Investigation of cellular and whole tissue infection, proliferation, and shedding of Coxiella burnetii in bovine mammary gland tissue.

Abstract

Coxiella burnetii is a gram negative obligate intracellular bacterium that is the causative agent of Q fever in humans and coxiellosis in animals. The bovine mammary gland may have a key role in C. burnetii pathogenesis as it can be excreted persistently in milk throughout a lactation. Such shedding may pose an ongoing zoonotic transmission risk or impact milk production, and therefore, this thesis aimed to investigate C. burnetii excretion in milk and disease manifestation in the mammary gland. A longitudinal study was conducted in a C. burnetii endemically infected Australian dairy cow herd to describe bacterial excretion and humoral and cellular immune responses. Blood, placenta, faeces, vaginal fluid, milk were collected between three weeks prior to calving until ~200 days in milk. Next, the impact of C. burnetii infection on milk volume, total solids, and somatic cell count (SCC) was explored. A variety of C. burnetii excretion routes and immune responses were described, with excretion of C. burnetii occurring through milk at 200 days after calving in 13/133 sampled cows. These cows had high antibodies titres throughout the entire seven-month sampling period which together with the low observed Interferon gamma response may indicate persistent infection in cows is driven by a T helper 2 preference. Cows with a placental infection produced 2.4 L/d less volume and cows with C. burnetii detected in the vaginal fluid produced 0.2 kg/d less total solids, compared to cows without C. burnetii detected in the placenta and vaginal fluid, respectively. Furthermore, the presence of C. burnetii in gland quarter milk was associated with significantly higher quarter SCC compared to quarters without C. burnetii. Ultimately, this thesis helps expand understanding of C. burnetii pathogenesis in cattle, including an improved insight into the different infection states in cows and the impact of these for the public health and dairy industry sectors.