THE VARIED ROLES OF SNAILS - National Universities Commission

one miracidium the potential of producing many thousand
cercariae which, when released from the secondary
sporocysts, migrate through the tissues into the blood
sinuses and escape from the snail through the walls of the
superficial blood vessels of the mantle, gill or pseudobrach.
The net result of prolonged production of cercariae
increases the chance that one cercaria will eventually infect
man or other suitable hosts.
It is important to note that the snails do not accept infection
entirely passively (Loker et al., 1984). Amoebocytic cells
originating from the heart epithelium attack invading
foreign bodies, possibly in conjunction with soluble factors
which themselves are derived from the amoebocytes. This
reaction helps to destroy many primary sporocysts within a
few days of penetration. In many snail species, the young
snails are susceptible, but progressive changes in the
composition of the haemolymph result in increasing
resistance with age (Michelson, 1986). It is further
important to note that some cercariae migrating through the
snail tissues are trapped and destroyed in granulomatous
reactions.
These infection processes of the snail borne trematodes can
be broadly divided into oral and(or) transdermal routes of
the worms’ larval stages which have been previously
released from the infected snails. During development
within the human body clinical cymptoms appear.
These symptoms include uticaria, fever, ascites,
haematemesis, carcinoma and hepatosplenomegaly. After
maturation and depending upon the species of infection, the
resultant adult worms reside within the blood vasculature
system or internal body cavities of the human hosts. Unlike
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