The Liver Flukes
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p697-699
2025-11-10
163
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 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 2 depicts the general life cycles of the liver and lung flukes.
Fig1. Cercaria of a liver fluke. (Photo courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Fig2. Life cycle 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. Table1 shows some diagnostic characteristics of the liver and lung flukes.
Table1. Characteristics of Liver and Lung Trematodes
The adult worms of Clonorchis are elongated and narrow, and a transparent reddish-yellow color. Adult Clonorchis may vary in size from 10 to 25 mm × 3 to 5 mm. The eggs of Clonorchis are 28 to 30 µm × 14 to 18 µm. The eggs have shouldered opercula and a small knob at the end opposite the operculum, are yellow-brown in color, and are embryonated when they leave the body (Figure 3).
Fig3. Clonorchis sinensis egg. (Photo courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Like Clonorchis, the adult worms of Opisthorchis are elongated and narrow, and a transparent reddish-yellow color. Adult worms of Opisthorchis, however, are much smaller in size: 5 to 10 mm × 0.8 to 1.9 mm. The size of Opisthorchis eggs is slightly smaller than those of Clonorchis; Opisthorchis eggs are 19 to 29 µm × 12 to 17 µm. Also like Clonorchis, the eggs have shouldered opercula and a small knob at the end opposite the operculum, are yellow-brown in color, and are embryonated when they leave the body (see Figure 3).
The adult worm of Fasciola is much larger (2 to 5 cm × 0.8 to 1.3 cm), with a cephalic cone at the anterior end that contains the oral sucker. The eggs are 130 to 150 µm × 70 to 90 µm, operculated, brownish-yellow, and unembryonated when they leave the body. Because the eggs of Fasciola and Fasciolopsis are virtually indistinguishable, it may also be necessary to recover eggs from bile specimens, or to recover adult worms. Definitive identification of Fasciola is important because the treatment is different than that for Fasciolopsis. There is also serologic testing available in the United States for diagnosis of Fasciola. Enzyme immunoassay (EIA) and enzyme-linked immunosorbent assay (ELISA) serum IgG antibody testing is performed at private references laboratories; cross reactivity with other trematodes, such as the schistosomes, may be an issue.
THERAPY AND PREVENTION
The drug of choice for treatment of infections with Clonorchis and Opisthorchis is praziquantel given orally three times for 1 day. An alternative drug is albendazole, a benzimidazole group drug, given once daily for 7 days. The drug of choice for Fasciola is bithionol (praziquantel is not effective) given orally every other day for 10 to 15 doses. A promising drug that has not yet been approved for human use in the United States or Canada, but is approved for veterinary use in the United States, and is recommended by the World Health Organization (WHO) for treatment of Fasciola, is triclabendazole, a benzimidazole compound.
Human infection can be prevented by ensuring that fish and aquatic vegetation are properly cooked before consumption, as well as by the improvement of sanitary conditions along with the education of good personal hygiene.
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