It's possible the giardia protozoa was introduced into the environment by wild birds. Chickens constantly peck the soil and pick up all kinds of nasty parasites, especially if the soil is wet or damp. It is normally transmitted through feces contaminated water. Giardia cant be completely be eliminated from soil. Keeping everything as dry as possible, removing poultry feces,ensuring fresh water is available at all times, and keeping feeders free of feces will help deter the protozoa. Putting sand in pens/runs helps alot as well.