoculated intravenously with the low virulence Streptococcus gallolyticus strain PDH 827, which belongs to serotype 1, supernatant phenotype A(-)T2. The birds did not develop clinical disease but shed S. gallolyticus in their faeces, and antibodies against the bacterium were detected in post-inoculation plasma samples of all birds. Seven weeks later, these pigeons, as well as 14 control pigeons, were challenged intravenously with the highly virulent S. gallolyticus strain STR 357, which also belongs to serotype 1 but to the supernatant phenotype A(+)T1. Post-inoculation morbidity in the immunized group amounted to 90%, demonstrating that no protective immunity had been built up after the first infection. These findings indicate that serotype-specific antigens are not, or at least not solely, involved in induction of protection against S. gallolyticus septicaemia in pigeons.