Detection of Coxiella burnetii in the bulk tank milk from a farm with vaccinated goats, by using a specific PCR technique

Q fever is a zoonotic disease, caused by the obligate intracellular bacterium Coxiella burnetii. Between 2007 and 2010, Q fever has been a major public health concern in the Netherlands, with almost 3500 human cases reported and dairy goats considered to be the most probable source. At the end of 2009, the Dutch government decided to cull all pregnant dairy sheep and dairy goats based on bulk tank milk C. burnetii positive farms, aiming to preventing shedding and to reducing environmental contamination. On bulk tank milk C. burnetii PCR positive farms, a life-time breeding ban was implemented for all remaining non-pregnant small ruminants. This study describes test results on a bulk tank milk C. burnetii PCR positive dairy goat farm on which all goats had been vaccinated against Q fever with an inactivated phase one vaccine since 2008. All pregnant dairy goats of this farm were culled in 2010, after which bulk tank milk was negative in the C. burnetii PCR. One year later, however, this farm became bulk tank milk C. burnetii PCR positive again. From all lactating animals on the farm (n = 350), individual milk samples were collected and tested using a commercial real-time PCR assay. Individual milk samples from five dairy goats appeared to be C. burnetii PCR positive. These positive goats had been born on the farm between 2002 and 2006. At postmortem examination, out of 33 mostly tissue samples per animal, only milk and mammary tissue samples were C. burnetii PCR positive. Moreover, immunohistochemical examination did not reveal the source of C. burnetii. After culling of these C. burnetii PCR milk positive animals, the bulk tank milk remained negative in C. burnetii PCR until the end of the observation period. The results indicate that vaccination of Q fever infected dairy goat farms does not completely prevent intermittent shedding of C. burnetii in probably previously infected goats. Further research is needed to investigate how and where C. burnetii multiplies in such intermittently shedding animals.