<i>Brucella abortus</i> <scp>RB51</scp> Persistent Infection and Milk Shedding in Vaccinated Dairy Cattle: A Public Health Risk

INTRODUCTION: Brucellosis, caused by a group of bacteria within the genus Brucella, is a zoonotic disease of significant public health concern. The RB51 strain vaccine is used to combat cattle brucellosis but has potential health consequences for exposed humans. METHODS: A retrospective review of NVSL's records prior to 2021 allowed identification of six cases of RB51 strain Brucella in the milk of dairy cattle through culture, followed up by PCR and whole genome sequencing (WGS). RESULTS: Six properly vaccinated dairy heifer calves actively shed strain RB51 in their milk as adults, years after vaccination. Two of these cows are linked to three human strain RB51 brucellosis cases (one cow to one human, one cow to two humans). All human cases were detected following consumption of unpasteurised ('raw') dairy products, highlighting the risk of raw dairy consumption. CONCLUSIONS: These cases demonstrate that milk-borne RB51 strain Brucella has been responsible for multiple human cases of Brucella. Dairy products from cows vaccinated with RB51 should not be consumed raw. The continued and widespread use of the RB51 vaccine should be evaluated critically, and authorities and veterinarians should revisit regulations and vaccination practices to ensure they are epidemiologically justified, particularly in regions or herds without documented brucellosis risk.