If you use it to add to the feed make sure it's not dry as the consistency is like flour and you need to be careful not to breathe it in as well as prevent your horses or other pets from doing so. I dusted my cocker when I discovered a couple of fleas in him and they were gone within a couple of days (also bathed him with blue dawn, it kills fleas). I use DE myself, I stir a large TB in fruit juice and drink it before going to bed. Just make sure you always buy FOOD grade, never POOL grade! I order mine on the net as the one at TSC is different, that one I only use to sprinkle outside, around the house (kills all bugs, incl spiders, fire ants etc) or to dry up the urine in my mini's stall as she always uses the same spot. Its really good for drying things up. Also, the main ingredient in the thrush treatment, I think it's called "no thrush" (and has great reviews on the net) is DE! There are tons of uses for it! Just google it you'll find lots of sites with plenty information.
As far as worming, it may help to add but I wouldn't solely rely on it as a dewormer. It doesn't kill worms on the ground only BUGS! But it does clean out the intestines and blood vessels and is supposed to get rid of heavy metals in the body, not sure how much of that is proven, but I do know that I'm feeling better, more energetic and have had many other benefits since taking it. Also lowers cholesterol and blood pressure.