Infections with helminths, or parasitic worms, affect more than two billion people worldwide. Pathogenic worms are classified into roundworms (nematodes) and two types of flatworms, flukes (trematodes) and tapeworms (cestodes). Immature forms invade humans via the skin or gastrointestinal (GI) tract and mature into adult worms with characteristic tissue distributions. With few exceptions, such as Strongyloides and Echinococcus, they cannot complete their life cycle and replicate themselves within the human host.
Anthelmintics are drugs that act locally to expel worms from the GI tract or systemically to eradicate adult helminths or developmental forms that invade organs and tissues. Because metazoan parasites generally are long-lived and have relatively complex life cycles, acquired resistance to anthelmintics in humans is not a major factor limiting clinical efficacy. The extensive use of anthelmintics in veterinary medicine ensures that the potential of drug resistance among helminths in humans cannot be discounted.
TREATMENT OF HELMINTH INFECTIONS Nematodes (Roundworms)
The major nematode parasites of humans include the soil-transmitted helminths (STHs) and the filarial nematodes. The STH infections, which include ascariasis, trichuriasis, and hookworm infection, are among the most prevalent infections in developing countries. Eradication programs use schools to administer broad-spectrum anthelmintics on a periodic and frequent basis. The most widely used agents for reducing morbidity are the benzimidazole anthelmintics (BZAs), either albendazole (albenza and zentel) or mebendazole (vermox) (see Table 41-1).
In addition to targeting STH infections among school-aged children, there is an ongoing attempt to employ anthelmintics that eliminate lymphatic filariasis (LF) and onchocerciasis (river blindness). The goal is to interrupt arthropod-borne transmission by administering combination therapy with either diethylcarbamazine and albendazole (in LF-endemic regions such as India and Egypt), or ivermectin and albendazole (in LF regions where onchocerciasis and/or loiasis are co-endemic). These drugs target the microfilarial stages of the parasite, which circulate in blood and are taken up by arthropod vectors where further parasite development occurs.
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