Mycobacterium bovis in California dairies: a case series of 2002-2013 outbreaks

From 2002 to 2013, bovine tuberculosis (bTB) caused by Mycobacterium bovis (M. bovis) has been detected on numerous dairies in California. In total, twelve herds had bTB detected and are included in the case series which describes these recent outbreaks and discusses potential pathways of introduction. Epidemiological investigations to determine the initial source of bTB in each herd included obtaining data on likely pathways of pathogen introduction. Pathways included purchasing cattle, use of heifer-raising operations, commingling of cattle at greater risk of exposure to infected cattle with cattle destined for California dairies, contact with infected wildlife, exposure to humans with bTB infections, community and neighboring herds and others. Epidemiologic and molecular typing data confirmed the source of infection in 3 herds and probable sources of infection in 2 herds. In the 7 remaining herds described in this case series an epidemiologic link to a source could not be determined and molecular typing results did not associate M. bovis isolates acquired from these herds with another specific U.S. herd or U.S.-born animal. Preventing new introductions of M. bovis onto California dairies will require rigorous epidemiologic investigation of all the potential pathways of introduction discussed here. The root cause(s) of bTB on California dairies is certainly multifactorial with complex interactions of herd management practices, importation of cattle at greater risk of exposure to infected cattle, and the potential of human M. bovis exposure. The extensive use of molecular typing has improved epidemiologists' ability to narrow the scope of potential sources.