Prevalence and persistence of Brucella abortus strain 19 infections and prevalence of other biotypes in vaccinated adult dairy cattle.

The prevalence and persistence of Brucella abortus strain 19 shedding from the udder were studied in adult cattle in infected dairy herds that were vaccinated by 4 methods. Specimens were cultured from 1,736 cows in 55 vaccinated herds. Of the 744 brucella isolates, 98 were strain 19 and were from cattle in 25 of the herds. Shedding was highest among conjunctival vaccinates (1.4%) and higher in those given the standard subcutaneous dose (0.83%) than in those given a reduced subcutaneous dose (0.45%). Isolations were not made from intradermally vaccinated cattle. The persistence of strain 19 shedding was related to the length of time after the initial isolation. Approximately 20% of the shedders that remained in the herds were seropositive 13-18 months later. Cattle were selected for bacteriologic studies largely on positive results of rivanol or complement-fixation tests inasmuch as the card and tube agglutination tests were unreliable. Of the 646 field strain isolates, 95.8% were biotype 1, 2.5% were biotype 2, and 1.7% were biotype 4.