What is a Fluke infection?

Fluke infections are an infection of a parasitic flatworm called trematode or fluke. There are many types of flukes that are able to infect humans, and they can be divided into two groups with several subgroups:

The blood flukes or schistosomes (Schistosoma haematobium, S. mansoni, S. japonicum, S. mekongi, S. intercalatum) cause schistosomiasis or bilharziasis.  Schistosomiasis fluke larvae are the only ones that infect humans through penetrating the skin. (Citation: 1) S. haematobium gives urogenital schistosomiasis, while the other types result in intestinal schistosomiasis. (Citation: 9)

The foodborne flukes are transmitted to humans through ingestion of metacercariae (encysted larvae). (Citation: 2) They can be divided into liver flukes, gastrointestinal (GI) tract flukes and flukes that live in other organs (e.g. lungs, central nervous system). (Citation: 1) The liver flukes Clonorchis sinensis, Fasciola hepatic and Opisthorchis species (viverrini and felineus) cause Clonorchiasis, Fascioliasis and Opisthorchiasis respectively. (Citations: 3, 4, 5)

The main GI tract flukes Fasciolopsis buski, Heterophyes heterophyes and related fluke species cause Fasciolopsiasis and Heterophyiasis. (Citation: 7)

The main Other organ fluke Paragonimus westermani and related fluke species cause Paragonimiasis.  (Citation: 8)

 

How common are Fluke infections?

Schistosomiasis is common in (sub)-tropical areas, especially where there is poor sanitation, with 92% of people who need treatment living in Africa. WHO estimated that in 2000 there were 200,000 annual global deaths because of schistosomiasis. (Citation: 9)

WHO estimates that the four species (Clonorchis, Fasciola, Opisthorchis and Paragonimus) that cause foodborne trematodiasis with potentially severe outcomes in humans together cause 200,000 illnesses and 7,000 deaths every year.  (Citation: 10) Paragonimiasis is found in Africa, Asia and Latin America, while Clonorchiasis and Opisthorchiasis are only found in Asia. Fasciolosis is found worldwide. (Citation: 9)

 

Are Fluke infections contagious?

Fluke infections are not directly transmitted between humans. People get infected with flukes if they eat fish, crustaceans or vegetables that are contaminated with foodborne fluke larvae; or in the case of schistosomiasis if the fluke larvae penetrates their skin. (Citation: 9, 10)

The foodborne trematodiases are zoonoses with complex life-cycles (Citations: 11, 12, 13, 14, 15, 16) that involve one or several intermediate hosts (like freshwater snails and fish), with as final host always a mammal. (Citation: 10)

Schistosomiasis larvae that penetrate the skin develop into adult schistosomes (worms) in the body where they stay in veins close to the intestines (and the urinary tract for S. Haematobium). The adult worms release eggs; some of these eggs are passed in the stool and urine and reach fresh water where they hatch into miracidia. Miracidia (larvae) penetrate snails in which they develop and are released as free-swimming cercariae (larvae) that are able to penetrate human skin.  (Citation: 1, 17)

 

What are the signs and symptoms of a Fluke infection?

Early and light foodborne fluke infections usually have few or no symptoms. Heavier infections with more worms are symptomatic by showing signs of general malaise, pain and other symptoms related to the location of the worms. If these infections are left untreated they become chronic within general severe complications again associated with the worm location.

Adult worms in clonorchiasis and opisthorchiasis live in the smaller bile ducts of the liver where they can cause inflammation, fibrosis and obstruction of surrounding tissues. (Citation: 10) This can lead to epigastric pain or discomfort, intermittent colic pain (gallbladder obstruction), enlarged liver, jaundice and diarrhea.

Fasciolosis flukes usually live in larger bile ducts and the gallbladder, resulting in infection, fibrosis, obstruction which give symptoms like a colicky pain and jaundice. (Citations: 3, 5, 10, 18, 19)

The final location of the paragonimiasis flukes is in the lungs, causing symptoms like a chronic cough with bloody sputum, chest pain with shortness of breath and fever. (Citation: 10, 20) In the acute phase (2-15 days after infection) there might be abdominal pain and discomfort, cough with a low-grade fever.  (Citation: 21)

In heterophyiasis and fasciolopsiasis, the adult worms live in the small intestine, which may cause abdominal pain and diarrhea, and in fasciolopsiasis also can cause fever, malabsorption or intestinal obstruction. (Citation: 6, 7)

Schistosomiasis infections are often without symptoms in the acute phase. There can be cercarial dermatitis: an itching rash with pimples (papules) because of cercariae (larvae) penetrating the skin. Katayama fever is a response to the first eggs, about 2-4 weeks after a heavy exposure. Katayama fever gives fever, urticarial rash, enlargement of liver and spleen, bronchospasm and a considerable increase of eosinophils in the blood. (Citation: 1, 22) The reaction of the body to eggs that get trapped in body tissues causes the symptoms in chronic schistosomiasis. This starts with bloody diarrhea or bloody urine due to mucosal ulceration but progresses to fibrosis, strictures, fistula and granulomatous growths around the eggs leading to many complications (see complications section below). (Citation: 1, 9)

 

How is the diagnosis of a Fluke infection in humans made?

