The word parasite comes originally from the Greek parasitos (para, alongside of; and sitos, food), meaning someone who eats at another’s table or lives at another’s expense. Although the same is true of many bacteria and viruses, the designation parasite is reserved, by convention, for helminths and protozoa. These organisms are larger and more complex than bacteria, with a eukaryotic cell structure similar to that of human host cells. Historically, this similarity has made it difficult to find effective antiparasitic agents that do not cause unacceptable toxicity to human cells. Fortunately, intensive research and modern techniques have now provided suitable agents for safe and effective treatment of most parasitic infections. See Chap. S14 for details on diagnostic procedures and Chap. 217 for details on treatment.
Internal parasites of human beings are divided into two types: helminths (worms) and protozoa. Helminths are multicellular organisms that can be seen with the naked eye (Chap. 225). There are two phyla: Platyhelminthes (flat worms) and Nemathelminthes (roundworms). Both phyla include some genera that mature in the gastrointestinal tract and others that migrate through the tissue after ingestion or skin penetration. Tables S14-1 and S14-2 present the helminthic genera, their definitive and intermediate hosts, geographic distributions, and the parasitic stages in the human body.
The key to understanding which helminths use humans as definitive hosts is to remember that helminth ova develop into larvae, and larval stages develop into adults. Humans serve as the definitive host when they ingest helminth larvae, which develop into adults in the intestine and usually cause mild disease, often without any symptoms. (The exception is ingestion of the late-stage larvae of the somatic or tissue flukes, as shown in Table S14-2.) In contrast, if humans ingest helminth ova and serve as the intermediate host, the ova develop into larvae, which penetrate the intestine, migrate through the tissue, and invade organs where they mature into adults. Intermediate hosts with parasitic invasion of organs may experience severe disease.
Protozoa are microscopic single-celled organisms. Among the many differences between helminths and protozoans, the most important is the ability of protozoa (like bacteria) to multiply within the human body and cause overwhelming infections. A major mechanism promoting unrestrained growth is evasion of the host immune response either by antigenic variation (Trypanosoma brucei) or by survival inside host cells (e.g., Plasmodium, Babesia, Cryptosporidium, Leishmania, and Toxoplasma). In contrast, almost all helminths require stages in other hosts to complete their life cycles and multiply. As a result, except for Strongyloides and Capillaria, which can complete their life cycle in humans, increases in the burden of infection with helminths require repeated exogenous reinfections. Thus permanent residents of endemic countries, who are exposed repeatedly, may have heavy severe infections, while most travelers with one or two exposures are unlikely to experience the full spectrum of chronic helminthic ...