Marc Caldwell presentation at 2019 Applied Reproductive Strategies in Beef Cattle

The fetus is the most susceptible to disease of any animal on the farm. The cow is basically a barrier for the fetus.
Good biosecurity practices are more impactful (important) than a vaccine protocol. But, in reality, the two work together.
Most disease problems are the results of purchasing practices. (Buying a BVD-PI cow for example.)
Modified live vaccines are not 100% safe. MLV vaccines can only be in pregnant animals if she has previously been vaccinated with a MLV. For pre breeding, should be used 30 days, and preferably 45 days, before the breeding season.
Killed vaccines are not 100% effective.
Two doses in time (2 to 4 weeks apart based on label) is still the best. The memory B and T cells created from two doses are what provide long term, lasting immunity.
Why do we use the same vaccines year after year? There is a new approach called Prime Boost. In this approach an animal receives two rounds of modified live vaccine. Then sometime in the future the animal receives an inactivated (killed) vaccine. Animals that receive a Prime Boost protocol have improved antibody titters and immune response.
Each farm must balance pathogen and vaccine risk. If the farm purchases bred replacement, the risk of BVD PI  entry is high. A MLV or combined approach is likely needed in this situation. If the vaccination status of the cows is unknown, a killed vaccine is safer.

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