<i>Coxiella burnetii</i> seroconversion in neonatal calves in an infected dairy herd attributable to ingestion of imported colostrum replacer rather than vertical transmission

In April-June 2017, a cluster of 4 third-trimester abortions caused by <i>Coxiella burnetii</i> was diagnosed on a dairy farm with 356 Holstein cows in Uruguay. We investigated 1) the seroprevalence of <i>C. burnetii</i> in cows delivering normal calves in this herd, 2) whether non-aborting cows gave birth to seropositive and/or PCR-positive newborn calves in the year of the abortion cluster, and 3) whether seronegative newborn calves seroconverted after ingesting colostrum replacer. In March-September 2017, serum samples were collected from 133 calving cows and their calves immediately after birth and before colostrum ingestion. All 266 sera were analyzed for anti-<i>C. burnetii</i> IgG by ELISA. Additionally, 179 blood samples from some of these animals (82 cows, 97 calves) were PCR-tested for <i>C. burnetii</i> DNA. Thirty of the 133 (22.5%) cows delivering normal calves were seropositive at parturition; 4 of 82 (4.9%) were PCR-positive. All calves tested at birth by ELISA and PCR were negative. Our results suggest that congenital infection is not an important route of <i>C. burnetii</i> transmission to live-born calves in this herd. We observed that 76 of 80 (95%) seronegative newborn calves in 2021 became seropositive for anti-<i>C. burnetii</i> IgG at 24-h-old after drinking an imported commercial colostrum replacer, which could confound seroepidemiologic surveys in herds using colostrum replacers.