Bot flies lay eggs on the horse, and the horse licks the larvae, and get infested with the bot fly's larvae. It does NOT spread from horse to horse, but a new fly has to be born and lay new eggs.
Dramatic weight loss in the spring is a lot of times tape worm. If you haven't dewormed against tapes for a long time, that would be my first thing to do. Get something that has praziquantel in it. You should se results in just a few weeks.