All horses sustain some level of worm or internal parasite burden. The species that most commonly affect horses are large and small Strongyles, Parascarus Equorum (large roundworms), Oxyuris Equi (pinworms), Strongyloides Westerii (threadworms), Dictyocaulus Arnfieldi (lungworm), and the Cestodes or tapeworms: Anoplocephala. Immunologists suggest that exposure to worms is a necessary part of acquiring immunity against internal parasites and therefor undesirable that an anthelmintics (anti worming) programme is too rigorous.
In recent years the pharmaceutical companies have convinced horse owners that regular treatment with chemical wormers, anything from quarterly to monthly, depending upon the efficacy of the drug, is an essential part of horse management. This has lead to resistance to specific chemicals and the search for increasingly powerful drugs. To some extent the pharmaceutical companies are correct because heavy levels of parasitism can cause devastating damage to horses resulting in anything from poor condition with unthrifty appearance e.g. starey coat, weight loss, distended abdomen, oedema of legs and sheath, colic, cough, diarrhoea, liver damage and even death. However, the requirement for the treating of horses against worms is dependent upon a number of factors that include the environment, age of the horse and genetic factors. While a natural worming system may not always entirely replace chemical wormers, a natural worming system together with careful management of the environment, can reduce reliance on chemicals that may have adverse effects.