Opisthorchis (Liver Flukes)

Opisthorchis (Liver Flukes): All About O. tenuicollis and O. viverrini

Opisthorchis species are trematode parasites commonly known as liver flukes that infect the biliary system of humans and various mammals. The two most important species are Opisthorchis tenuicollis (O. felineus) and Opisthorchis viverrini, both of which have complex life cycles involving snails and freshwater fish. This article covers their morphology, life cycle, pathogenesis, clinical signs, diagnosis, treatment, and prevention.

Opisthorchis tenuicollis (O. felineus)

• Family: Opisthorchiidae
• Common Name: Cat liver fluke / Siberian liver fluke
• Host: Dogs, cats, foxes, pigs, cetaceans, and humans
• Location: Bile ducts; less commonly in the intestine and pancreatic ducts
• Distribution: Eastern Europe, Poland, Germany, Siberia, and other parts of Asia

Opisthorchis viverrini

• Family: Opisthorchiidae
• Common Name: Southeast Asian liver fluke
• Host: Civets, domestic and wild cats, dogs, and humans
• Location: Bile ducts, intestine, and pancreatic ducts
• Distribution: Southeast Asia, particularly Thailand and Laos

Morphology

• Opisthorchis measure approximately 7–12 × 1.5–2.5 mm and are reddish in color.
• The tegument is smooth.
• The esophagus is short, and the intestinal ceca extend almost to the posterior end.
• The testes are lobed and situated near the posterior end. The excretory bladder passes between the testes.
• The prostate gland and cirrus sac are absent.
• The ovary is small and lies on the midline at the beginning of the posterior third of the body.
• The vitellaria occupy the middle third of the lateral fields and consist of a series of transversely arranged follicles.
• The transverse uterine coils do not extend behind the ovary.
• The eggs measure approximately 26–30 × 11–15 µm. They possess an operculum with a thickened shell rim, contain a miracidium, and are asymmetrical in shape.

Life Cycle

Life Cycle of O. tenuicollis

1. First I/H: Snails (Bithynia leachi and B. inflata)
2. Second I/H: Cercariae penetrate the skin and encyst in the subcutaneous tissues, particularly at the bases of the fins of cyprinid fish (Leuciscus, Blicca, Tinca, Idus, Barbus, Abramis), forming metacercariae.

Life Cycle of O. viverrini

1. First I/H: Snails (Bithynia goniomphalus, B. laevis, B. funiculata)
2. Second I/H: Cyprinid fish (Cyclocheilichthys, Hampala, Puntius)

Drinking river water or other non-potable water does not result in infection with Opisthorchis.

Adult flukes deposit fully developed eggs that are passed in the feces. After ingestion by a suitable snail (first IH), the eggs release miracidia, which undergo several developmental stages within the snail (sporocysts, rediae, and cercariae). Cercariae are then released from the snail and penetrate freshwater fish (second IH), where they encyst as metacercariae in the muscles or beneath the scales.

The mammalian definitive host (cats, dogs, and various fish-eating mammals, including humans) becomes infected by ingesting undercooked fish containing metacercariae. Following ingestion, the metacercariae excyst in the duodenum and ascend through the ampulla of Vater into the biliary ducts, where they attach and develop into adults. Egg production begins approximately 3–4 weeks later.

Humans become infected by consuming raw or undercooked fish containing the parasite. After ingestion, the liver flukes mature into adult worms that reside within the bile duct system. The life cycle is completed in approximately three months.

Pathogenesis

Dilation of the bile ducts with adenomatous thickening of the epithelium is common, and in advanced cases, marked fibrosis (periductal fibrosis) may occur.

Cases of liver and pancreatic carcinoma in cats and humans have been associated with O. tenuicollis infection.

The parasites cause catarrhal cholecystitis (inflammation of the gallbladder), desquamation of the bile duct epithelium, and increased mucus production.

Occlusion of the bile ducts with bile stasis and jaundice may occur. Secondary bacterial infections may result in cholangitis and cholangiohepatitis.

Cholangiocarcinoma may occur concurrently in severe cases. If the parasites are present in the pancreatic ducts, pancreatitis may develop.

Clinical Signs

In heavy infections, clinical signs in humans include indigestion, abdominal pain, diarrhea, or constipation. Severe cases may present with abdominal pain, nausea, diarrhea, icterus, ascites, and other signs associated with liver cirrhosis and impaired portal circulation.

O. viverrini infections may present with fever, facial swelling, enlarged lymph nodes, joint pain, and skin rash.

Diagnosis

• Clinical signs, history, and geographic location.
• Detection of eggs in feces.
• Serological tests (CFT, PHA, CIEP, and FAT).

Treatment

• Hexachloroparaxylene: 100 mg/kg in three divided doses, administered three times on alternate days.
• Chloroquine: 5 mg/kg/day for two months.
• Dihydroemetine: 2.5 mg/kg administered 30 times on alternate days.
• Niclosamide: 1–2 mg/kg for 2–3 days.
• Dithiazanine iodide.
• Niclofolan: 1–2 mg/kg for 2–3 days.
• Praziquantel: 50 mg/kg as a single dose.

Prophylaxis

• Prevent the consumption of raw fish. Thorough cooking to at least 145°F (63°C) or appropriate freezing of fish is essential.
• Eradication of snail intermediate hosts.
• Avoid adding human sewage or dog and cat feces to fish ponds.
• Treat sewage and fecal waste with ammonium sulfate to destroy fluke eggs.
• Improve sanitation and promote public health education.