About Opisthorchis

What is Opisthorchis?

Opisthorchis species are liver fluke parasites (trematodes or worms). Opisthorchis viverrini is known as the Southeast Asian liver fluke and O. felineus as the cat liver fluke.

What are the signs and symptoms of infection with Opisthorchis?

Most infected persons have no symptoms. In mild cases, symptoms may include indigestion, abdominal pain, diarrhea, or constipation. Signs and symptoms are related to inflammation and blockage of the bile ducts that may come and go. In severe cases, abdominal pain, nausea, and diarrhea can occur.

Untreated, infection may persist for up to 25–30 years, the lifespan of the parasite. In infections that last a long time, an enlarged liver and malnutrition may occur. In areas where liver flukes are found and a person may have multiple long-standing untreated liver fluke infections, the inflammation of the gallbladder and ducts caused by the parasite has been associated with liver and bile duct cancers (bile duct cancer is also known as cholangiocarcinoma). Liver fluke infection is one of many factors that have been associated with bile duct cancer. Other known risk factors for bile duct cancer include hepatitis B, hepatitis C, alcoholic liver disease, and other causes of bile duct inflammation. These risk factors are thought to be more common causes of bile duct cancer in the United States than liver fluke infection. Most patients in Western countries do not have an identifiable risk factor for bile duct cancer.

In addition to the symptoms listed above, infections due to O. felineus can also present with fever, facial swelling, swollen lymph glands, sore joints, and rash—similar to the signs and symptoms of schistosomiasis. Chronic O. felineus infections may also involve the pancreatic ducts.

How does one become infected with Opisthorchis?

The eggs of Opisthorchis are ingested by freshwater snails. After the eggs hatch, infected snails release microscopic larvae that then enter freshwater fish. People become infected when eating infected raw or undercooked fish containing the larvae. After being ingested by humans, the larvae grow into adult worms and live inside the human bile duct system. The life cycle takes three months to complete in humans. Infected people will then pass eggs in their stool or may cough them up.

Who is at risk for Opisthorchis infection?

Liver fluke infections occur mostly in people living in areas where the parasites are found. Travelers to Asia who consume raw or undercooked fish are at risk for liver fluke infection. Chlonorchis is found in several areas in Asia including Korea, China, Taiwan, Northern Vietnam, Japan, and Asian Russia. Clonorchis infection has been reported in Asian immigrants in non-endemic areas. Some cases have been found in people who had ingested freshwater fish (undercooked or pickled) containing parasitic cysts; the fish were imported to the US from areas where Clonorchis can be found.

What should I do if I think I might be infected?

See your health care provider if you have traveled to a country where Opisthorchis parasites are found and think you may have been exposed. Research has shown the liver fluke infections can persist for up to 25–30 years, the lifespan of the parasite.

How is infection with Opisthorchis diagnosed?

Diagnosis of Opisthorchis infection is based on microscopic identification of eggs in stool specimens. More than one stool sample may be needed to detect the eggs. The eggs of Opisthorchis are very similar to those of Clonorchis, another liver fluke, but can be distinguished by microscopic features. Additionally, cysts containing the parasite can sometimes be detected by ultrasound, CT, or MRI. Serologic testing (testing the blood) for Opisthorchis is not useful for patient management and is not available in the United States.

Can Opisthorchis be transmitted person to person?

No. Opisthorchis cannot be directly transmitted from person to person.

How is infection with Opisthorchis treated?

Praziquantel or albendazole are the drugs of choice to treat Opisthorchis infection.

Patient management

Patients with signs and symptoms of Opisthorchis infection or liver disease should see a health care provider.

Examination of the stool for liver fluke eggs is the only test available for the diagnosis of Opisthorchis infection. Stool examination is unlikely to result in a diagnosis in persons whose only exposure to Opisthorchis took place more than 25–30 years ago (the life span of a liver fluke), as the liver fluke must be living in order to produce eggs. In the absence of detection of liver flukes, there is no test available that can determine if liver fluke infection is the underlying cause of cholangiocarcinoma or other conditions of the liver, bile ducts, or gallbladder.

How can I prevent Opisthorchis infection?

Opisthorchis infection can be prevented by avoiding raw or undercooked freshwater fish. Lightly salted, smoked, or pickled fish can contain infectious parasites. Opisthorchis infection does not result from drinking river water or other non-potable water.

The FDA recommends the following for fish preparation or storage to kill any parasites.

  • Cooking
    • Cook fish adequately (to an internal temperature of at least 145° F [~63° C]).
  • Freezing (Fish)
    • At -4°F (-20°C) or below for at least 7 days (total time); or
    • At -31°F (-35°C) or below until solid, and storing at -31°F (-35°C) or below for at least 15 hours; or
    • At -31°F (-35°C) or below until solid and storing at -4°F (-20°C) or below for at least 24 hours.

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Page last reviewed: August 25, 2020