PARASITOLOGY
posterior sucker, located a short way behind the anterior sucker. Fasciolahas a blind-
ending gut, hence faeces pass out via the oral aperture.
The excretory system consists of flame cells. Each flame cell encloses cilia which beat
and drive fluid from the cell into collecting tubules. The tubules lead into a network of
ducts that finally ends in a secretory vesicle.
The tegument is used for both absorption and excretion. The surface of the tegument
is continuously shed and replaced. The antigenicity of the fluke changes with the chang-
ing tegument.7.5.7.1 Life-cycle
An adult Fasciolalives in the large bile duct and migrates to the smaller bile ducts to
feed (see Fig. 4.11). The adult is hermaphrodite, eggs are excreted into the bile and exit
the host via the faeces. The eggs are embryonated and take up to 15 days to mature in
fresh water. A free-swimming ciliated miracidium hatches out of the egg. When the
miracidium makes contact with an aquatic Lymnaeid mollusc, it penetrates the foot of
the snail and migrates to the hepatopancreas (digestive gland) where future development
takes place. Each miracidium contains germ and somatic cells. The next generation, the
sporocysts, develops from the germ cells. Germ cells within the sporocyst form the next
larval phase, the redia.
The cercariae develop from germ cells within the redia and escape from the snail via
an exit pore. Cercariae at first are free-swimming and then settle onto vegetation and
each encysts into a metacercaria which has a life span of 2–3 months. Development from
egg to metacercaria takes about 4–7 weeks.
Metacercariae when eaten by the definitive host (an herbivorous mammal) excyst in
the duodenum releasing a juvenile stage. The juvenile stage penetrates the intestinal wall
and migrates through the peritoneal cavity to the liver where it matures into an adult fluke.
Adult flukes can produce 500–700 eggs per day, with more eggs being produced in
the morning, which possibly hints at periodicity. The most common definitive host is
sheep. However a range of mammals including man can also serve as the definitive host.7.5.7.2 Prevalencen BOX 7.3
Fascioliasis is a disease of economic importance. In the 19th century Fasiciola hepatica
was estimated to be responsible for loss of one million sheep per year. During 1956–57
there was an outbreak of human fascioliasis (about 500 cases) in Europe. In 1969 in Australia
sheep farmers suffered large losses due to fascioliasis. The most common cause of human
fascioliasis is eating contaminated watercress.
Fascioliasis as an infection has been known for about 600 years, and was first recorded
in 1379 in sheep by De Brie.n1532 The liver fluke was described by Fitzherbert
n1698 Fluke eggs were discovered by Bidloo
n1758 The genus Fasciolawas created by Linnaeus, who named the fluke Fasciola
hepatica
n1818 Rediae were discovered in freshwater snails by Ludwig Bojanus