Evaluation of C oxiella burnetii Status in Dairy Cattle Herds with Bulk‐tank Milk Positive by ELISA and PCR

Summary Bulk‐tank milk ( BTM ) samples are frequently used to evaluate the health status of dairy livestock. A large‐scale investigation carried out in BTM samples from dairy cattle herds from a Q fever‐endemic region in N orthern S pain revealed a high degree of exposure to C oxiella burnetii . This study was aimed at assessing the value of BTM samples analysis as an indicator of the C . burnetii status in dairy cattle herds. Three herds with BTM samples positive for C . burnetii by ELISA and PCR were selected, and blood, faeces and individual milk and BTM samples were analysed by serology and PCR . In spite of the high antibodies titres found in BTM samples, only one of the three farms presented an active infection by C . burnetii , as revealed by the presence of bacterial DNA in vaginal mucus and in environmental samples collected in the calving area, a seroprevalence around 40% in heifers and the seroconversion rate observed in cows. Results obtained indicated that the analysis of BTM samples is a good epidemiological tool at the population level that can be used to discriminate between seropositive and seronegative herds, but at the herd level, additional tests are necessary to evaluate whether Q fever is a potential problem in the farm. When Q fever is suspected in a cattle herd, sera from a small group of 1‐ to 3‐year‐old animals need to be analysed to investigate recent contact with C . burnetii .