Persistence of Brucella abortus strain 19 infection in adult cattle vaccinated with reduced doses.

Brucella abortus strain 19 was recovered at necropsy from cows vaccinated with reduced doses of strain 19, three to six months previously. Strain 19 was isolated from four of nine cows vaccinated with 5.8 x 10(9) viable cells during pregnancy. In two of these nine cows vaccination had led to colonisation of the uterus with erythritol-utilising mutants of strain 19 and consequent abortion or premature calving. In one of another group of nine pregnant vaccinated with 2.8 x 10(8) viable cells, the vaccine strain was recovered from the colostrum. There was no interference with the course of pregnancy in over 700 cattle vaccinated with 3 x 10(8) viable cells. Thirteen of these cows, serologically positive approximately three months after vaccination, were necropsied and strain 19 was isolated from six. This lower dose has already been shown to produce immunity equal to that of higher doses and is recommended as suitable for whole herd vaccination.