Diatomaceous Earth (DE) is a type of soil that is formed from the fossilised remains of diatoms, tiny hard-shelled protists. DE is usually high in silica, and is a granular powder. DE has also been touted as an organic anthelmintic, with the hypothesis that the sharp, silaceous material of the fossils slashes parasites, causing death. Look up an organic farming/animal health blog and it would appear that DE is a wonder material, ready to fix all your parasite problems.
Unfortunately, wonder-materials do not occur, otherwise we would all be using them. There is almost* no scientific evidence to prove that feeding livestock DE has any impact on internal parasite burdens. Studies have been conducted comparing FEC, blood values (such as haemoglobin and red blood cell count – indicators of infection) and weight gain against the addition of DE to feed in sheep, cattle, goats and poultry. Results from these studies have consistently shown no differences in parasite burdens between animals that were fed DE and those that were not. Some studies did show that DE may** increase weight gain – however that is likely to be due to the increase in dietary minerals supplied by DE.
One anti-parasitic effect that DE has been shown to have, is that when used externally it can aid in the prevention and removal of ectoparasites (mites/lice) in birds. However, the authors of one study warned that the effects of DE as a topical treatment is gradual, rather than the instant removal of ectoparasites one would observe with a chemical compound.
In conclusion, all studies that investigate DE state that DE alone is not enough to control parasites, however inclusion in the diet may have other benefits unrelated to parasite infection. A parasite control regime should focus on correct diagnosis, manure management and appropriate treatment with chemical anthelmintics, or if an organic property is run, with supplements shown to have anti-parasitic effects, such a certain plants and plant extracts.
If you would like to look up these topics yourself, I would suggest visiting Google Scholar rather than Google. Google scholar will lead to you published scientific papers, that although you may not get full access to, you can get the drift of the study from the abstract paragraph.
*almost is included as without reading every paper ever published I cannot say that there is definitely no evidence.
**may does not mean it does. May means that more research is needed.
References:
Osweiler, Gary D. and Carson, Thomas L., "Evaluation of diatomaceous earth as an adjunct to sheep parasite control in organic farming" (1997). Leopold Center Completed Grant Reports. 102.
Bernard, G., Worku, M., & Ahmedna, M. (2009). The effects of diatomaceous earth on parasite-infected goats. Bull. Georgian Natl. Acad. Sci, 3, 129-135.
Fernandez, M. I., Woodward, B. W., & Stromberg, B. E. (1998). Effect of diatomaceous earth as an anthelmintic treatment on internal parasites and feedlot performance of beef steers. Animal Science, 66(3), 635-641.
McLean, B., Frost, D., Evans, E., Clarke, A., & Griffiths, B. (2005). The inclusion of diatomaceous earth in the diet of grazing ruminants and its effect on gastrointestinal parasite burdens. ADAS Agricultural Research and Consulting Report.
Tello-Velamazán, Julia, et al. "Use of diatomaceous earth as a dietary supplement in organic hens and its effects on parasite load, egg production and egg quality." WPSA conference. Malaga. Spain. 2015.
Beltran, M. A. G., & Martin, R. (2016). DIATOMACEOUS EARTH INHIBITED THE IN VITRO MIGRATION OF Oesophagustomum dendatum LARVAE. Philippine Journal of Veterinary and Animal Sciences, 41(2).
Nkwana, A. T., Hoon, J. H., & Fourie, P. J. THE EFFECT OF DIFFERENT INCLUSION LEVELS OF DIATOMACEOUS EARTH IN FEEDLOT DIETS ON THE PERFORMANCE OF LAMBS.
Isabirye, R. A., Waiswa, C., Kabi, F., Nanyeenya, W. N., Biryomumaisho, S., Acai-Okwee, J., ... & Nasinyama, G. W. (2019). Efficacy of Diatomaceous Earth on Ascaridia galli, Blood Parameters: And on Ectoparasites In Chicken.