Most horses should be dewormed at least once yearly for tapeworms and usually twice a year for other parasites. However, the deworming frequency should be determined by the results of a fecal egg count, not from past experiences.
Many people also want to use nonchemical dewormers as they are afraid of chemicals and feel that a natural approach is better. However, natural substances are also chemicals and can be just or more toxic, and they are not regulated by the FDA as no testing is performed.
Dr. David Ramey reports in Veterinary Practice News that there is no effective method of killing internal parasites without chemicals. Certainly, you can take steps to control parasites, including removing manure from the pasture. However, some parasites will remain and must be treated. Some may point out that wild horses do not suffer from parasite problems but the difference is that wild horses roam thousands of acres and are not confined to a small area where manure and parasite eggs can accumulate.
Many natural supplements are claimed to control intestinal parasites, including pumpkin seeds, carrots, turnips, beets, garlic, bacterial supplements, wheat bran, and wheat germ oil. However, all of these natural remedies have been shown to be ineffective in killing intestinal parasites. Herbal dewormers are also available and regardless of their claims, there is no evidence they control internal parasites.
Lastly, the most used may be diatomaceous earth for external insect parasites, which is about 80% silica. Used in gardens, it absorbs moisture from the external skeleton of insects and dries them out. However, the environment must be dry, which cannot be achieved in a horse’s intestine. Studies in goats, sheep, and cattle have shown diatomaceous earth to be ineffective and no studies have actually been performed in horses.