Screw worms/maggots are very effectively killed with a product widely available called "Screw Worm Spray". You can find it at any farm store or vet supply catalog.
They enjoy an environment of moist dirty skin, so keeping sheep "clean and dry" is important. Most often found around tail area of long tailed lambs or around wounds. They will infect about any animal that has an open wound that is not kept clean or dry.
Note: if you are in a really quiet area and put your ear within 10-12 inches you can hear them work on the skin....