Longitudinal surveillance of Coxiella burnetii following an abortion storm in domestic goats

Abstract

Q fever is a disease caused by Coxiella burnetii, which can cause serious illness in humans and abortions in goats. A Q fever outbreak among an unvaccinated goat herd led to a 65% loss of the kid crop in spring 2018. To assess the impact of the outbreak on the herd and environment, longitudinal surveillance of the ranch was conducted across three samplings in September 2018, April 2019, and May 2022. Antibodies against C. burnetii were monitored by an indirect immunofluorescence assay. Shedding was monitored through analysis of vaginal/fecal swabs and milk. Environmental swabs and bulk soil were collected from various locations around the ranch. Animal and environmental samples were analyzed for C. burnetii DNA by PCR. Herd-level seroprevalence decreased from 89% in 2018 to 84.3% in 2019, and 64.5% in 2022. Overall herd shedding was 14.4% in 2018, 7.4% in 2019, and 6.7% in 2022. The percentage of C. burnetii-positive environmental samples was 83.7% in 2018, 51.7% in 2019, and 28.6% in 2022. Serological evidence suggests that new infections were occurring in the herd 4 years post-abortion storm. This study demonstrates the presence of C. burnetii shedding and environmental contamination in a goat operation at least four kidding seasons after an outbreak. A better understanding of management practices that can improve outcomes for infected herds, particularly in areas without access to vaccines against C. burnetii, is needed to better protect operators and the public.

Keywords: Q fever; coxiellosis; livestock; one health; zoonosis.

Figure 1

Study timeline. Timeframe from the initial abortion event marking the beginning of the outbreak in 2018 through the final sampling in 2022 is depicted. Kidding seasons are noted as purple squares.

Figure 2

Serological analysis of goats following a C. burnetii abortion event. (A) Antibody titers against PhI C. burnetii are plotted against the PhII titers for each sampling year. Titers ≥128 are considered positive. This cutoff is indicated as a red dashed line. (B) GMT ± range is given for PhI (black) and PhII (blue) across the study period. **p < 0.01, ****p < 0.0001 as determined by Welch’s one-way ANOVA with Dunnett’s T3 multiple comparisons test.

Figure 3

Anti-C. burnetii titers over time post-abortion storm. Transformed (log₂) antibody titers against C. burnetii (A) PhI and (B) PhII are paired and plotted on the left y-axis for the 58 seropositive goats that were available for testing in both 2018 and 2019. One goat whose PhI titer fell to <16 in 2019 is not included in the PhI analysis. Titers ≥128 (red dashed line) are considered positive. Box and whiskers plot (± range) of the difference between the means for each pair are plotted on the right y-axis. ***p < 0.001, ****p < 0.0001 as determined by paired t-test. Serial testing for anti-C. burnetii antibody titers against (C) PhI and (D) PhII for the four goats tested across all periods.

Figure 4

Environmental burden and spatial distribution of C. burnetii following an abortion event. Environmental samples were collected from various locations around the ranch. (A) Bulk samples were mapped by longitude and latitude and relevant geographic features from satellite imagery were overlayed. Negative samples are displayed as open circles. Positive samples are colored based on GE per gram of soil. One negative sample collected in 2019 from the boundary of the property is not displayed for better visualization of the remaining data points. Samples were analyzed by IS1111 PCR for the presence of C. burnetii DNA. Data from positive (B) bulk soil and (C) environmental swabs are presented as the transformed (log₁₀) quantity estimates for individual positive samples along with the geometric mean (bold horizontal bar) and geometric standard deviation (error bars). No environmental samples were positive in 2022.