Chickens maintain their feathers and skin and control parasites by dust bathing, which is no more than a dry dirt bath. Some claim that adding DE to the dust bathing area combats external parasites (mites, lice, fleas) and that adding it to their feed controls internal parasites (worms).
Members of my flock, enjoying a nice, communal dust bath.
As a lawyer, when reading about the benefits of any product, a red flag always goes up in my mind when I read product label claims prefaced by "may, might or could," particularly when written by a company selling the product. For example: "...the addition of (DE) to chicken feed may also provide additional benefits including increased body weight, egg production and egg quality as well as decreased internal parasites (in poultry with a lower natural resistance)." Okay, it may. But it may not. Where's the scientific proof that it does?
According to Gail Damerow in The Chicken Encyclopedia, adding diatomaceous earth, wood ashes or lime-and-sulfur garden powder to their dust bath is hazardous to their respiratory health and should be avoided unless they are "seriously infested" with parasites. Even in that case, she writes, "the benefit may outweigh the danger of TEMPORARILY adding such materials." (p. 93 emphasis added)
Damerow also states: "Diatomaceous earth is also sometimes fed to chickens as a dewormer, which supposedly causes dehydration and death to internal parasites. But when combined with a chicken's saliva, diatomaceous earth softens and loses its cutting edge. The only way it could dehydrate internal parasites would be if it contained a hydrophilic substance that draws moisture from a parasite's body, and such a substance would equally affect the chicken's innards."
In The Chicken Health Handbook, 2ed, Damerow states, "Some types of respirable dust physically stick to lung tissue...(r)espirable particles of diatomaceous earth is an example of dust that sticks, causing scar tissue to form that impairs respiration." (pg. 158 emphasis added)
The theory is that DE is dehydrating because of the sand content, and because the algae are microscopically jagged, they scratch the waxy coating on parasites, allowing them to be killed by dehydrating the worm, tick, cocci or flea. It is touted as a "natural" worming medication and external parasite medication. What you need to realize is that the gut is full of water (making it very hard to dehydrate anything), and that it takes a long exposure time to kill multicellular parasites (ie ticks and mites).
I'm also a little leery of any product that claims to do everything....diatomaceous earth claims to be useful in animals, on animals, in the walls of barns, coops and houses, as a pasture treatment, in the yard and garden, and as a treatment for granaries, kills bacteria, viruses, absorbs and neutralizes mercury, eliminates drug residues, absorbs organophosphate pesticides, etc, etc. Really? To be honest, all the scientific literature is very equivocal on the usefulness. Some studies find "trends", but very little statistical significance, some studies say that DE is poorly effective at a relative humidity above 85%. One study showed more parasites in one breed of hen fed DE and less parasites in another breed of hen fed DE.
At the end of the day, my feeling is that if you want to treat the parasites in your flock, use a treatment that works....has been designed to work and has been proven to work. A well-conceived parasite program that uses different classes of drugs, observes withdrawal times and will effectively control parasites as well as resistance.
I apologize to any "anti-drug" advocates that may be offended by my stance in this regard, but I have seen too many instances of treatments that served only to allow animals to suffer infections until regular treatments are finally commenced."
Dr. Mike Petrik, DVM, MSc,
The Chicken Vet