Infections with helminths, or parasitic worms, affect more than 2 billion people worldwide (Figure 51-1). In regions of rural poverty in the tropics, where prevalence is greatest, simultaneous infection with more than 1 type of helminth is common.
Relative incidence of helminth infections worldwide.
Worms pathogenic for humans are Metazoa and can be classified into roundworms (nematodes) and 2 types of flatworms, flukes (trematodes) and tapeworms (cestodes). These biologically diverse eukaryotes vary with respect to life cycle, bodily structure, development, physiology, localization within the host, and susceptibility to chemotherapy. Immature forms invade humans via the skin or GI tract and evolve into well-differentiated adult worms with characteristic tissue distributions. With few exceptions, such as Strongyloides and Echinococcus, these organisms cannot complete their life cycle and replicate within the human host to produce mature offspring. Therefore, the extent of exposure to these parasites dictates the number of parasites infecting the host. Second, any reduction in the number of adult organisms by chemotherapy is sustained unless reinfection occurs. The burden of parasitic helminths within an infected population is not uniformly distributed, and it typically displays a negative binomial distribution whereby relatively few persons carry the heaviest parasite burden, resulting in increased morbidity in these individuals who also contribute disproportionately to transmission.
Anthelmintics are drugs that act either locally within the gut lumen to cause expulsion of worms from the GI tract, or systemically against helminths residing outside the GI tract. Therapy for many tissue-dwelling helminths, such as filarial parasites, is not fully effective. There is increasing appreciation of the impact of helminth infections on the health and education of school-aged children. In a massive public health effort, international health organizations are promoting the periodic and frequent use of anthelmintic drugs in schools as a means to control morbidity caused by soil-transmitted helminths and schistosomes in developing countries. Control programs employing anthelmintics rank among the world's largest health efforts, and hundreds of millions of people receive treatment annually.
This chapter is divided into 2 main parts:
HELMINTH INFECTIONS AND THEIR TREATMENT
The major nematode parasites of humans include the soil-transmitted helminths (STHs; sometimes referred to as "geohelminths") and the filarial nematodes. The major STH infections (ascariasis [roundworm], trichuriasis [whipworm], and hookworm infection) are among the most prevalent infections in developing countries. The agents most widely employed for reducing morbidity are the benzimidazole (BZ) anthelmintics, either albendazole (Albenza and Zentel) or mebendazole (Vermox, others) (Figure 51-2).
Structure of the Benzimidazoles.