Yes it's contagious. I think I am right that the bug can persist for 3 weeks on pasture.
Lime in gateways and around troughs will help a lot.
Maybe one of the vets can comment about Footvax - my understanding is that Footvax immunises against footrot, which is a different bug to scald. Footrot can sometimes opportunely infect where there is already scald to weaken the foot, but if your problem is pure scald then I am not sure if Footvax would be of any value.
Yes blue spray clears up scald.
You won't want to hear this with new pedigrees.... but if we have to repeatedly treat any sheep for scald, she's marked not for breeding. Proclivity to foot problems is highly hereditary.
Anyone's allowed to get scald - or even footrot - once, but they need to get better after that.
(And I'd have better foot health in the flock if I could get BH to follow the same protocol with the tups
)
However, I would certainly allow your newcomers time to adapt to the local buglife - it could be that this bug is on your farm, rather than that they brought it with them, but having not come across it before it's hitting them hard. They may be able to build up immunity to it.
Finally, yes footbathing can help control scald. Formulin is, however,
intensely painful on open flesh, so although walking them through a 2% solution every 2-3 weeks (and then standing them on clean dry cement for an hour) will stay on top of the problem, you would have to steel yourself to watch them in apparent agony for a few minutes after the treatment each time. Golden Hoof might be less painful and as effective, but much more of a palaver to administer as they have to stand in it for some minutes.