Abstract
The RB51 vaccine is an attenuated live strain of B. abortus and has been used in the United States since 1996 as part of the State-Federal Brucellosis Eradication Program. It is estimated that 4,000,000 calves are vaccinated annually with RB51 and an unknown proportion of these vaccinated animals may become chronic shedders of RB51. Since August 2017, the U.S. Centers for Disease Control and Prevention (CDC) has reported three confirmed human cases of brucellosis in Texas, New Jersey and New York due to B. abortus RB51. These cases were each associated with consumption of domestically acquired unpasteurized (raw) dairy products. The epidemiological investigation of a human RB51 case from New York led to a dairy farm in Pennsylvania (PA) as the potential source of infection. Laboratory testing by USDA’s National Veterinary Services Laboratory (NVSL) on animal samples collected from the PA farm identified one cow to be shedding RB51 in milk. Whole- genome sequencing (WGS) analysis of the RB51 strains cultured from the PA cow identified two distinct RB51 strain types that match the genetic profiles of RB51 strains isolated from two human RB51 brucellosis cases- one reported in October 2017 from New Jersey and another reported from New York in November 2018. CDC acquired the PA cow under an approved research protocol to investigatecurrently unknown questions related to the molecular diversity of RB51 in a host, patterns of shedding, host factors associated with chronic infection, and exposure risk to humans. From March to September 2019, we collected samples of quarter milk, blood, urine, rectal and vaginal swabs for culture, bacterial quantification and sequencing as well as antibiotic susceptibilty tesitng. Findings from this testing show intermittent shedding patterns of RB51 in the milk, genetic and phenotypic variations of the RB51 vaccine strain isolated in the mammary system and host immune response to infection with the RB51 vaccine strain.
