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SCHISTOSOMA SPECIES :PARASITOLOGY
The schistosomes are a group of closely related flukes that inhabit the portal vascular sys-tem of a number of animals. Of the five species known to infect humans, three, S. mansoni,S. haematobium, and S. japonicum, are of primary importance. They infect 200 to 300 million individuals in Africa, the Middle East, Southeast Asia, the Caribbean and South America, and kill 1 million annually. The remaining two species are found in lim-ited areas of West Africa (S. intercalatum) and Southeast Asia (S. mekongi), and will not be discussed in detail.
The adult worms can be distinguished from the hermaphroditic trematodes by the an-terior location of their ventral sucker, by their cylindric bodies, and by their reproductive systems (ie, separate sexes). They are differentiated from one another only with difficulty. The 1- to 2-cm male possesses a deep ventral groove, or gynecophoral canal, in which it carries the longer, more slender female in lifelong copulatory embrace. After mating in the portal vein, the conjoined couple use their suckers to ascend the mesenteric vessels against the flow of blood. Guided by unknown stimuli, S. japonicum enters the superior mesenteric vein, eventually reaching the venous radicals of the small intestine and as-cending colon; S. mansoni and S. haematobium are directed to the inferior mesenteric system. The destination of the former is the descending colon and rectum; the latter, how-ever, passes through the hemorrhoidal plexus to the systemic venous system, ultimately coming to rest in the venous plexus of the bladder and other pelvic organs.
On reaching the submucosal venules, the worms initiate oviposition. Each pair de-posits 300 (S. mansoni, S. haematobium) to 3000 (S. japonicum) eggs daily for the re-mainder of its 4- to 35-year life span. Enzymes secreted by the enclosed miracidium dif-fuse through the shell and digest the surrounding tissue. Ova lying immediately adjacent to the mucosal surface rupture into the lumen of the bowel (S. mansoni, S. japonicum) or bladder (S. haematobium) and are passed to the outside in the excreta. Here, with appropri-ate techniques, they may be readily observed and differentiated. The eggs of S. mansoni are oval, possess a sharp lateral spine, and measure 60 by 140μm. Those of S. haematobium differ primarily in the terminal location of their spine. The eggs of S. japonicum, in contrast, are more nearly circular, measuring 70 by 90μm. A minute lateral spine can be visualized only with care.
When the eggs are deposited in fresh water, the miracidia hatch quickly. On finding a snail host appropriate for their species, they invade and are transformed over 1 to 2 months into thousands of forked-tailed cercariae. When released from the snail, these infectious larvae swim about vigorously for a few days. Cercariae coming in contact with human skin during this time attach, discard their tails, and penetrate. During a 1- to 3-day sojourn in the skin, the outer cercarial membrane is transformed from a trilaminar to a heptalaminar structure, an adaption that is thought to be critical to the survival of the par-asite within the human body. The resulting schistosomula enter small venules and find their way through the right side of the heart to the lung. After a delay of several days, the parasites enter the systemic circulation and are distributed to the gut. Those surviving passage through the pulmonary and intestinal capillary beds return to the portal vein, where they mature to sexually active adults over 1 to 3 months.
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