Page 188 - The Veterinary Laboratory and Field Manual 3rd Edition

Parasitology 157

Fasciolosis diate host (Figure 3.30). Adult flukes are found
in the bile ducts of the liver of the host and lay
Fasciolosis has a worldwide distribution and
is common in rice growing areas. Liver fluke eggs which pass into the intestine to be voided
infection can be caused by Fasciola hepatica with the faeces. Eggs develop into minute imma-
(common liver fluke or sheep liver fluke) or F. ture worms (miracidia). The miracidia penetrate
gigantica (liver fluke). F. hepatica is usually found certain species of snail (Lymnaea spp.) which act
in temperate areas whereas F. gigantica is seen in as the intermediate host. The immature flukes
tropical/sub-tropical zones. amplify within snail tissue and develop for 3
weeks or more until they are released into the
SuScEPtIbLE doMEStIc SPEcIES water (as cercariae) to encyst (as metacercariae)
Cattle, sheep, buffaloes, goats and pigs are sus- on aquatic vegetation. Each egg can potentially
ceptible. The disease is rare in other species develop into 1260 metacercariae; each worm can
although the number of reported cases in humans shed around 4000 eggs per day; and each animal
has been rising in some parts of the world. could be infected with, on an average, 100 adult
worms.
LIFE cycLE oF FASCIOLA HEPATICA Infection of sheep (and other hosts) takes
Fasciola sp., in common with other flukes, are place in damp, marshy and swampy areas where
hermaphrodite (both sexes present in same snails are found. Livestock become infected
individual), flat, un-segmented, leaf like para- when metacercaria encysted on vegetation
sites. They are greyish-brown in colour. The life are consumed. In sheep, the larval stages bur-
cycles of F. hepatica and F. gigantica are similar row through the wall of the intestine into the
and complex involving an invertebrate interme- abdominal cavity from where they migrate to

Figure 3.30 The life cycle of the liver fluke (Fasciola hepatica). (A) The adult liver flukes live in the bile
ducts of the liver. (B) Eggs are passed in the host’s faeces (most common in sheep). (C) Egg containing
the first immature stage, the miracidium. (D) Miracidium penetrating the snail intermediate host (Lymnaea
sp.). Without the snail the life cycle cannot continue. (E) The snails prefer wet areas and boggy land, this is
why liver fluke disease is more common in these environments. (F) The sporocyst forms within the snail
and develops into the next stage, the redia (G). (H) Redia develop into cercaria which are shed from the
snail. (I) The cercaria encyst on the vegetation as metacercaria where they are ingested by the definitive
host (J) sheep host. Cattle and other grazing animals can also be infected but liver fluke disease is less
commonly seen in these other species. Illustration: Louis Wood.

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