Treating eye worms in Chickens

[FriendFarm]

We have several chickens that have worms in their eyes, not the white ones in the corner, but small black ones on the edges of the lens. My question is what should I use to treat them? I have heard that Ivermectin will work, but how do I dose it and which do I use? Also, will the local vet sell Vet RX? I know that there are a lot of questions about this, but I see a lot of different answers and was hoping that someone could explain the differences. Also, can we treat the chickens that don't have the worms as a preventative? Does anyone know what causes these worms? We don't have cockroaches up here (it's too cold), and thats the only thing that I've seen can cause eye worms. As newcomer to chicken raising, I have no clue about any of this stuff
Thanks everyone
The FriendFarm