Dr. Damien Barrett:

They have to get it from somewhere. Cattle to cattle transmission is something we are probably not dealing with enough. The spread within species is far greater than the spread between species. The spread between badgers is greater then the spread from badgers to cattle and the spread between cattle is greater than the spread between badgers and cattle. Animals do not get it out of nowhere. There may well be animals that are latently infected or anergic and they may not react to the test. A feature of the disease is that certain animals do not react. It is long-standing animals that probably pose the greatest risk as they may not necessarily react. There has to be a source somewhere.