Worm eggs are everywhere in the soil, on the soil, on grass etc...it's impossible to get rid of them. Look up a diagram of roundworm lifecycle and you'll understand how easy it is for chickens to pick up infective eggs. One roundworm lays thousands of eggs in one day, then the eggs are excreted onto/into the soil. Rotating pasture, keeping grass cut short, and keeping everything as dry as possible are good deterents. Some folks lime soil in chicken pens, but some types of lime will burn the chickens feet and can be a problem if inhaled. I've never used lime, I prefer sand in my pens, 5-6 inches deep. However it's best to set up a routine worming schedule keeping your soil conditions in mind. I worm my birds every 3 months, sometimes sooner when it rains alot like last year. Keep in mind that insects can carry worm eggs as well; ants, flies, termites, earthworms, beetles, grasshoppers, snails, wild bird droppings etc etc etc.
