I'm no expert but I did go to a pasture management session the other day, lectured by an expert who understood small farms. He said they are permanently in our environment, at a very low level, just waiting for a host. They overwinter in a stage of their life cycle. So apparently there is no such thing as 'clean' pasture, only 'safe' pasture.
That being said, I think some species (nematodirus?) only last in their 'worm' and therefore infective stage for three weeks (they hatch, don't find a host then die) but hopefully someone more knowledgable can expand on this!