when my boy used to get them I used to use bonjella! Do you know why they are there? Eliminating the reason is always a start! I couldn't as my boy used to get them eating the thistles and thorny bushes in his field!!!
Are they rough and circular with aflatish surface?? If so it sounds like a reaction to certain bot flys- the best way to deal with it is to worm with a wormer specific to bots- they go quite quickly after that.
no sure about them being rough and circular...look a bit more like flesh woundy things....well got the vet coming later to vet my other mare so shall ask him them!!