Comparison between persisting anti-lipopolysaccharide antibodies and culture at postmortem in salmonella-infected cattle herds.

Herds with recent clinical outbreaks of Salmonella dublin (7 herds) and S. typhimurium (4 herds) infections were followed serologically in O-antigen ELISAs over about one year, divided in four equal sampling phases. Animals found to be persistent high-reactors or seronegative at the end of the study were slaughtered and subsequently cultured for salmonella in a selected number of organ samples. Approximately 3% of all animals had high seroreactions up to 17 months after the outbreaks, and less than half of the seropositive animals in the S. dublin-infected herds were salmonella culture positive at slaughter (14/31). However, one persistently seronegative animal was also culture positive. Furthermore, as much as 70% of the male calves investigated at postmortem in the S. dublin-infected herds were high-reactors, among which approx. 56% were culture positive. Surprisingly, 2 of the 14 animals found culture positive turned out to be culture positive for S. typhimurium only. In the S. typhimurium study, none of the 17 animals investigated at postmortem were salmonella culture positive. All sera from these animals were negative in the O:9 blocking ELISA, and no serum sample was positive in the S. dublin ELISA, alone. In conclusion, although serology based on the O-antigens appears to be useful to identify salmonella-infected herds, it seems to be insufficient for identification of persistently infected animals.