I use dirt in every cage I have reptiles in except beardies under 3 months old. Every monitor I have has the same dirt from a local creek bank mixed with field dirt and varying amounts of sandbox sand. Sand is a lousey substrate, it holds 0 moisture, it does not hold a burrow, and allows all moisture to evaporate quickly. As far as parasites etc they are almost all species and host specific, no problems for your reptiles, if your that paranoid about a parasite your not allowing your animal to have an immune system, which naturally defends it from germs, cool temps, etc. If you are that paranoid I suppose you could bake it little by little until youve baked hundreds or thousands of pounds of dirt for a cage. Monitors live in, on, and around dirt, doesnt matter if they are arboreal or semi-aquatic they all use dirt and its important to them, something they recognize and use. If youve ever visited a desert youd find out that sand and dirt makes up a desert, and if you have one of the desert species (some aussie, or Iranian desert monitors) youd still need dirt just a higher percentage of sand in it. Once you keep a monitor in dirt youll never want to consider keeping them in anything else.