Anthelmintics

Summary

This article describes the drugs used in helminthic infections and their therapeutic indications, mode of action, toxicity and other details of each of the recommended drugs, and discusses the nature and treatment of infection by helminths important in human medicine. Infestation due to the roundworms Enterobius vermicularis, Ascaris lumbricoides and the hookworms, Ancylostoma duodenale and Necator americanus can all be treated effectively with pyrantel pamoate. For Enterobius vermicularis, however, a newer drug, mebendazole, is equally as effective. The advantage of these drugs in the indicated circumstances is that they can be administered in a single dose. Unfortunately, pyrantel pamoate is not a panacea and in the case of Necator it is not as effective as in the other roundworms. In that situation the use of tetrachlorethylene is preferable. For treatment of Strongyloides stercoralis, an important human parasite, because it can become disseminated and lead to fatal infections in immunoin-competent hosts, the only effective drug is thiabendazole. In treatment of Trichuris trichiura infection, mebendazole, administered over a period of 3 days, is the most effective available drug.

For the roundworms inhabiting tissues —either as aberrant infections of man or as the normal part of their life cycle in man —therapy tends to be largely non-specific. For example, in visceral larva migrans, caused by the dog roundworm Toxocara canis, only palliative therapy with systemic anti-inflammatory agents and corticosteroids may be helpful. Cutaneous larva migrans, caused by the dog hookworms Ancylostoma brasiliensis and Ancylostoma caninum, is also treated primarily with symptomatic measures, but there is a suggestion that thiabendazole may kill the larvae and thus be effective. Trichinella spiralis may cause severe, even fatal infections in man, but only symptomatic therapy can be offered.

Therapy for fllarial infections is regrettably complicated and not completely effective. Diethylcarbamazine remains the best available drug, but in some of these infections local surgical excision may also be used. It is important to bear in mind that release of antigens from dying or dead worms may cause systemic inflammatory and allergic reactions that may require therapy with corticosteroids. Therapy for Cestodes is achieved most effectively with niclosamide, but the antimicrobial agent paromomycin has also been effective. For the aberrant cestode infections of man, such as echinococcal cysts or Taenia solium cycticerci, treatment is surgical if the affected areas are accessible.

Treatment of schistosomal infections is quite toxic and, therefore, it is mandatory to determine viability of the worms before recommending therapy. If therapy is required, then Schistosoma mansoni infections are treated with stibophen and S. japonicum with antimony potassium tartrate, taking care in both of these instances to watch for the early signs of antimony toxicity; therapy of S. haematobium infections is based on administration of niridazole.

Infections with other flukes tend to be less portentous than those due to the schistosomes. Fasciolopsis buski can be treated effectively with tetrachlorethylene. Fasciola hepatica with bithionol, as is Paragonimus westermani. Clonorchis sinensis cannot be usually eradicated, although chloroquine phosphate or emetine has led to a transient reduction in the egg output of the worm.

A table provides details of drug dosage, but familiarity with side-effects and indications for therapy is a major requirement for success.