Foodborne Animal Parasites 693
the rediae through a birth pore. A cercaria is a miniature fluke with a tail. Cercariae leave the snail
and swim through water in search of their next host—usually fish, clams, and the like. They bore into
the new host, shed the tail, and become surrounded by a cyst. Within the cyst, further development
leads to metacercariae, which develop further in the final host, usually a vertebrate, including humans.
Upon ingestion of metacercariae-containing fish, the cyst wall dissolves in the intestine, and the young
flukes emerge. They then migrate through the body to their final site, the bile ducts of the liver in the
case of C. sinensis where, among other problems, they may cause cirrhosis (see below).
Liver flukes are common in China, Korea, Japan, and parts of Southeast Asia. It is estimated that
more than 20 million persons in Asia are infested with this parasite.
45 In China, it is often associated
with the consumption of a raw fish dish called ide. Over 80 species of fish are known to be capable of
harboring C. sinensis.
45
Symptoms, Diagnosis, Treatment, and Prevention
Symptoms may not occur if the infection is mild, but in severe cases, damage to the liver may occur.
The liver damage may lead to cirrhosis and edema, and cancer of the liver is seen occasionally.
79
Diagnosis is made by repeated microscopic examinations of feces and duodenal fluid for eggs. An
ELISA test is useful, but cross-reactions with other trematodes may occur. Praziquantel is an effective
chemotherapeutic agent.
The prevention of this syndrome is achieved by avoiding the deposition of human feces in fishing
waters, but this seems unlikely in view of its wide distribution. The avoidance of raw fish products and
the proper cooking of fish are more realistic alternatives. C. sinensis can be inactivated in fish by the
same procedures as for roundworms and flatworms. According to Rodrick and Cheng,
84 all captured
fish must be considered to be potential carriers of parasites. This applies to all flukes, flatworms and
roundworms, and protozoa.
Diphyllobothriasis
This infection is contracted from the consumption of raw or undercooked fish, and the causative
organism, Diphyllobothrium latum, is often referred to as the broad fish tapeworm. The definitive hosts
for D. latum are humans and other fish-eating mammals; intermediate hosts are various freshwater
fish and salmon, where plerocercoid (or metacestode) larvae are formed.
When humans consume fish flesh that contains plerocercoid larvae, the larvae attach to the ileal
mucosa by two adhesive grooves (bothria) on each scolex and develop in 3–4 weeks into mature forms.
As a worm matures, its strobila, made up of proglottids, increases in length to 10 m or nearly 20 m,
and each worm may produce 3000–4000 proglottids that are wider than they are long (hence, broad
fish tape) (see Figure 29–1). Over 1 million eggs may be released each day into stools of victims. Eggs
are more often seen in stools than proglottids, and they are not infective for humans.
When human feces are deposited in waters, the eggs hatch and release six-hooked, free-swimming
larvae or coracidia (also known as oncospheres). When these forms invade small crustaceans (copepods or microcrustaceans such as Cyclops or Diaphtomus), they metamorphose into a juvenile stage
designated metacestode or procercoid larvae. When a fish ingests the crustacean, the larvae migrate
into its muscles and develop into plerocercoid larvae. If this fish is eaten by a larger fish, the plerocercoid migrates, but it does not undergo further development. Humans are infected when they eat fish
containing these forms.
the rediae through a birth pore. A cercaria is a miniature fluke with a tail. Cercariae leave the snail
and swim through water in search of their next host—usually fish, clams, and the like. They bore into
the new host, shed the tail, and become surrounded by a cyst. Within the cyst, further development
leads to metacercariae, which develop further in the final host, usually a vertebrate, including humans.
Upon ingestion of metacercariae-containing fish, the cyst wall dissolves in the intestine, and the young
flukes emerge. They then migrate through the body to their final site, the bile ducts of the liver in the
case of C. sinensis where, among other problems, they may cause cirrhosis (see below).
Liver flukes are common in China, Korea, Japan, and parts of Southeast Asia. It is estimated that
more than 20 million persons in Asia are infested with this parasite.
45 In China, it is often associated
with the consumption of a raw fish dish called ide. Over 80 species of fish are known to be capable of
harboring C. sinensis.
45
Symptoms, Diagnosis, Treatment, and Prevention
Symptoms may not occur if the infection is mild, but in severe cases, damage to the liver may occur.
The liver damage may lead to cirrhosis and edema, and cancer of the liver is seen occasionally.
79
Diagnosis is made by repeated microscopic examinations of feces and duodenal fluid for eggs. An
ELISA test is useful, but cross-reactions with other trematodes may occur. Praziquantel is an effective
chemotherapeutic agent.
The prevention of this syndrome is achieved by avoiding the deposition of human feces in fishing
waters, but this seems unlikely in view of its wide distribution. The avoidance of raw fish products and
the proper cooking of fish are more realistic alternatives. C. sinensis can be inactivated in fish by the
same procedures as for roundworms and flatworms. According to Rodrick and Cheng,
84 all captured
fish must be considered to be potential carriers of parasites. This applies to all flukes, flatworms and
roundworms, and protozoa.
Diphyllobothriasis
This infection is contracted from the consumption of raw or undercooked fish, and the causative
organism, Diphyllobothrium latum, is often referred to as the broad fish tapeworm. The definitive hosts
for D. latum are humans and other fish-eating mammals; intermediate hosts are various freshwater
fish and salmon, where plerocercoid (or metacestode) larvae are formed.
When humans consume fish flesh that contains plerocercoid larvae, the larvae attach to the ileal
mucosa by two adhesive grooves (bothria) on each scolex and develop in 3–4 weeks into mature forms.
As a worm matures, its strobila, made up of proglottids, increases in length to 10 m or nearly 20 m,
and each worm may produce 3000–4000 proglottids that are wider than they are long (hence, broad
fish tape) (see Figure 29–1). Over 1 million eggs may be released each day into stools of victims. Eggs
are more often seen in stools than proglottids, and they are not infective for humans.
When human feces are deposited in waters, the eggs hatch and release six-hooked, free-swimming
larvae or coracidia (also known as oncospheres). When these forms invade small crustaceans (copepods or microcrustaceans such as Cyclops or Diaphtomus), they metamorphose into a juvenile stage
designated metacestode or procercoid larvae. When a fish ingests the crustacean, the larvae migrate
into its muscles and develop into plerocercoid larvae. If this fish is eaten by a larger fish, the plerocercoid migrates, but it does not undergo further development. Humans are infected when they eat fish
containing these forms.