I've used DE for putting on their coats as the last time I used a flea collar on them, both dogs got visibly agitated - and then I read something on FB about certain products, i.e flea pour-on products/capsules and the like, causing bad allergies and even death to one poor little dog.
So I don't use chemical stuff any more.
I also use DE on the chooks and on my cob for his feather mites.
Have heard of its use internally, but not tried it.
This is where I get mine from, there is info and some links available from this page.
Editing to say there are 2 different types of DE. The food grade stuff is very different to the type used in swimming pool filters, which contains glass particles. The link above should explain more but what I buy is human food grade, so very good quality and safe. Suitable for internal and external use as discussed above, I think, for various animals, oh and they also do worm count tests at very reasonable prices (compared to my vet anyway).
Hope there is something useful on there for somebody.