Page 144 - Medical Parasitology_ A Textbook ( PDFDrive )

Blood Flukes 137

Life Cycle (Fig. 12.2)
(1) The eggs are passed in urine (S. haematobium) and in faeces (S. mansoni and S.
japonicum) from infected humans. (2) Eggs hatch in water releasing miracidia. (3)
Miracidia penetrate tissue of freshwater snail (intermediate host). (4) Sporocysts
develop in snail. (5) Free swimming cercariae are released by snail into water.
(6) Cercariae penetrate skin of human. (7) Cercariae lose their tails during penetra-
tion and become schistosomulae. (8–9) The schistosomulae are carried in blood
circulation and migrate to portal blood in liver and mature into adults. (10) Paired
adult worms migrate to venous plexus of bladder (S. haematobium) or mesenteric
venules (S. mansoni and S. japonicum) where the female lay their eggs.

Pathogenesis and Clinical Features
Cercarial dermatitis presents with transient itching and petechial lesions at the site
of entry of the cercariae, more often seen in visitors to endemic areas than among
locals who may be immune due to repeated exposure. Acute systemic schistosomia-
sis may cause Katayama fever which presents with leucocytosis, eosinophilia and
hepatosplenomegaly. Clinical features during oviposition include painless terminal

i = Infective Stage
d = Diagnostic Stage Cercariae released by snail
5 into water and free-swimming
Sporocysts in snail 4
(successive generations) i
Cercariae lose tails during
7 penetration and become
Penetrate schistosomulae
skin 6

8 Circulation

3 Miracidia penetrate
snail tissue A
Migrate to portal blood
B in liver and mature
into adults 9
in faeces d in urine C
2
Eggs hatch
releasing miracidia 10
Paired adult worms migrate to:
A B mesenteric venules of bowel/rectum
S. japonicum
A A (laying eggs that circulate to the
S. mansoni S. haematobium liver and shed in stools)
B 1 C C venous plexus of bladder
Fig. 12.2 Life cycle of blood flukes (Reproduced from https://www.cdc.gov/dpdx/schistosomia-
sis/index.html)