Coxiella burnetii shedding in dairy cattle and its correlation with humoral and cell-mediated immune responses

Coxiella burnetii is the intracellular bacterium responsible for the zoonotic disease Q fever, with cattle among the reservoirs capable of shedding the bacterium for prolonged periods. This longitudinal study examined temporal variation in C. burnetii shedding and immune responses among 192 Australian cows in a naturally infected Australian dairy herd. Blood, placenta, faeces, vaginal mucus, and milk were collected, with the same cows sampled at up to five time points from before calving to mid-lactation (∼200 days in milk). Coxiella burnetii DNA was detected by multiplex qPCR, serum C. burnetii antibody evaluated by ELISA, and IFNγ and IL-10 responses to C. burnetii antigen stimulation assessed using a cytokine recall assay. Associations between antibody or cytokine response and shedding were analysed using logistic regression models, while differences in the prevalence of PCR-positive milk samples between sampling time points were assessed using generalised estimating equations. At calving, C. burnetii DNA was detected in 52% of placental specimens. Shedding declined in early lactation, remained low thereafter, but increased in milk at mid-lactation. Early lactation antibody titre was associated with mid-lactation C. burnetii milk shedding; for every 10-unit antibody titre increase, the odds of shedding increased by a factor of 2.14 (95% CI: 1.37 - 3.34; p = 0.001). No cow in the study had both a high antibody titre and a high IFNγ response at early lactation. These findings highlight the role of the host immune response in modulating C. burnetii shedding, particularly intermittent excretion through milk.