Common Liver Fluke, Fasciola hepatica

The common liver fluke (Fasciola hepatica), also known as the sheep liver fluke, is a parasitic flatworm in the Trematoda class. This species can infect sheep, cattle, humans, and other animals across the world. This species is one of the largest of its kind, reaching an average body length of 1.1 inches, with a width of up to .5 inches. This species is shaped like a worm and is typically wider at the front end, although some individuals have wider back ends. The front end holds a cone like extension, under which the oral sucker can be found. The acetabulum of this species is larger than its oral sucker, but is also located near the front of its body. Its body, known as a tegument, is covered with large spines that resemble spines.

The life cycle of the common liver fluke begins when an adult lays its eggs inside of a definitive host, or main host. These eggs are released from the host’s body in feces and then fall into the water where the feces is dumped. If the water is warm, the eggs develop into miracidia and hatch nine to ten days after entering the water, but they only have one day to find an intermediate or secondary host to enter in order to develop further and if the water s too cold, the eggs may not hatch. The intermediate hosts are freshwater snails of the family Lymnaeidae, including Radix rubiginosa, Lymnaea neotropica, and Lymnaea cousin, and many others.  Once the miracidia have entered a snail, they can develop into “mother” sporocysts, which then develop into the “daughter” rediae. These rediae develop into small larvae known as cercariae that swim through the small pools and drinking water of ruminants in fields. The larvae undergo one more development within these waters, turning into metacercariae that the ruminants then consume. Humans occasionally contract the worm at this stage as well, when eating undercooked foods such as the watercress plant.

Once the common liver fluke larvae have entered a definitive host, it can begin the process of maturing into an adult fluke. However, if the host has a low PH balance within its stomach, the larvae will become dormant. When the metacercariae enters the first part of the small intestine, also known as the duodenum, the larvae will detach and travel into the peritoneal cavity. Although the larvae are developing at this stage, they will not eat until they reach the liver parenchyma. After spending one day in this area, the larvae will begin to feed on the tissue of the liver. This stage is known as the pathogenic stage and may cause the first symptom of infection, anemia, in the host. After about six weeks of feeding and growing in the liver, the larvae will travel into the bile duct where they will mature into adult flukes that can reproduce. If an infection is small, the flukes will lay around 25,000 eggs in a day, but if the infection is large, each adult fluke can lay up to 500,000 eggs each day.

The symptoms of a common liver fluke include atrophy of liver parenchyma, reduced liver functions, jaundice, anemia, gall bladder damage, weight loss, submandibular oedema, and occasionally diarrhea. It is common for ruminants like sheep and cows in the United Kingdom to contract diseases from this worm.

In order to diagnose an infection of the common liver fluke a stool sample must be taken. However, because the eggs that appear in the sample can also occur from a human consuming infected liver, a clear diagnose can only be made if the patient has not consumed liver. Another test, known as enzyme linked immunosorbent assay or ELISA, can be obtained commercially. The ELISA test is able to detect anti-hepatica antibodies within a serum or milk. However, experts are researching this type of test using feces in order to make the testing process more efficient. Infections of this species, known as fascioliasis, can be cured in both humans and livestock by using many types of medicines. These include rafoxanid and triclabendazole, a type of anthelmintic that is most often used for treating this type of infection. Despite this, the common liver fluke has shown signs of resisting the treatment in Ireland and Australia.

Image Caption: Adult of Fasciola hepatica. Credit: Flukeman/Wikipedia (CC BY-SA 3.0)