The main forms of human schistosomiasis are caused by five species of the flatworms, or blood flukes, known as schistosomes:
This disease, also called bilharzias, is transmitted by a sweet water snail. The larvae, called cercariae will penetrate through pores of skin, enter the blood stream and develop into adult schistosoms. Adult worms, pair, mate and females lay hundreds of eggs each day for many years in humans. Eggs are excreted in the faeces or urine and the parasite lifecycle is completed when these hatch into miracidiae (upon contact with water), which then infect the intermediate snail host. Disease is often caused when, rather than being excreted, eggs become lodged in host tissues such as the intestine, liver or bladder, where they stimulate an inflammatory granulomatous response
Schistosoma mansoni eggs in a patient from Egypt. These eggs are large (length 114 to 180 µm) and have a characteristic shape, with a prominent lateral spine near the posterior end. The anterior end is tapered and slightly curved. When the eggs are excreted, they contain a mature miracidium (especially visible in A).
Eggs of S. mansoni
in faeces showing characteristic lateral spine.
Schistosoma mansoni eggs in intestinal wall.
Schistosoma japonicum eggs
Schistosoma japonicum eggs in liver
Schistosoma japonicum eggs in colon
Schistosoma haematobium eggs
Egg of S. haematobium
in the urinary sediment.
Note the terminal spine.
The fork-tailed larval stage
of Schistosoma mansoni, known as a cercaria, emerges from an infected aquatic snail and swims about seeking contact with the skin of a potential host.
When contact is made, the cercaria initiates infection by penetrating the epidermis, at which time it sheds its tail, and heading deeper into the dermis.
The slender female worm lies in a groove
(the gynecophoral canal)
in the lateral edge of the body of the male. Scaning electron micrograph