Red Lake Diatomaceous Earth is registered as a feed additive for use in livestock feed as an anti-caking agent and pelleting aid. However, food grade diatomaceous earth is also known to work well as a natural insecticide. DE works to kill pests such as bed bugs, mites, fleas, ticks and many other crawling insects by lacerating their exoskeletons and dehydrating them.
The reason that diatomaceous earth is safe for animals to ingest but also works so well to kill insects has to do with the fact that it works as an abrasive only at a microscopic level. DE particles are very tiny and their razor-sharp edges only function to lacerate surfaces at a microscopic level. Because insects are also small, the diatomaceous earth can easily cut their exoskeletons. The movement of their bodies across the DE, as well as their waxy exoskeletons, creates conditions in which the DE rubs against their bodies and lacerates their outer shells. A large, strong surface, on the other hand, such as the tissue of the intestines and stomach are not affected by the sharp edges of the diatomaceous earth, as the particles are too tiny to cause any damage.
So, while food grade DE has the ability to kill insects due to the razor-sharp edges of its particles, it is completely safe for animals to consume.