Introduction:
Which trematode may cause serious disease, with loads of
over 25,000 parasites per patient? If you answered the Clonorchis sinensis, you are correct! Clonorchis sinensis, also known as the Chinese liver fluke, lives
in the liver of humans and is commonly found in the bile ducts and gall bladder [1].
The geographic distribution of this parasite is in endemic areas in Asia
including China, Japan, Korea, Taiwan, and Vietnam [1]. It is found mainly in
Asian immigrants, or following ingestion of imported, undercooked, or pickled
freshwater fish. C.
sinensis has also been reported in non-endemic areas including the United
States.
Figure 1. Clonorchis sinensis: Chinese liver fluke
http://www.suite101.com/view_image.cfm/2299692
Do Hyun Park, M. D., Ph. D. and Hyun-Young Son, M. D. from the
New England Journal of Medicine presented a clinical case dealing with the disease
associated with C. sinensis. Briefly,
an elderly man reported fatigue, fever, and abdominal pain for a week along
with the consumption of raw pond smelt [2]. A duodenoscopy was performed and
after the cannulation of the common bile duct, numerous leaf-shaped worms popped
out (see video below)—subsequently identified as C. sinensis [2]. The worm
burden and the duration of infection tend to reflect the clinical manifestations
of the disease clonorchiasis [2].
Video 1. Cannulation of the common bile duct displaying numerous trematode parasite C. sinensis being popped out of a patient with the disease clonorchiasis.
http://www.nejm.org/doi/full/10.1056/NEJMicm054461#t=article
Symbiont Description:
Clonorchis sinensis
is a trematode in the phylum Platyhelminthes [3]. Because this parasite exhibits
a three-host sequence, part of its life cycle takes place in aquatic habitats
while the other part takes place in terrestrial habitats. The physical
appearance of C. sinensis includes
many forms: egg, miracidium, sporocyst, redia, cercaria, metacercaria, and
adult [3]. C. sinensis is classified
as hermaphroditic; as a result, every single sexually mature form will produce
eggs [3]. Interestingly, each adult yields a daily production of 4000 eggs for
at least six months [3]! C. sinensis is
currently infecting an estimated 30,000,000 humans and is believed to be the
third most prevalent trematode parasite in the world [3].
Host Description:
C. sinensis
includes three species of hosts: the mollusk, the fish, and the mammal. Eggs
are evacuated with feces and ingested by a freshwater gastropod mollusk. A dozen
species have been reported as vectors, but the most frequent one is Parafossarulus manchouricus [4].
Numerous swimming larvae, also known as cercariae, are produced by asexual
multiplication and then leave the mollusk [4]. The cercariae penetrate a
variety of fish species—almost a hundred—where they then encyst in the muscle
as metacercariae [4]. Once the metacercariae are eaten by the fish, they excyst
in the mammal’s stomach and mature in the bile ducts of the liver [4].
Life Cycle:
DPDx—a website
developed and maintained by CDC’s Division of Parasitic Disease and Malaria—provided
an outstanding photo depicting the life cycle of C. sinensis. First, embryonated eggs are discharged in the biliary ducts
of humans [1]. Second, eggs are ingested by a suitable snail intermediate host
[1]. Each egg releases a miracidia, which go through four developmental stages:
miracidia, sporocysts, rediae, cercariae [1]. The cercariae are then released
from the snail and after a short period of free-swimming time in water, they
come in contact and penetrate the flesh of freshwater fish, where they encyst
as metaceriae [1]. After ingestion, the metacercariae excyst in the duodenum
and ascend the biliary tract via the ampulla of Vater [1]. The cycle repeats
over and over again from this point on.
Figure 2. The three-host sequence life cycle of C. sinensis.
http://dpd.cdc.gov/dpdx/html/Clonorchiasis.htm
Ecology:
C. sinensis has
had large detrimental effects on humans, specifically those in areas such as
Asia, where eating raw or undercooked fish is a part of their cultural upbringing.
A human host with an average of infection will have or three dozen worms, and
heavily infected individuals have been found with as many as 20,000 worms [3]. These
parasites reside in the biliary system of the liver and occasionally in the
pancreas. Although the liver or pancreas is not attacked, they are greatly
damaged due to the parasites’ migration through the biliary system [3]. Damages
include erosion of the epithelial lining of bile ducts, which leads to the development
of blockage in the ducts due to the thickening of scar tissue [3]. This erosion,
along with the effects of C. sinensis’
perforation into the parenchyma of the liver leads to possible disturbance of
normal hepatic functions [3].
An Example of the Three-Host Sequence:
Several unrelated species of hosts may be successively
exploited as the parasite develops over its life cycle. The sequence of hosts
consists of the series of hosts utilized during the life cycle and is often
referred to as the longitudinal component of the cycle [4]. This sequence involves
two dimensions: ecological (the succession of environments) and ontogenetic
(the various developmental states of the parasites) [4]. The parasite undergoes a
metamorphosis at each change of host because each time that the parasite
changes host, some genes are turned off while others are activated, resulting
in a morphologically, anatomically, physiologically, and ethologically distinct
organism [4]. Adults of C. sinensis
demonstrate an example of the three-host sequence: the mollusk, the fish, and
the mammal.
References:
[1] Laboratory Identification of Parasites of Public Health Concern. “Clonorchiasis.”
Centers for Disease Control and Prevention. Online. Accessed 24 Feb. 2012. <http://dpd.cdc.gov/dpdx/html/Clonorchiasis.htm>.
[2] Park, D. H. and
H. Y. Son. “Images in Clinical Medicine: Clonorchis sinensis.” The New England Journal of Medicine. (2008);
358: e18.
[3] Eckroad, E. and H. Lee. 2001. "Clonorchis sinensis."
Animal Diversity Web. Online. Accessed 24 Feb. 2012. <http://animaldiversity.ummz.umich.edu/site/accounts/information/Clonorchis_sinensis.html>.
[4] Combes C. 2001. Parasitism: The Ecology and Evolution of Intimate
Interactions. Chicago: University of Chicago Press. 36-38 p.
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