Intestinal Trematodes

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Intestinal Trematodes

1. List the clinically significant intestinal trematodes.

2. Describe the general life cycle of the intestinal trematodes and identify the life cycle stage infective for humans.

3. Describe the diagnostic methods used to identify intestinal trematodes.

4. Explain the pathogenesis of intestinal trematode infections.

5. List the drugs of choice for intestinal trematode infections.

6. Describe the environment where intestinal trematodes are found, the route of transmission, and preventive measures.

Parasites to be Considered

Trematodes (Flukes)

Fasciolopsis buski

Heterophyes heterophyes

Metagonimus yokogawai

The intestinal trematodes (flukes) are members of the phylum Platehelminthes (flatworms), are dorsoventrically flattened, and require at least one intermediate host (a freshwater snail). Human infection occurs by ingestion of metacercariae encysted on freshwater vegetation or fish. Most trematodes are hermaphroditic (both ovaries and testes are contained within each adult worm). The parasites are typically identified from eggs shed in the feces.

The adult worms are located in the small intestine, where they lay eggs that may be embryonated or remain unembryonated until they are shed from the body via feces. The egg continues developing after reaching the water, and a ciliated, free-swimming miracidium larva is released. The miracidium enters a snail host and develops into a redia (cylindrical larvae), followed by development into tailed cercariae. The cercariae emerge from the snail and encyst as a metacercariae (encrusted larvae) on water plants or fish. A human host ingests raw or undercooked plants (Fasciolopsis buski) or fish (Heterophyes heterophyes, Metagonimus yokogawai) containing the metacercariae, which exycyst in the intestinal tract, attach, and mature into adults (Figure 56-1).

Figure 56-1 Life cycle of trematodes acquired by humans through ingestion of raw fish, crabs, or crayfish and vegetation.

Fasciolopsis Buski

General Characteristics

The adults of F. buski have an elongated shape and range from 20 to 75 mm long to approximately 8 to 20 mm wide (Figure 56-2). They have an oral sucker at the anterior end and a ventral sucker located about midway to the posterior end. The eggs, which are indistinguishable from those of Fasciola hepatica (Figure 56-3), are oval and elongated, transparent, and yellow-brown with an operculum (lid) at one end, and they range in size from 130 to 140 µm long to 80 to 85 µm wide and may be unembryonated.

Figure 56-2 Whole mount of Fasciolopsis buski. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)

Figure 56-3 Fasciola egg. The eggs of F. buski and F. hepatica are indistinguishable morphologically. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)

Epidemiology

Fasciolopsis buski is found in Bangladesh, Cambodia, China, India, Indonesia, Laos, Malaysia, Pakistan, Taiwan, Thailand, and Vietnam, and is prevalent in school-aged children. Contaminated feces drains into the water from farm lands, where feces is used for fertilization, and defecation occurs in or near water sources. Reservoir hosts include pigs, dogs, and rabbits.

F. buski is the largest of the intestinal trematodes, and infection is acquired by ingestion of raw water chestnuts or caltrop. The definitive host is the pig, and fish-eating wild and domestic animals may serve as reservoir hosts. The water vegetation may become contaminated when feces is used for fertilization or where disposal of farm animal feces is inadequate.

Pathogenesis and Spectrum of Disease

The intestinal attachment site of the adult worms often becomes locally inflamed and ulcerated, and may hemorrhage. Moderate to heavy infections may cause abdominal pain, diarrhea, intestinal obstruction, and edema of the abdomen and lower extremities, and may result in inadequate absorption of vitamin B₁₂. Eosinophilia is commonly observed.

Prevention

Infection can be prevented by making sure that water plants and fish are properly cooked before eating. In addition, changes are needed in agricultural practices and health education in the endemic areas.

Heterophyes Heterophyes and Metagonimus Yokogawai

General Characteristics

Adult H. heterophyes worms range in size from 1.0 to 1.7 mm in length by 0.3 to 0.4 mm in width, and have a broadly rounded posterior. M. yokogawai adults range in size from 1.0 to 2.5 mm long to approximately 0.4 to 0.8 mm wide. The adult H. heterophyes also has an additional sucker, the genital sucker, which surrounds the genital pore. The eggs are small, yellow-brown, embryonated, and operculated and may have minimal opercular shoulders. Eggs range in size from 26 to 30 long µm to 15 to 17 µm wide, and may be indistinguishable between the two species.

