The persistence of Brucella abortus infection in calves: a retrospective study of heavily infected herds.

A retrospective inquiry into 170 herds which were slaughtered in the first three years of the brucella eradication scheme found that in the 115 herds where all animals were slaughtered there were no subsequent breakdowns, other than in herds which were restocked with imported animals. In the 46 herds in which only the adult stock were slaughtered, 11 herds suffered breakdowns. In 10 of these 11 herds the sources of reinfection were possibly reprieved heifers. Seven out of the 11 initial reactors in these 10 herds were known to be offspring of reactor dams, one was born of a serologically negative dam and in the other three cases the dams were not identified. Out of the 11 initial reactors, six could have acquired infection at any time up to and including puberty, but in five cases infection could only have been acquired in utero or in early calfhood. It is estimated that under the circumstances of the British eradication scheme there is a risk of 2.52 per cent of heifer calves born of serologically positive dams reacting in early adulthood and thus constituting a risk to the re-established herds.