Effect of dry period immunization of Salmonella Dublin latent carriers with a commercial live culture vaccine on intrauterine transmission based on the presence of precolostral antibodies in offspring

Salmonella Dublin latent carrier cows represent a high risk for infection of newborn calves via intrauterine transmission and shedding of bacteria in feces and colostrum at calving. Vaccination of these latent carrier dams during late gestation boosts immunity against Salmonella Dublin. This could reduce the activation of the dormant bacteria during the periparturient immune dysfunction period, thereby reducing the risk of early-life infection in the offspring. Thus, the objective of this study was to evaluate the extent to which vaccinating Salmonella Dublin latent carrier cows at dry-off with a commercial live culture vaccine reduces bacterial shedding at calving and intrauterine infection to calves. To identify latent carriers, we screened 1,084 cows in 4 Michigan commercial dairy farms with a history of Salmonella Dublin. Cows were defined as latent carriers when they showed 3 consecutive positive milk antibody ELISA tests conducted every 2 mo. Subsequently, 148 latent carriers were randomly allocated to the vaccine or control group. Vaccine cows received a commercial live culture vaccine s.c. at dry-off and a booster 2 wk later. Control cows received saline s.c. at the same times. At calving, we collected fecal and colostrum samples from the dam and a precolostral serum sample from the calf. Bacterial shedding was evaluated in feces and colostrum both qualitatively and quantitatively through bacterial culture and quantitative PCR (qPCR), respectively. Intrauterine transmission was defined when a calf was positive for serum antibody ELISA at birth. Vaccination decreased the likelihood of calves being born Salmonella Dublin seropositive (relative risk [95% CI]) = 0.19 [0.04-0.84]). However, no Salmonella Dublin positive isolates were identified through either bacteriological culture or qPCR in feces or colostrum. Vaccination of Salmonella Dublin latent carrier cows at dry-off reduced intrauterine transmission to calves. Further research is warranted into the potential of vaccination to decrease vertical transmission of Salmonella Dublin in dairy farms. Additionally, the absence of Salmonella Dublin positive fecal and colostrum samples warrants further evaluation of the detection methods for identifying latent carriers or Salmonella Dublin isolation, as well as the role of latent carriers in infecting newborn calves in the maternity area at birth.