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Intestinal Trematodes 183

called metacercariae, which can infect a host who feeds FASCIOLOPSIS BUSKI
on the animals or plants.
The correct handling, concentrating, and preserving of
specimens as well as the competence of the laboratory
Liver and Lung Fluke Diseases
worker and the timeliness of the specimen collection is vital
More than 80 other species of flukes other than in the detection and subsequent accurate identification
Schistosoma are considered capable of infecting humans. of parasites and their eggs. An example of easily identifi-
Humans may be infected either during the adult stage or able eggs (ova) is that of the giant intestinal fluke Facio-
larval stage, but only the most common species that rou- lopsis buski (Figure 8-1). Different life stages of a number
tinely impact human health are considered in this sec- of parasites may require that several samples be collected
tion: Paragonimus westermani, the lung fluke that causes and even pooled before steps toward identification of
paragonimiasis; Clonorchis sinensis, the liver fluke that the parasite begins. This is particularly true for some of
causes clonorchiasis; and Opisthorchis spp., which the protozoan diseases where certain stages of infection
causes opisthorchiasis. are only visible during a critical period when collection
Virtually all the important discoveries and ad- should be accomplished. This will be fully discussed dur-
vances regarding these parasites were made during the ing the presentations regarding specific species. F. buski is
period from 1874 to 1918. Discoveries of these medi- prevalent in Vietnam, Thailand, and all of Southeast Asia.
cally important parasite life cycles and other biological
data often occurred as a result of observations of other Morphology
parasitic flukes such as Fasciola hepatica in sheep and
F. buski is an organism known as the giant intestinal
several other agricultural or zoological flukes rather
fluke. This species is the largest fluke known to inhabit
than medical interest in humans. The life cycles of these
the human body and lives in the small intestine of in-
other flukes are essentially the same as those described
fected individuals. The adult fluke itself ranges from 2 to
for each of the Schistosoma spp. discussed previously.
7 cm in length. The egg is broad, ellipsoid, and contains
However, there is an added complexity because some
a small operculum, or lid, at the more pointed end of the
species require an additional intermediate or secondary
egg, which has a transparent eggshell and is unembryo-
host during development. Whereas snails are secondary
nated when passed. The egg measures 130 to 140 μm in
hosts, humans become infected both internally (diet) or
length and 80 to 85 μm in width.
externally (skin penetration) when the cercariae encyst.
Humans become infected only when they ingest the in-
fected second intermediate host. Symptoms
These various discoveries were made by a large Bowel obstruction may occur in heavy infections and
number of people and often in independent and special- diarrhea and abdominal pain and cramping may be
ized reports that were initially shared with only a few
biologists who had an interest in the topic. No attempt is
made here by the author to list these individual achieve-
ments. The learner is encouraged to refer to a multitude
of resources provided by Internet sources. Significant
knowledge of the pathological effects of clonorchiasis
and opisthorchiasis has emerged slowly and over quite a
period of time. Few startling and interesting discoveries Source: Centers for Disease Control and Prevention (CDC)
have occurred recently, except for finding an association
with bile duct cancer called cholangiocarcinoma when
clonorchiasis or opisthorchiasis infections are present.
The basic flukes that cause human disease are described
as follows, with the exception of the schistosomes, which
are of such importance as to deserve a separate section FIGURE 8-1 Egg (ova) from the intestinal fluke
for the organisms causing schistosomiasis. Fasciolopsis buski