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188 CHAPTER 8

Source: Centers for Disease Control and Prevention (CDC)

FIGURE 8-2 Life cycle of Clonorchis sinensis, the causal agent of clonorchiasis

surrounding them. They no longer require ingestion, but asymptomatic, but are still able to transmit the organ-
actively seek out a fish as a new host, where they bore into ism to others. These metacercariae are not released
the fish’s body and become parasites in their host. These into the water where they can invade the skin of people
redia are now cercariae and form a protective cyst in the entering the water, as is the case with some species of
fish muscle before becoming known as metacercariae. metacercariae.
This cyst is resistant to stomach acids of the human host
so the metacercariae are able to reach the small intestine Laboratory Diagnosis
after the fish is eaten. From there they travel to the liver
where they feed on bile produced by the liver and reach Using the oil immersion objective providing a micro-
sexual maturity. These organisms are hermaphroditic scopic power of 1000×, structural features are somewhat
adults, as are other flukes, and are able to produce eggs at easily visualized in a Clonorchis sinensis egg. C. sinensis is
a rapid pace leading to a quick saturation of the liver with a trematode known as the Chinese or oriental liver fluke.
adults producing eggs. The eggs of this organism are relatively small and range
from 25 μm to 35 μm by 10 μm to 20 μm. The eggs are
Disease Transmission oval shaped with a sharply curved and convex operculum
that rests on a rounded rim at the smaller end of the egg.
The ingestion of raw or undercooked fish containing At the larger end, a stem-shaped knob is usually visible
metacercariae is the most likely cause of a C. sinensis (Figure 8-3). A miracidium is visible inside the eggs of
infection. Those with light infections are sometimes C. sinensis, which are capable of floating in fresh water.