Brucellosis – Booster Vaccination with RB51

By Charles C. Price, Region IV Vice President, Wyoming Stock Growers Association

On Jan. 1, 2011 the Wyoming Livestock Roundup published a guest opinion where I wrote about a experiment that my son and I were doing to determine if we could safely booster vaccinate pregnant cows with RB51 at our fall pregnancy test.

In cooperation with the state veterinarian, we booster-vaccinated all of our yearling heifers and all of our adult cows with RB51 while they were open in Spring 2010, prior to breeding. When we pregnancy tested these cattle in the fall we had a good conception rate and found no adverse effects. At the fall pregnancy test we had our veterinarian give a second booster vaccination of RB51 to 20 head of our pregnant commercial replacement heifers.

On May 19, 2011 the last of our 88 replacement heifers calved. All of the heifers carried their calves to the full term, a little unusual for us because we often have one or two that will abort their calf or show up in heat by the end of calving. We lost one calf in the registered heifers because we waited too long to pull it.

Of the 20 commercial replacement heifers we had given the second booster vaccination to at the time of their pregnancy test, all produced a healthy live calf. Based on this result, we believe that cattle that receive their booster vaccination while they are “open” (not pregnant) can then be booster vaccinated at a convenient time when they are pregnant with a minimum risk for RB51 vaccine induced abortion. This following booster shot must be done within a time period when the last booster shot is still effective. We plan on booster vaccination of all the bred cows that we will retain at our 2011 fall pregnancy test.

While we are confident that giving the first booster shot to the cows while they are open allows the next booster to be given to them while they are pregnant with minimal risk of a vaccine induced abortion, a number of questions regarding booster vaccination remain:

What time interval is acceptable between booster shots with RB51 to minimize the risk of a vaccine induced abortion?

If we follow this procedure for booster vaccination, will it provide a sufficient reduction in risk for contracting the field strain of brucellosis from wildlife and constrain the spread of the disease within a herd that does become infected?

Time and further testing will provide an answer to the first question. This can be done by taking a herd of cattle of sufficient size and vaccinating one group annually, a second group every other year, another group every third year, etc, until the vaccine induced abortion become evident. From this information a reasonable booster vaccination interval can be determined.

The second question and the most important one will be the most difficult to answer. A partial but uncertain answer may be found when booster vaccinated cattle in high risk areas comingle with infected elk. If there is a significant reduction in the instances of the field strain of brucellosis crossing over to the cattle, this could indicate that booster vaccination is effective. However, the best way to determine if the booster vaccination with RB51 provides a significant protection against contracting or spreading brucellosis is a well planned test where cattle with different levels of vaccination are challenged by the field strain of brucellosis.

In summary, we know that the effectiveness of a vaccination with RB51 declines rather rapidly with time. For this reason it is necessary to booster vaccinate the cattle to maintain a high level of immunity. We have shown that by giving the first booster vaccination while the cows are open they can be vaccinated later when they are pregnant with a minimal risk of a vaccine induced abortion.

It may be that booster vaccination with RB51 is sufficiently effective against the field strain of brucellosis. This would be a welcome result. However, if it is not, we are wasting a lot of time and money following this route to an extreme without testing to determine its effectiveness. We need to consider the possibility that the booster vaccination with RB51 is inadequate and only offers a bridge to a more effective vaccine that can come from research at the University of Wyoming BSL-3 lab. It is my opinion that a challenge test is imperative and planning for it should start now.