Lancet Liver Fluke, Dicrocoelium dendriticum
The lancet liver fluke (Dicrocoelium dendriticum) is a parasitic worm that is classified within the Platyhelminthes phylum. It is thought to be native to over thirty countries including Switzerland, Spain, Germany, Iran, China, Vietnam, Japan, Ghana, and Nigeria, among many other areas. It is also found in South and North America and in Australia. It is typically found in cattle or other grazing species, so it is thought to prefer a habitat that supports these species. It is similar in appearance to the Chinese liver fluke, although its testes are lobe shaped and occur in the front of the body.
Rudolphi discovered the lancet liver fluke in 1819, at which point it was known to live in sheep, but there was no other information available about the species. It was not until after Loos discovered D. hospes in 1899 that both species could be properly studied and understood. Krull and C.R. Mapes published their findings about the lifecycle of the lancet liver fluke in many papers between the years of 1951 and 1953. Krull and Mapes discovered that the worm requires an intermediate host, specifically the land dwelling snail Cochlicopa lubrica. After the discovery of a possible transference method from the snail to a sheep, the pair discovered that the snail was actually the first of two intermediate hosts. These discoveries laid the groundwork for modern knowledge of the species.
The life cycle of the lancet liver fluke begins when the eggs are deposited in the feces of an intermediate host. A land snail, like Cochlicopa lubrica in the United States, consumes the feces and the larvae, which are in the miracidium stage. The larvae will penetrate the snail’s stomach lining and move into the digestive tract where they can mature into young flukes. The snail’s body reacts to the parasite by encasing the larvae in cysts, which are excreted onto the ground.
The second intermediate host for the lancet fluke is an ant, like Formica fusca in the United States. The ants, attracted by the moisture left behind in the snail trail, will consume a remaining cyst. The cyst can contain hundreds of young flukes that will travel to the digestive tract of the ant and mature into metacercariae larvae. Once a sheep or other grazing animal consumes the ant, which is moving about in grass, the larvae will mature into adult flukes that can reproduce, continuing the cycle of the species.
Because the lancet liver fluke is specialized to live in grazing animals, human infections are very rare. When this species does infect a human, it travels to the bile duct where it can feed off the bile and reproduce. Infections do not usually create large problems for human hosts, because the worms are long and will most often reside in an area of the bile tree that is farther away from the liver. As a result, the symptoms of an infection appear to be mild. These include abdominal issues like diarrhea and bloating. If the parasite is located in or close to liver, however, symptoms like inflammation of the liver or internal issues can occur.
Traditional diagnosis of a lancet liver fluke infection was conducted by looking at the feces of an infected animal or human. However, humans often ingest the raw or undercooked liver of an infected animal, producing a false positive result of infection. Modern techniques for diagnosis include examining the duodenal fluid or bile duct fluid for the presence of eggs. Diagnosis of animals has typically been done by examining stool samples, but modern methods like the ELISA test are becoming more popular due to efficiency and earlier detection.
Treating a lancet liver fluke infection in humans is relatively easy, despite its rarity. Medicines like Praziquantel and Triclabendazole, if used in the correct dosage for a defined period, are typically successful in getting rid of liver flukes. The drug Mirazid can also be used and has proven to be effective in sheep and goats, as well as humans. Preventing an infection in humans can be accomplished by avoiding uncooked liver. However, the infection can be contracted by other means, so the World Health Organization added it to a list of targeted creatures known as the Foodborne Disease Burden Epidemiology Reference Group in 2007.
Image Caption: Dicrocoelium dendriticum, from teaching slides at the University of Edinburgh. Credit: Adam Cuerden/Wikipedia