Schistosoma mansoni is a significant parasite of humans, one of the major agents of schistosomiasis. Of the trematodes, schistosomes are atypical in that the adult stages have two sexes (dioecious) and are located in blood vessels of the definitive host. Most other trematodes are hermaphroditic and are found in the intestinal tract or in organs, such as the liver. The lifecycle of schistosomes includes two hosts: a definitive host (i.e., man) where the parasite undergoes sexual reproduction, and a single intermediate snail host where there are a number of asexual reproductive stages. S. mansoni is named after Sir Patrick Manson, who first identified it.
Schistosomes, unlike other trematodes, are long and slim worms. The male S. mansoni is approximately 1 cm long (0.6 to 1.4 cm) and is 0.11 cm wide. It is white, and it has a funnel-shaped oral sucker at its anterior end followed by a second pediculated sucker. The external part of the worm is composed of seven layers and is continually renewed as the outer layer is continually shedding. The tegument bears a large number of small tubercules. The suckers have small thorns in their inner part as well as in the buttons around them. The male genital apparatus is composed of 6 to 9 testicular masses, situated dorsally. There is one deferent canal beginning at each testicle which is connected to a single deferent that dilates into a reservatory, the seminal vesicle, located at the beginning of the gynacophoric canal. The copula happens through the coaptation of the male and female genital orifices.
The female has a cilindric body, longer and thinner than the male (1.2 to 1.6 cm long by 0.016 cm wide). The female parasite is darker, and it looks gray. The darker color is due to the presence of a pigment (hemozoin) in its digestive tube. This pigment is derived from the digestion of blood. The ovary is elongated and slightly lobulated and is located on the anterior half of the body. A short oviduct conducts to the ootype which continues with the uterine tube. In this tube it is possible to find 1 to 2 eggs (rarely 3 to 4) but only 1 egg is observed in the ootype at any one time. The genital pore opens ventrally. The posterior two-thirds of the body contain the vittelogenic glands and their winding canal, which unites with the oviduct a little before it reaches the ootype.
The digestive tube begins at the anterior extremity of the worm, at the bottom of the oral sucker. The digestive tube is composed of an esophagus which divides in two branches (right and left) and that reunite in a single cecum. There isn't any anus.
Of the infected patients, 20 million suffer severe consequences from the disease, and 120 million are symptomatic. Some estimate that there are approximately 20,000 deaths related to schistosomiasis yearly. In many areas, schistosomiasis infects a large proportion of children under 14 years of age. An estimated 500–600 million people worldwide are at risk from the disease.