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