|MadSci Network: Microbiology|
Well, to start of, brucelosis is not caused by a virus. It is cause by a bacteria, which is called Brucelosis abortus. In bisons it doesn't do much harm, they don't really get ill. However in cows, it will prevent little calves from being born, also known as abortus Bang. Since big money is involved in the cow-bussiness, farmers wouldn't like bisons to bring this disease to their cows.
Now the question is how fast can this bacterium grow? That is hard to answer, since I don't know the bacterium. However, it is considered a slow grower in the laboratorium. It can take up to six (!) weeks to grow it. This is very slow, because many bacteria can be grown in one day. Of course the bacterium may grow faster in the bison or cows.
The first is some quite technical info in Australia from and for vet's.
The second is a news-article from CNN.
Try the links in the MadSci Library for more information on Microbiology.