Calves do not respond adequately to lepto vaccination until they are over 3 months of age. In problem situations, cattle should be vaccinated for lepto every 6 months. If a cow aborts with brucellosis, it is usually during the last third of gestation. Less dramatic symptoms, but symptoms which occur more frequently than abortion, include retained placenta, lowered fertility, and reduced milk production. Cows that become infected after reaching sexual maturity usually remain infected for a long time, perhaps for life.
Calfhood infection is usually transient.
However, there is evidence that some calfhood infections go through a latent period and reactivate following puberty. Vaccination is a helpful tool in maintaining resistance to brucellosis in a cattle population. Two-thirds of cattle properly vaccinated will resist moderate exposure. No animal can be considered immune to severe exposure. Vaccination is required for breeding females to be moved into some states.