What to use on my horse for mosquito's?

my horse is allergic to mosquito's that's what the vet said they said to buy something to keep the mosquito's away from her i have buy lots of stuff but it seems to only last for a few minutes or so. I want to know if there is something stronger that i can use on her because when its summer and there are lots of mosquito's she gets some fetlock wounds due to mosquito's so is there something i can use that would be stronger and last longer?