Joel V. Weinstock Tufts Medical Center, Boston, MA, USA Helminths are multicellular eukaryotic organisms that are common parasites of the human gastrointestinal tract, biliary tree or liver. As members of the animal kingdom, helminths are large enough to be visible to the human eye during their life cycle. Thus, it is not uncommon to encounter some of these organisms during endoscopic procedures performed on high‐risk populations. Throughout human history, infestation with intestinal helminths has been a normal part of the human condition. Helminths are categorized into three major groups: nematodes, trematodes, and cestodes. Nematodes are roundworms with a tubular gut, including both mouth and anus. There are two groups of flatworms: trematodes and cestodes. Trematodes, or flukes, are flat, leaf‐shaped organisms with a blind gut. All trematodes require an obligate freshwater snail host. The cestodes, or tapeworms, typically have two distinct forms. The adult stage is a tapeworm in the gut of the definitive host with an attachment organ, the scolex, and segments, termed proglottids. The external cestode surface tegument performs many of the same functions as the mammalian digestive tract. The proglottids are hermaphroditic, containing both ovaries and testes. Most helminths need to reside in soil or some intermediate host before they can colonize humans. This work was supported by NIH grants DK38327 and DK058755. Figure 70.1 Ascaris lumbricoides. (a) The adult female worm measures up to 40 cm in length. (b)Ascaris ova are often bile‐stained and demonstrate a thick mammillated shell. (c) Endoscopic extraction of an Ascaris located at the papilla. (d) Visualization of Ascaris in the common bile duct during endoscopic retrograde cholangiopancreatography (ERCP). (e) Loeffler syndrome (or pneumonia) develops in some individuals during the acute stage of infection with A. lumbricoides. Chest x‐ray may reveal unilateral or bilateral abnormalities ranging from diffuse interstitial pneumonitis to nodular densities. Source: (a) Centers for Disease Control and Prevention (www.dpd.cdc.gov/DpDx/HTML/ImageLibrary/Ascariasis_il.htm). (b) GrahamColm, http://en.wikipedia.org/wiki/File:Ascaris_lumbricoides.jpg. CC BY‐SA 3.0. (c,d) Astudillo J, Sporn E, Serrano B, Astudillo R. J Am Coll Surg 2008;207:527–32. Reproduced with permission of Elsevier. (e) Lau S, Woo P, Wong S, et al. J Clin Pathol 2007;60(2):202–3. Reproduced with permission from BMJ Publishing Group Ltd. Figure 70.2 The life cycle of Ascaris lumbricoides. Adult worms (1) live in the lumen of the small intestine. A female may produce approximately 200 000 eggs per day, which are passed with the feces (2). Unfertilized eggs may be ingested, but are not infective. Fertile eggs embryonate and become infective after 18 days to several weeks (3), depending on the environmental conditions (optimum: moist, warm, shaded soil). After infective eggs are swallowed (4), the larvae hatch (5), invade the intestinal mucosa, and are carried via the portal, then systemic circulation to the lungs (6). The larvae mature further in the lungs (10–14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat and are swallowed (7). Upon reaching the small intestine, they develop into adult worms (1). Between 2 and 3 months are required from ingestion of the infective eggs until there are adult female worms in the gut of the host that can produce eggs. Adult worms can live 1–2 years. Source: Centers for Disease Control and Prevention (www.dpd.cdc.gov/DpDx). Figure 70.3 (a) Adult Trichuris trichiura inhabit the colon and can be visualized during colonoscopy. (b) Massive infection can be associated with diarrhea, colonic bleeding, or (d) rectal prolapse. (c) T. trichiura ova are characterized by polar plugs at both ends. Source: (a) Courtesy of Richard Goodgame, (c) Centers for Disease Control and Prevention (www.dpd.cdc.gov/DpDx). Figure 70.4 Human hookworms, Necator americanus and Ancylostoma duodenale, are common intestinal parasites that attach to intestinal villi and feed on blood. Low‐burden infections are typically asymptomatic, but heavy infection may cause iron deficiency anemia. (a) Electron microscopy of N. americanus reveals two pairs of teeth for adhesion to bowel mucosa. (b) Hookworm ova typically display segmented larvae within a thin clear shell. Source: (a) Jian X, Shu‐Hua X, Hui‐Qing Q, et al. Exp Parasitol 2003;105(3–4):192–200. Reproduced by permission of Elsevier. (b) Centers for Disease Control and Prevention (www.dpd.cdc.gov/healthypets/diseases/hookworm.htm). Figure 70.5 Human hookworm life cycle. Eggs are passed in the stool (1), and under favorable conditions (moisture, warmth, shade), larvae hatch in 1–2 days. The released rhabditiform larvae grow in the feces and/or the soil (2), and after 5–10 days (and two molts) they become filariform (third‐stage) larvae that are infective (3). These infective larvae can survive 3–4 weeks in favorable environmental conditions. On contact with the human host, the larvae penetrate the skin and are carried through the blood vessels to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed (4). The larvae reach the small intestine, where they reside and mature into adults. Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the host (5). Most adult worms are eliminated in 1–2 years, but the longevity may reach several years. Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle). In addition, infection by A. duodenale may occur by the oral and transmammary route. N. americanus, however, requires a transpulmonary migration phase. Source: Centers for Disease Control and Preventionwww.dpd.cdc.gov/DpDx). Figure 70.6 (a) An adult pinworm, Enterobius vermicularis, alive in the colon as seen through a colonoscope. (b) At night, the adult worms emerge and lay ova on the perianal skin. (c) The ova, which are found on the perianal skin, are flattened on one side. Source: (a) Courtesy of Richard Goodgame, (b) From Zaiman H/ASTMH, “A pictorial presentation of parasites” www.astmh.org/education‐resources/zaiman‐slide‐library?page=4&fileid=1771. Figure 70.7 (a) The adult females of Strongyloides stercoralis burrow into duodenal mucosa and shed their ova that hatch in the gastrointestinal tract, and (b) rhabditiform larvae are later voided into stool.
CHAPTER 70
Parasitic diseases: helminths
Acknowledgment
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