What Are Liver Flukes?

The name “fluke” describes a group of species of parasitic flatworms that are able to infect mammals including humans. Parasites are organisms that live on or in another organism (known as the “host”). They depend on their hosts for their survival, getting their food either from their hosts or at the hosts’ expense, and they are specially adapted to live in this way.

Because parasites need their hosts to be able to survive, many species do not kill the host directly but often carry diseases that can be life-threatening to the host. Once they have infected a person, blood flukes tend to reside in the blood vessels. Liver flukes are small enough to travel around the body in the blood circulation. They often end up in the liver, gallbladder, and bile ducts where they can cause disease.

What Do the Liver and Bile Ducts Do?

The liver has a number of roles in the body, including cleaning the blood to remove harmful substances and metabolizing proteins, fats, and carbohydrates so that the body can use them. The liver also makes a fluid called bile that helps the body to break down fats from food, which can be stored in the gallbladder or can travel directly from the liver to the small intestine.

Most of the digestion of the food we consume takes place in the small intestine and it is here that nutrients and minerals from our food are absorbed into the blood. The bile ducts are part of the digestive system and are small tubes that connect the liver to the gallbladder and small intestine.

Where Are Flukes Found?

Different types of fluke have different life cycles (the different stages that organisms go through during their lives) and are found in different areas of the world. Some types of liver fluke are endemic (this means “native to” or regularly occurring and/or present) in areas of southern and southeastern Asia, but other types can be found in all continents except Antarctica. Although health authorities in areas where flukes are endemic have made major efforts to prevent and control their presence, with some successes, they are still a problem and many people become infected.

How Are Fluke Infections Linked to Cancer?

Although the symptoms caused by fluke infections are often mild, flukes can survive in the human body for several decades if the infection is not treated. This can lead to chronic (long-term) inflammation, the process by which your body responds to an injury or a perceived threat.

Liver fluke infection can also lead to an increase in the number of cells lining the ducts and the passageways connecting the liver, gallbladder, and small intestine (known as “epithelial hyperplasia”), and thickening or scarring of the ducts (known as “periductal fibrosis”). These symptoms can cause further complications over time including the formation of stones and the development of hepatobiliary cancers (the prefix “hepato” refers to the liver and “biliary” refers to the gallbladder and bile ducts).

Which Flukes Are Linked to Cancer?

There are a number of flukes that have been linked to the development of hepatobiliary cancer. Three of the most common are Schistosoma japonicum, Clonorchis sinensis, and Opisthorchis viverrini.

Schistosoma japonicum

S. japonicum is a blood fluke that is responsible for a disease called schistosomiasis, which is estimated to affect 200 million people worldwide. People become infected with this type of fluke through contact with fresh water in which the parasite is present, either through work and agriculture, or activities of daily living. During its life cycle, S. japonicum can cause blockages in small blood vessels in the liver and cause cirrhosis (scarring of the liver tissue that causes long-term damage and prevents the liver from working properly).

It is suspected that S. japonicum infection is directly linked to the development of a type of liver cancer called hepatocellular carcinoma, because rates of this type of cancer are much higher in areas where S. japonicum is endemic. The International Agency for Research on Cancer (IARC) has classified this fluke as a group 2B carcinogen (a substance or organism that can cause cancer), which means that the IARC has brought together a panel of experts on the subject that has evaluated all of the available published evidence and agreed that it is “possibly” able to cause cancer in humans.

Clonorchis sinensis

C. sinensis is endemic in China and Korea, and is estimated to affect 35 million people worldwide. People can become infected with this liver fluke by eating raw or undercooked infected fish, crab, or crayfish. Similarly to S. japonicum, C. sinensis has been linked to the development of cholangiocarcinoma, the name given to a group of cancers that form in the bile ducts, because the incidence of cholangiocarcinoma is much higher in areas where C. sinensis is endemic.

In one study, C. sinensis infection was shown to increase the risk of developing cholangiocarcinoma by 14 times compared with individuals with no history of C. sinensis infection. The IARC has classified this liver fluke as a group 2A carcinogen, which means that it is “probably” able to cause cancer in humans.

Opisthorchis viverrini

O. viverrini is known by some as the “South East liver fluke”, is endemic in northern Thailand, and is estimated to affect 10 million people worldwide. Similarly to C. sinensis, people can become infected with this liver fluke by eating raw or undercooked infected fish, crab, or crayfish. The evidence that O. viverrini infection is linked to the development of cholangiocarcinoma is so strong that the IARC has classified it as a group 1 carcinogen, which means that the IARC views O. viverrini as “definitely” being carcinogenic.

Take-Home Message

It is clear that fluke infections can have serious long-term implications that go beyond the initial effects of the parasite on the host. It is therefore essential that fluke infections are recognized and treated as soon as possible after infection to reduce the risk of hepatobiliary cancer developing in the future.

Note: This post is based on an article that is not open-access; i.e., only the abstract is freely available.