Definition. Fascioliasis is an infectious disease caused by Fasciola parasites. Two Fasciola species (types) infect people. The main species is Fasciola hepatica, which is also known as “the common liver fluke” and “the sheep liver fluke.” A related species, Fasciola gigantica, also can infect people. It is a common parasite, cosmopolitan in distribution.
Epidemiology. Fascioliasis is a waterborne and foodborne zoonotic disease caused by two parasites of class Trematoda, genus Fasciola; namely Fasciola hepatica and Fasciolagigantica. Fascioliasis is more common in livestock and other animals than in humans, because humans are incidental hosts and become infected by ingesting contaminated watercress or water. It is large (3 cm in length). Adult worms reside in the large biliary passages and gall bladder. The illness occurs worldwide, particularly in regions with intensive sheep or cattle production. Incidence of human infection has increased over the past 20 years. Although F. hepatica and F. gigantica are distinct species, “intermediate forms” that are thought to represent hybrids of the two species have been found in parts of Asia and Africa where both species are endemic. These forms usually have intermediate morphologic characteristics (e.g. overall size, proportions).
Life cycle and development. The adult is hermaphrodite, eggs are excreted into the bile and exit the host via the feces. The eggs are embryonated and take up to 15 days to mature in fresh water. A free-swimming ciliated miracidium hatches out of the egg. When the miracidium makes contact with an aquatic Lymnaeid mollusk, it penetrates the foot of the snail and migrates to the hepatopancreas (digestive gland) where future development takes place. Each miracidium contains the germ and somatic cells. The next generation, the sporocysts, develops from the germ cells. Germ cells within the sporocyst form the next larval phase, the redia. The cercariae develop from germ cells within the redia and escape from the snail via an exit pore. Cercariae at first are free-swimming and then settle onto vegetation and each encysts into a metacercaria which has a life span of 2–3 months. Development from egg to metacercaria takes about 4–7 weeks. Metacercariae when eaten by the definitive host (an herbivorous mammal) excyst in the duodenum releasing a juvenile stage. The juvenile stage penetrates the intestinal wall and migrates through the peritoneal cavity to the liver where it matures into an adult fluke. Adult flukes can produce 500–700 eggs per day, with more eggs being produced in the morning, which possibly hints at periodicity. The most common definitive host is sheep. However, a range of mammals including man can also serve as the definitive host.
Symptoms. The symptoms of this disease are loss of weight; accumulation of fluids in the abdomen due to liver damage; ‘bottle-jaw’ or collection of fluid in the lower jaw; diarrhea; lethargy followed by death.
Treatment. The drug of choice is triclabendazole. The drug is given by mouth, usually in two doses. Most people respond well to the treatment.
References:
Arora, H.S. and Steele, R.W. Fascioliasis [Updated: Jun 14, 2018]. Available from: https://emedicine.medscape.com/article/997890-overview#a4
Centers for Disease Control and Prevention. Fasciola. Available from: https://www.cdc.gov/parasites/fasciola/gen_info/faqs.html
Chernin, J. Parasitology – Lifeline (Modules in life sciences). CRC Press, London: 2000.
