Opisthorchiasis is a parasitic disease caused by liver flukes, a type of flatworm. The condition is a foodborne illness transmitted to humans through the consumption of raw or undercooked freshwater fish containing the parasite’s infectious larvae. The species Opisthorchis viverrini and Opisthorchis felineus are primarily responsible for human infections. Once ingested, these parasites settle in the host’s liver, gallbladder, and bile ducts.
The Liver Fluke Lifecycle
The lifecycle of the Opisthorchis parasite is complex and involves multiple hosts. The cycle begins when eggs from adult flukes, residing in a host like a human, are passed into the environment through feces. These eggs must reach a freshwater source to be ingested by the first intermediate host, an aquatic snail.
Inside the snail, the eggs hatch and develop into a free-swimming larval stage known as cercariae, which are then released into the water. These cercariae seek out and penetrate the skin of a second intermediate host, a cyprinoid fish.
Inside the fish, the larvae encyst in the tissues and become metacercariae, the stage infectious to mammals. After an infected fish is consumed, the larvae travel from the intestine to the biliary ducts, where they mature into adult flukes, completing the cycle.
Symptoms of Infection
The clinical signs of opisthorchiasis vary depending on the number of flukes and the duration of the infection. A large percentage of infected individuals, about 80%, show no symptoms, particularly with a low worm burden. These asymptomatic cases may be identified through the presence of eosinophilia, an increase in a type of white blood cell.
In cases of acute or heavy infection, symptoms can appear within 10 to 26 days of consuming contaminated fish. Initial symptoms may include high-grade fever, right upper quadrant abdominal pain, fatigue, lack of appetite, and diarrhea.
Chronic infection occurs when flukes live in the biliary system for many years, leading to persistent issues like indigestion and fatigue. Over time, the inflammation can result in an enlarged liver, jaundice, and fluid accumulation in the abdomen and legs. The disease often progresses to this chronic stage unnoticed due to the lack of initial symptoms.
Medical Diagnosis and Treatment
The standard method for diagnosing an active Opisthorchis infection is the microscopic identification of parasite eggs in a fecal sample.
Medical imaging can also be used to assess the infection’s impact on the liver and bile ducts. Techniques like ultrasound, CT scans, or MRIs can reveal abnormalities such as bile duct dilation or fibrosis. Blood tests that detect antibodies against the parasite can suggest an infection but may not distinguish between past and current infections.
The primary treatment for opisthorchiasis is the oral medication Praziquantel. This drug is highly effective at killing the adult flukes and is administered in a single dose. While the treatment eliminates the parasites, it does not provide future immunity, so reinfection is possible if individuals continue to consume contaminated fish.
Associated Cancer Risk
Chronic infection with the liver fluke Opisthorchis viverrini is a significant risk factor for developing cholangiocarcinoma, a cancer of the bile ducts. The International Agency for Research on Cancer (IARC) has classified O. viverrini as a Group 1 carcinogen.
The mechanism driving this cancer risk involves chronic inflammation. The physical presence of the flukes attached to the walls of the bile ducts, along with their feeding activities and metabolic byproducts, creates a state of persistent inflammation. This response causes mechanical damage to the ductal epithelium and can lead to changes in the DNA of the host’s cells.
Over many years, this sustained cycle of tissue damage and repair can promote cellular proliferation and genetic mutations, leading to the uncontrolled cell growth that characterizes cancer. The risk is particularly high in individuals with heavy and long-standing infections.
Prevention and Global Distribution
Preventing opisthorchiasis relies on food safety practices. The most effective strategy is to ensure all freshwater fish is cooked thoroughly to a temperature that kills the parasite. Freezing fish at -10°C for at least five days can also destroy the infective larvae.
Educational programs in endemic areas are a component of control efforts, aiming to change cultural traditions of eating raw or fermented fish dishes. Improved sanitation to prevent human feces from contaminating freshwater sources is another measure that can help break the parasite’s lifecycle by stopping eggs from reaching snail hosts.
Opisthorchiasis has a distinct geographical distribution based on the parasite species. Opisthorchis viverrini is primarily found in Southeast Asia, with high rates of infection in Thailand, Laos, Cambodia, and Vietnam. In contrast, Opisthorchis felineus, also known as the cat liver fluke, is endemic to parts of Eastern Europe and Siberia, including Russia, Ukraine, and Kazakhstan.