The diagnosis of Clonorchiasis, opisthorchiasis and heterophyiasis is through finding eggs upon microscopic stool examination, the eggs of these three fluke types are very similar. (Citations: 3, 5, 7, 23, 24, 25)

Fasciolosis and Fasciolopsiasis can also be detected by finding eggs in the stool, the eggs of both types are very similar. Antibody tests in the blood (serology) might be useful in people who are not passing eggs in Fascioliasis, for example during the acute phase or in light infections during the chronic phase. (Citations: 4, 6, 16, 27, 28)

Paragonimiasis is usually diagnosed by finding eggs in the sputum, although eggs (coughed up and then swallowed) are sometimes found in the stool. It is useful to do an antibody test in the blood as the eggs are released intermittently and only in small amounts. (Citation: 8, 29)

The diagnosis of schistosomiasis is made through microscopic detection of eggs in stool, urine (S. haematobium, sometimes S. Japonicum) or tissue biopsies or detection of antibodies in the blood (serology). (Citation: 1, 30)

 

What is the treatment for Fluke infections?

Treatment for Clonorchiasis and opisthorchiasis is with oral Praziquantel (25 mg/kg 3 times daily for 2-3 consecutive days), which is also used in a single dose (40 mg/kg) as preventive chemotherapy. Treatment for Fascioliasis is with oral Triclabendazole (10 mg/kg single dose), and in the same dosage as a preventive chemotherapy. Paragonimiasis is also treated with oral Triclabendazole (2 doses of each 10 mg/kg for one day), which is also used as preventive chemotherapy (20 mg/kg single dose). (Citation: 10)

The treatment for schistosomiasis is with oral Praziquantel (20 mg/kg two times a day, three times a day for S. japonicum and S. mekongi), usually for 1-2 days, with retreatment if living eggs are still present at treatment check-ups. These treatment check-ups are done for example after 1 to 2, 3 and 6 months. (Citations: 1, 31, 32)

 

Are there complications of Fluke infection in humans?

The main complication of clonorchiasis and opisthorchiasis is the development of a fatal form of bile duct cancer: cholangiocarcinoma. In fact, both C. sinensis and O. viverrini (but not O. felineus) are considered carcinogenic agents. (Citation: 10)

Complications in heterophyiasis and fasciolopsiasis are rare. (Citation: 6, 7)

Fascioliasis complications include anemia, pancreatitis, gallstones, secondary bacterial infections and liver fibrosis. (Citation: 10, 33) Stray flukes might result in ectopic lesions in the bowels, lungs, pharynx and other organs. (Citation: 4)

Complications in paragonimiasis include pneumothorax and a chronic pulmonary infection that is often confused with tuberculosis. (Citations: 8, 10, 18) Stray flukes might cause brain and skin lesions. (Citations: 8, 10)

Complications of intestinal schistosomiasis are liver and spleen enlargement due to cirrhosis and fibrosis, hypertension of abdominal and esophageal veins and accumulation of fluid in the abdominal cavity. (Citation: 1, 9) Complications of urogenital schistosomiasis are among others genital lesions, infertility, kidney damage and bladder cancer. (Citation: 1, 9) Both types of schistosomiasis lead to many deaths every year. Other complications are anemia due to blood loss and secondary bacterial infections. Eggs in the lungs might result in hypertension in the pulmonary blood vessels and subsequent right-sided heart failure. (Citation: 1) Stray eggs can lead to spinal cord inflammation and brain lesions with seizures. (Citation: 1)

 

What are the preventive measures for Fluke infections?

Foodborne flukes infections are prevented by not eating contaminated food (fish, crustaceans, plants). (Citation: 2) Clonorchiasis, opisthorchiasis and heterophyiasis can be prevented by not eating raw or undercooked fish. (Citations: 34, 35, 7) Fascioliasis and Fasciolopsiasis are prevented through not eating raw watercress and other water plants. (Citations: 36, 37) Paragonimiasis is prevented through not eating raw freshwater crabs or crayfish.  (Citation: 38)

Schistosomiasis is prevented through avoiding contact with contaminated water. Contaminated water for bathing should be boiled first (1 minute) or kept in a storage tank (1-2 days). Vigorously towel drying after accidental contact with contaminated water might remove parasites before they penetrate the skin. (Citation: 1, 39)