This decision is largely based on the classification of the herd as a closed, modified open or open herd. A closed herd has no new additions except for bulls and there is no animal to animal contact with neighboring herds. New bulls are isolated prior to introduction into the herd, and appropriately tested for and vaccinated against transmissible diseases during the isolation period. The level of immunization necessary for a closed herd is less than that for other herds because the risk of exposure is low. Modified open herds have a higher risk of exposure to pregnancy-wasting diseases through limited additions of bulls and replacement females, animals moving in and out of the herd to exhibitions, or through contact with neighboring herds.
Open herds have a high risk of exposure to pregnancy-wasting pathogens.
This may be due to the frequent introduction of replacements or exposure of the breeding herd to stocker cattle or other recently purchased, stressed cattle. As brucellosis is a disease that can be spread to humans, its control in animals is especially important. Brucella abortis strain 19 vaccine is a live bacterial product and confers long-term cell mediated protection in healthy animals vaccinated properly. Brucellosis is also controlled by a monitoring system of blood testing and identifying market cattle, cattle at livestock sales and cattle moving across state lines. Whether or not to continue with a brucellosis vaccination program should be determined only after considering interstate movement, risk of exposure and legal responsibility.