The helminths, or parasitic worms, comprise the nematodes (roundworms),
trematodes (flukes), cestodes (tapeworms) and acanthocephalans
(thorny-headed worms). Most anthelmintics were
discovered and developed for use in the veterinary field, where helminths
significantly impact health and productivity. Few companies
are searching for new compounds for use in human medicine,
but commercial competition has produced a steady supply of new
products for the veterinarian. Three new classes of anthelmintic
(cyclic octadepsipeptides, amino-acetonitrile derivatives and paraherquamides)
have entered or will soon enter the veterinary market
and these have potential for human medicine. In addition, some
members of established classes have exhibited promising results in
preliminary investigations (moxidectin for filariasis; artemisinins for
schistosomiasis).
Despite the lack of new anthelmintics for human treatment,
satisfactory results can be achieved with current products for nearly
all helminth infections. In some cases such as for filariasis, combinations
of available drugs are being used to enhance treatment. Side
effects usually include gastrointestinal upsets, but these are as likely
to be related to the worm burden as to the drug. As helminths are
often large and/or present in large numbers, their death and disintegration
after chemotherapy can result in an obstruction or an
allergic–anaphylactic type reaction.
The biggest problems still remaining to be solved are treatment of
infections with larval cestodes, especially Echinococcus spp. There is
no satisfactory drug against these parasites; although benzimidazole
carbamates have useful activity, they are poorly absorbed from the
gut when administered orally and painful injection-site reactions preclude
parenteral use. Praziquantel also may be useful in some cases
for larval cestodes. Another problem is the treatment of disseminated
strongyloidiasis, which occurs when patients with a latent infection
are immunosuppressed. No satisfactory chemotherapy
is available for the macrofilarial stages in filarial infections, or
to treat Guinea worm (Dracunculus medinensis) infection, although
an eradication program has greatly reduced its incidence.
Although drug resistance is common in the veterinary field it is
not yet a problem in human medicine, except with regard to schistosomiasis
where resistance is known for hycanthone and praziquantel.
This discrepancy is because anthelmintics are very widely
and frequently used in the veterinary world but much less so in
human medicine, in part due to the poverty of most of the people
who are infected. With increasing wealth in tropical countries
where these infections are common, anthelmintics are being used
much more widely and there is concern that drug resistance will
develop.
