Calves which have received no antibodies against Br abortus in their colostrum may respond to strain 19 vaccination even on the first day of life. However, calves with such antibodies may completely suppress any response to S19 vaccination. In calves which have received very high titres of passive antibodies from their dams complete suppresson of vaccinal response may be expected to occur up to about 153 days of age. Thus, if vaccination is restricted to between 90 and 180 days many calves in infected herds will not be protected by routine vaccination with S19. In calves born of and reared on infected dams the colostral antibodies also have a marked protective effect. In spite of heavy exposure to Br abortus at birth and later from infected milk only six out of 64 calves (9.4 per cent) developed persistent positive titres. The remaining calves reverted to zero titres within 12 months and an anamnestic test indicated that of these up to 15 per cent may have been latent carriers but even more remarkably, 85 per cent of such calves were not even "primed" by such heavy exposure.
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