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186 CHAPTER 8

Disease Transmission Morphology
Transmission of either of these two diseases happens Members of this family are large and leaf-shaped parasites
when uncooked or inadequately cooked fish are eaten. of mammals including humans but mostly affect herbi-
The eggs of H. heterophyes and M. yokogawai may be vores (cattle). The adult parasitic worm reaches a length
confused with those of the liver fluke Clonorchis sinensis. of 2 to 3 cm. The eggs of Fasciola hepatica are oper-
Adult worms of these species are almost never detected culated and can be as large as 130 to 150 μm by 63 to
in fecal waste. 90 μm. This parasite also has characteristically branched
reproductive organs. The Fasciola hepatica organism
Laboratory Diagnosis also has oral suckers used to effectively anchor the para-
site, as it frequently inhabits the bile duct. Adult flukes
Because the adult fluke is almost never seen in a fecal feed on the lining of the bile ducts and pass their eggs
specimen, the presence of eggs provides the diagnostic from the bile duct by which it reaches the intestines.
tool for determining infection. When seen, the adult stages It should be noted that the organism does not directly
of both species are tiny, measuring 1 to 2 mm in length. affect the liver, but the close proximity of the gallbladder
The diagnosis of either heterophyiasis or metagonimiasis and the bile ducts produces symptoms similar to those
is most often accomplished by detecting the character- that directly impact the liver.
istic ova during a routine examination for ova and para-
sites in a stool specimen. The ova must be differentiated
from those of Clonorchis sinensis, because the eggs of Symptoms
H. heterophyes and M. yokogawai are indistinguishable Abdominal cramps and pain, diarrhea, and indigestion
from each other. However, purported differences in are symptoms of this parasite. Migration through the liver
the thicknesses of the egg shells of the two species may may cause damage when the number of worms infecting
be possible, but differentiation of the two species is merely the liver and the bile duct are excessive. Inflammation
academic, as the treatment is the same for either species. of the bile duct may mimic other diseases of the biliary
system, and complete obstruction of the common bile
Treatment and Prevention duct may occur, leading to serious consequences.

As for a number of other trematodes, praziquantel is
the choice for treatment of infections from both H. het- Life Cycle
erophyes and M. yokogawai. Proper disposal of human
F. hepatica has a similar lifestyle to that of F. buski, with
wastes is paramount in preventing the diseases, where
water runoff into bodies of water is controlled. Thor- the passage of unembryonated eggs in the feces. Follow-
ing the passage of the unembryonated eggs in the stool,
oughly cooking fish, especially those from endemic areas
of the world, will also minimize the number of cases of micacidia develop within 2 weeks, escape from the eggs,
and infect the snail, which acts as the first intermediate
infection by H. heterophyes and M. yokogawai.
host penetrated by the miracidia. Cercaria are produced
in the snail from these miracidia and are released into
FASCIOLA HEPATICA fresh water where feces is dumped or runs into the water.
There they encyst and form metacercariae on freshwater
F. hepatica is known either as the common liver fluke or vegetation, which is considered the second intermedi-
sheep liver fluke and is a parasitic flatworm of the class ate host. Humans and other mammals are infected after
Trematoda that infects the livers of various humans eating the contaminated vegetation. The metacercariae
and other mammals. The term hepatica always refers excyst in the duodenal portion of the intestine and the
to the liver, hence the common name of liver fluke. The larvae penetrate the intestinal wall and enter the gallblad-
disease is called fascioliasis, and the causative organism, der and bile ducts of the human host. The adult worms
F. hepatica, is distributed over a wide portion of the world. attach themselves in the large bile ducts and the gallblad-
Besides infecting humans, the organism causes great der of humans rather than in the intestines as other para-
economic losses in sheep and cattle. Eggs of F. hepatica sites do, but eggs are passed from the bile duct into the
are virtually indistinguishable from those of F. buski. intestine and are excreted in feces.