Coxiella BurnetII infections in intensively-managed dairy goats: elucidation of transmission dynamics and vaccination strategies

Abstract

Coxiella burnetii causes Q fever, a zoonotic disease commonly associated with exposure to infected animals, particularly ruminants. In this study, the dynamics of C. burnetii infections were elucidated by detecting serological responses to C. burnetii and DNA of the organism in samples from adult and kid goats on a large dairy enterprise in Australia on which 24 cases of Q fever had occurred since 2013. An indirect immunofluorescence assay (IFA), initially developed for human serology, was adapted and standardised to detect IgG and IgM antibodies to phase 1 and to phase 2 antigens of C. burnetii. A cross-sectional study was then undertaken in 164 pregnant goats of different parities (86 primiparous and 78 multiparous) to determine the changes in seroprevalence over the kidding period. Following kidding, the seroprevalence in adult goats increased by 17.7%; the increase was higher (26.7%) in primiparous than in multiparous goats (7.7%) as 47.4% of the multiparous goats were seropositive pre-partum compared to 4.7% of the primiparous goats. However, the risk of seroconversion in multiparous and primiparous goats that were seronegative pre-partum was comparable; 24.4% and 26.8%, respectively, suggesting a constant risk of infection with C. burnetii to all susceptible goats across the kidding period, irrespective of parity status. A longitudinal study in 95 kids from two successive kidding seasons was performed to elucidate the infection dynamics of C. burnetii. Maternally-derived antibody was detected after feeding pooled goat colostrum but this subsided within 9 weeks and new infections (shown by a rise in phase 2 IgM) occurred soon after. As kid goats were infected very early in life and well before they were mated, it was concluded that there is a need to vaccinate young goats, before they are 9 weeks of age, in addition to the conventional strategy of vaccinating adult goats shortly before breeding. Since a vaccine registered for use in goats is not currently available in Australia, an inactivated autogenous vaccine, prepared from an isolate from the goat farm, was trialled in 8-week-old goats. Two injections, four weeks apart elicited specific IgM and IgG responses in all vaccinated goats (n = 11), while no antibodies were detected in two control groups (n = 22). Swelling at the site of inoculation was observed in all of the vaccinated and in 91% of the control group but receded after 3 weeks. The data indicated that this autogenous vaccine could be suitable for immunising young goats (< 3 months of age) to break the cycle of infection with C. burnetii on this property although further trials to determine level of protection against field challenge are required.