Not sure about Dorset but it is very bad on the Forest this summer, it's normally only seen in winter around here, but there is a lot around. My yard have just had to report a pony to the verderers as it was stood outside our yard with full on bastard strangles.
It has been contained to one yard as far as i know. Only one horse, all the others have been blood tested clear. I think the infected one just has to have one more clear test then the yard will be open. Was alarming to start with but not too much of a drama as they caught it quickly and were very responsible they way it was dealt with.
Well we have it in our area near the pony sales at Beaulieu Rd, alos Newforester mentioned it was around her area at New Park a few weeks back. The trouble is it's endemic in New Forest Ponies so pops up all the time, just unusual at this time of year. As long as you don't play with the wild ponies you are normally fine.