Changes in the dynamics of Coxiella burnetii infection in dairy cattle: an approach to match field data with the epidemiological cycle of C. burnetii in endemic herds.

This study aimed to evaluate changes in the epidemiological status of Coxiella burnetii in dairy cattle herds to better understand the epidemiology of the infection and to predict its evolution. Bulk-tank milk (BTM) and serum samples were collected from 94 dairy cattle herds and analyzed by ELISA (BTM and sera) and PCR (BTM) in study 1 (S1). Two years later (study 2; S2), the same farms were visited with a similar sampling approach. To estimate seroconversion during this period, blood samples were collected from the maximum possible number of animals surveyed in S1. Environmental samples were collected in S2 to identify active shedding. Farms were allocated into 3 different categories in each study according to PCR and ELISA results: category A, with BTM ELISA and PCR positive herds and at least 1 seropositive animal; category B, with BTM ELISA or PCR positive herds or individual sera positive; and category C, with all negative results among herds. Changes in herd category between S1 and S2 were grouped in 9 classes. Two statistical models, one to search for drives of within-herd changes in C. burnetii infection status and another to look for variables modulating individual changes in C. burnetii antibody level, were built. Several herds in category A in S1 remained in that category 2 yr later, indicating that C. burnetii can remain within a herd for a long time. Most of the herds with seroconversion and detection of the bacterium in the environment belonged to category A, suggesting active and recent infections. Changes in the epidemiological status of herds were driven by local densities of domestic ruminants, showing the implication of neighbor reservoirs; whereas individual changes in antibody levels were modulated by variation in the epidemiological status of herds. Observed changes in epidemiological status allowed depiction of the hypothesized life cycle of C. burnetii within dairy cattle herds, which should be tested by future long-term series studies on C. burnetii infection to help fitting control measures (e.g., vaccination) to within-herd C. burnetii status.