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INTRODUCTION

Schistosomiasis, or bilharziasis, is a chronic debilitating disease associated with significant morbidity and mortality. The disease affects more than 190 million1 people in 78 countries2 worldwide and is second only to malaria among parasitic diseases in socioeconomic and public health importance in tropical and subtropical areas.1 Most human disease is caused by three species of blood flukes of the genus Schistosoma: S. mansoni, S. haematobium, S. japonicum. S. mekongi, S. intercalatum, S. guineensis, and some rare zoonotic species can also infect people.

BIOLOGY AND LIFE CYCLES

The schistosome requires an intermediate and a definitive host to complete its life cycle. Asexual reproduction takes place in the molluscan intermediate host and sexual reproduction in the definitive vertebrate host. Briefly, free-swimming miracidia hatch from eggs deposited in freshwater during defecation or urination by an infected definitive host. These miracidia penetrate the snail host and develop into primary sporocysts, each of which produces multiple secondary sporocysts. Each of the secondary sporocysts produces a great number of cercariae, resulting in the production of hundreds to thousands of cercariae from an individual miracidium. The fork-tailed cercariae migrate out of the snail under appropriate environmental stimuli, usually heat and light, and move toward the surface of the water. Of note, both free living stages (miracidia and cercariae) have a limited life span in the absence of an appropriate host (6–24 hours under experimental conditions).3 Sporocysts, on the other hand, can remain dormant in estivating snails during adverse conditions and resume cercarial production with the return of a favorable environment.

When humans contact schistosome-infested water, cercariae penetrate the skin, lose their tails, and transform into the schistosomula life stage. After several days, the schistosomula enter a venule or lymphatic vessel and migrate to the right side of the heart, then to the lungs, and finally to the liver sinusoids, where they begin to mature. On reaching maturity, adult male and female worms pair and migrate to their final destinations, which differ by schistosome species. There, eggs are released from adult females into the venules of the intestine or urinary bladder, break through the submucosa and mucosa into the lumen, and are evacuated through the feces or urine, again depending on the species, to complete the life cycle.

The size of adult schistosomes varies by species. Mature female S. haematobium measure up to 20 mm in length by 0.25 mm in width at the widest point, whereas the mature males measure from 10 to 15 mm in length and 0.8–1 mm in width. Adult worms of S. mansoni are smaller, with females measuring 7.2–17 mm long and males 6.4–12 mm in length.4 Adult male and female schistosomes live in copula an average of 5–8 years but sometimes for as long as 30 years (Fig. 128-1).5 Adult S. japonicum and S. mekongi parasites are generally found ...

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