Prevention of Coxiella burnetii shedding in infected dairy herds using a phase I C. burnetii inactivated vaccine.

The main objective of this study was to assess the efficacy of a monovalent inactivated vaccine containing phase I Coxiella burnetii to prevent Coxiella shedding in susceptible dairy cows within infected herds in comparison to a placebo. A total of 336 dairy cows and heifers, from six spontaneously infected herds, were followed over a 1-year period. Before treatment (i.e. vaccination or placebo), the C. burnetii infection status of the cows was determined on the basis on PCR results on milk, vaginal mucus and faeces and serological analysis performed 2 weeks apart. A cow was considered susceptible (i.e. non-infected) when all results were negative, and was considered infected otherwise. The allocation of treatments was performed randomly within pregnant and non-pregnant cows. After treatment (D0), the animals were subject to systematic sampling (milk, vaginal mucus and faeces) on D90, D180, D270 and D360 to detect putative shedding. In addition, the same samples were taken within 15 days after calving. An animal was considered as a shedder at a given time t, if at t, it was found PCR-positive on at least one test taken among the samples (milk, vaginal mucus and faeces). The effect of the treatment on the probability for an initially susceptible animal of becoming shedder was assessed using survival analysis techniques (Cox regression model). Almost all heifers were detected as susceptible before treatment. When vaccinated while not pregnant, an animal had a five times lower probability of becoming a shedder than an animal receiving placebo. An animal which was vaccinated while pregnant had a similar probability of becoming shedder as an animal receiving the placebo. There was no significant farm effect in this multi-centric trial. These results highlight the value of implementing vaccination, if possible, in non-infected herds. In infected herds, the vaccination should be implemented in quite all presumably susceptible animals, i.e. at least the heifers. The vaccination of the dairy cows should be performed when the within-herd seroprevalence is low, i.e. in herds where the infection has not spread widely yet.