how to prevent mange ??

Im no expert but from what I understand, the best preventative for mange is simply maintaining a healthy dog and a clean kennel. Good hygiene in other words.
Use diluted bleach in a peach sprayer once a month in and around your kennels and be sure to keep your dogs on a good worming program.

Ivomec is used topically to successfully treat mange and I would imagine that giving it to them as a heart worm preventative would only aid in the same for mange but I cant be positive on that. I have nothing to back this up so just consider it a guess on my part.

I've never had an issue with mange other than a pup I brought home one time with Red mange. I cured him and that was that.

Sorry I couldn't give you anything more definitive but maybe someone with more insight will post soon.