Abstract
Studies were carried out in South America to assess the effect of maternally derived antibody (MDA) on the responsiveness of calves to FMD vaccination. It was found that calves with MDA did not merely fail to respond to vaccination, but that their serum titres were depressed. This depression was proportional to the level of pre-existing MDA at the time of vaccination and following primary vaccination it persisted for a least 60 days. High MDA titres interfered with both primary and secondary responses. Animals with relatively low MDA titres were able to respond to vaccination, or at least to be sensitized so that on revaccination they showed a satisfactory response. The half-life of MDA was shown to be approximately 22 days, suggesting that under field conditions significant MDA titres are likely to persist for 4-5 months. A trial carried out in Brazil in which the primary course of two inoculations, 4 weeks apart, was initiated when the calves were 5-6 months of age, resulted in the reduction of FMD in the calf population from 11% to 0.9% over a 12-month period. The use of vaccination programmes of this type to lessen the incidence of FMD in young bovines is discussed.