Liver and Lung Trematodes

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Liver and Lung Trematodes

The parasites within this chapter are typically food-borne and may result in serious economic impact. Clonorchis sp., Opisthorchis sp. and Fasciola sp. live in the biliary ducts of humans. Paragonimus spp. are found in the lungs and in other body sites.

The Liver Flukes

General Characteristics

The adults of these trematodes live in the biliary ducts and in heavy infections may be also found in the gallbladder. Two of these, Clonorchis sinensis (the Chinese liver fluke) and Opisthorchis viverrini (the Southeast Asian liver fluke), are elongated and narrow and much smaller than Fasciola (the sheep liver fluke). These flukes also all require a freshwater snail as an intermediate host.

Epidemiology and Life Cycle

Clonorchis sinensis is found in China, Japan, Korea, Taiwan, and Vietnam. Opisthorchis viverrini is found in Cambodia, Laos, Thailand, and Vietnam. Reservoir hosts include dogs and cats. Fasciola hepatica has worldwide distribution and impacts the economics of the sheep and cattle industries. Reservoir hosts include dogs, pigs, and rabbits. Infected feces enter the water system as a result of improper drainage and unsanitary practices.

The life cycle of the liver flukes is very similar to that of the intestinal flukes. The adult worms produce eggs in the biliary ducts that are then excreted from the body in the feces. The free-swimming miracidium is released from the egg in freshwater and enters the snail host where it develops into a redia and then a cercariae, which leaves the snail and enters the water (Figure 57-1). The cercariae of Clonorchis and Opisthorchis are ingested by a second intermediate host, a freshwater fish. The cercariae then encyst and develop into the metacercariae within the intermediate host. The metacercaria is the infective stage for humans. When infected freshwater fish are eaten raw or undercooked, the metacercariae will excyst in the duodenum and then travel to the bile duct where they mature. The cercariae of Fasciola encyst on freshwater vegetation, such as watercress and water chestnuts, and develop into metacercariae. When the infected vegetation is eaten raw, the metacercariae will excyst in the duodenum and then travel to the bile duct and mature. Figure 57-2 depicts the general life cycles of the liver and lung flukes.

Pathogenesis and Spectrum of Disease

Light infections with C. sinensis or O. viverrini are most common, and may be asymptomatic. Heavier infections with these flukes may present with fever, abdominal pain, and jaundice. Eosinophilia and increased serum levels of immunoglobulin E (IgE) may be observed. Severe infections may cause obstruction of the biliary ducts, resulting in enlargement and tenderness of the liver, cirrhosis, cholecystitis (inflammation of the gallbladder), and cholangiocarcinoma (cancerous growth in bile duct epithelium).

Even light infections with Fasciola may cause fever, abdominal pain, nausea, diarrhea, enlargement and tenderness of the liver, jaundice, nonproductive cough, eosinophilia, and elevated serum IgE levels. More severe infections may result in obstruction of the biliary ducts, cirrhosis, cholecystitis, and cholangiocarcinoma. During migration in the human body, the larvae may penetrate the peritoneal cavity, and adult flukes may then be found in the intestinal walls, lungs, heart, or brain.

Laboratory Diagnosis

Identification of the liver flukes is primarily made by recovery of the eggs in feces using a sedimentation method and a wet mount with or without iodine staining. Table 57-1 shows some diagnostic characteristics of the liver and lung flukes.

TABLE 57-1

Characteristics of Liver and Lung Trematodes

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Trematode Adult Location Food Source Size of Egg Description of Egg
Fasciola hepatica Bile ducts Freshwater vegetation 130-150 µm × 70-90 µm Operculated, brownish-yellow, unembryonated
Clonorchis sinensis Bile ducts Freshwater fish 28-34 µm × 14-18 µm Operculated with shoulders, opposite end knob, yellow-brown, embryonated
Opisthorchis viverrini Bile ducts Freshwater fish 19-29 µm × 12-17 µm