Epidemiology

H. heterophyes is found in China, Egypt, India, Iran, Israel, Japan, Korea, Sudan, Taiwan, the Philippines, Tunisia, and Turkey. M. yokogawai is found in the Balkans (a cultural region of southeastern Europe), China, Indonesia, Israel, Japan, Korea, Russia, Spain, and Taiwan, and is considered the most common intestinal fluke infection in the Far East. Reservoir hosts include cats, dogs, and birds, and a variety of freshwater fish may serve as the second intermediate host.

These are two very small trematodes acquired through ingestion of pickled or uncooked fish. A variety of fish-eating mammals may serve as reservoir hosts.

Pathogenicity and Spectrum of Disease

Infections with a small number of worms may be asymptomatic. Symptoms in heavy infections may include abdominal pain, diarrhea with a large amount of mucus, and ulceration of the intestinal wall. Eggs may gain entry into intestinal capillaries and lymphatics, where they can be carried to the heart, brain, spinal cord, or other tissues, causing emboli or granuloma formation.

Prevention

Avoid ingestion of raw, inadequately cooked, and pickled or salted fish. The risk of infection could be greatly reduced by improved sanitary conditions and health education programs.

Laboratory Diagnosis

Identification of the intestinal trematodes is made by recovery of eggs, or in rare cases adults, from stool specimens using a sedimentation method such as formalin-ethyl acetate. The sediment may be examined in a wet mount with or without iodine. Because the eggs of Fasciolopsis buski are identical to those of Fasciola hepatica, and those of Heterophyes heterophyes and Metagonimus yokogawai are very similar, diagnosis may also require assessment of symptoms, obtaining a travel history, and/or recovery of adult worms. Table 56-1 shows the diagnostic characteristics of the intestinal trematodes.

TABLE 56-1

Diagnostic Characteristics of Intestinal Trematodes

Intestinal Trematode	Food Source	Size of Egg	Description of Egg
Fasciolopsis buski	Freshwater vegetation	80-85 µm × 130-140 µm	Operculated, yellow-brown, unembryonated
Heterophyes heterophyes	Pickled or uncooked fish	15-17 µm × 26-30 µm	Operculated with slight opercular shoulders, yellow-brown, embryonated
Metagonimus yokogawai	Pickled or uncooked fish	15-17 µm × 26-30 µm	Operculated with slight opercular shoulders, yellow-brown, embryonated

Treatment

The drug of choice for treatment of intestinal trematode infection is praziquantel (Biltricide), an isoquinoline derivative administered orally in three doses for 1 day. There may be some mild side effects, but these usually disappear within 48 hours, and may be more severe in those with heavy infections. An alternative drug is niclosamide (Niclocide), which is given for 1 to 2 days.

1. Fluke eggs are equipped with a lid at the top of the shell called a/an:

2. The infective life cycle stage of a fluke is the:

c. Metacercariae

d. Pleurocercariae

3. What intermediate host is required in the life cycle of all trematodes?

a. Freshwater snail

c. Aquatic vegetation

d. Freshwater crab

4. Which two of the following are small flukes whose eggs are generally undistinguishable?

a. Fasciolopsis

b. Heterophyes

c. Metagonimus

d. Paragonimus

5. What laboratory finding is common in F. buski infections?

a. Increased levels of vitamin B₁₂

b. Increased serum bilirubin levels

c. Decreased number of red blood cells

d. Increased number of eosinophilia

6. The drug of choice for an intestinal trematode infection is:

b. Praziquantel

d. Triabendazole

Case Study 56-1

The husband of a 32-year-old woman is employed in the foreign service and the couple has recently been on assignment to Africa. The wife complained of a 2-month history of abdominal pain, vomiting, diarrhea, and weight loss. While in Africa she had often eaten the locally grown watercress. A stool specimen was collected for culture and ova and parasite examination. The bacterial culture was negative. A wet mount made during the parasite examination showed large, oval, operculated, and unembryonated helminth eggs.

1. What parasite is the probable cause of the patient’s symptoms?

2. Another parasitic worm has indistinguishable eggs. How would the infections caused by these two worms differ?

3. How did the patient most likely acquire this infection?

4. What would be the preferred treatment for this infection?

Related

Bailey _ Scotts Diagnostic Microbiology 13e