The tegument of the adult worm differs in each species. S. mansonihas a tegument
covered with papillae, in S. haematobiumthe papillae are shorter and more widely spaced
and S. japonicumhas a smooth tegument.
The intermediate hosts are freshwater snails and each species of worm has a different
genus or species of snail host. S. mansoniuses the snail host Biomphalariaspp; S. japonicum
has Oncomelaniaspp and both S. haematobiumand S. intercalatumhave Bulinus.
S. mansoniadults live in the inferior mesenteric blood vessels, S. japonicumin the
superior mesenteric blood vessels and S. haematobiumadults are found in the venous
plexus associated with the urinary bladder.
n Although trematodes have a mouth and gut they also absorb food through their tegu-
ment and most of it will be amino-acids and sugar associated with blood.
n Both male and female worms have suckers which are used for attachment. In the
female the birth pore is above the posterior sucker which can penetrate the endothe-
lium cells of the walls of the blood vessels.
n The S. mansoni, S. japonicumand S. intercalatumeggs enter the connective tissue through
damaged blood vessels. They use the spine on the exterior shell as well as proteolytic
enzymes secreted by the embryonic miracidium within the egg to burrow through the
sub-mucosa of the gut into the gut lumen and then escape to the exterior via the faeces.As many as 50% of the eggs of the above species do not exit from the host but via the
circulation end up in the liver where they remain and eventually the host reaction forms
a granuloma around the egg. S. haematobiumeggs burrow through the blood vessels into
the bladder and pass out of the host via the urine. Eggs that do not pass out of the host
become lodged in the bladder wall and urinary tracts and granulomas form round them.
n From the eggs that pass out of the host into fresh water with temperatures of about
25–28°C and average amount of light, a free-living ciliated miracidium hatches (see
Fig. 4.10).
n The miracidia remain active for 6–8 h, and swim until they come across a specific species
of aquatic snail. Once they encounter the snail they actively penetrate the foot of the
snail and migrate to the snail’s digestive organ, the hepatopancreas.
n The germ cells develop into sporocysts, which can produce a second generation of sporo-
cysts as well as the next generation, the cercaria.
n Cercariae exit from the snail and are free-living. They have a tail to help them propel
themselves through the water to seek out their next and definitive host. Each cercaria
is either male or female and can survive in water for about 8 h.
n Once a cercaria comes in contact with mammalian skin (in particular human skin), they
actively penetrate via a hair follicle, shed their tail and enter the dermis, a process which
can take up to 2 h. In the dermis they transform into juveniles known as schistosomules
and about 2 days later enter the bloodstream and migrate to the lungs (the lung stage
worms). From the lungs they migrate to the liver and eventually to mesenteric or
bladder wall blood vessels where maturation takes place. Eggs are usually produced
after about 28 days.n 4.8.2FASCIOLA HEPATICA
Fasciola hepaticais commonly known as the liver fluke and, like the schistosomes, is a dige-
nean trematode with two hosts and two free-living stages (see Fig. 4.11). This parasite
PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE