Dicrocoelium dendriticum
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The Lancet liver fluke (Dicrocoelium dendriticum) is a parasite fluke that tends to live in cattle or other grazing mammals.
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History of discovery
Much of what is presently known about D. dendriticum is the result of the work of the naturalist Wendell Krull. While D. dendriticum was discovered by Rudolphi in 1819 and D. hospes was discovered by Loos in 1899, the full life cycle was not known until Krull and C.R. Mapes published a series of papers from 1951-1953 detailing their observations and experiments. It was known that D. dendriticum affected sheep, but everything else was a mystery. The first link in the chain was the discovery of the first intermediate host, the land snail Cionella lubrica. Next came the discovery that the slime balls coughed up by the snails could be a potential method of transfer of the parasite. Shortly thereafter, the ant, Formica fusca was found to be the second intermediate host by which sheep were infected. The knowledge they generated is the foundation of our understanding of the parasite today.
Clinical presentation in humans
D. dendriticum along with D. hospes are part of a group of trematodes that can infect the bile ducts of humans. Because the bodies of these parasites are long and narrow, infections are generally confined to the more distal parts of the bile ducts. As a result most D. dendriticum infections of the biliary tree produce only mild symptoms. These symptoms can include biliary colic and general digestive disturbances, including bloating and diarrhea. However, in heavier infections, bile ducts and the biliary epithelium may become enlarged in addition to the generation of fibrous tissue surrounding the ducts, and as a result, hepatomegaly and inflammation of the liver (cirrhosis) may occur. In one unique case, an infection with D. dendriticum was associated with acute urticaria.
Transmission
Due to the highly specific nature of this parasite’s life cycle, human infections are generally rare. Ruminants such as cows and sheep are usually the definitive host, but humans and other herbivorous mammals can also serve as definitive hosts through ingestion of infected ants. One definitive case involved a man who ingested bottled water contaminated by infected ants.
Reservoirs
The main reservoirs for D. dendriticum are sheep, cows, land snails and ants. However, D. dendriticum has also been found in goats, pigs and even llamas and alpacas.
Vector
There is no currently identified vector for D. dendriticum.
Incubation Period
The incubation period for D. dendriticum is currently unknown.
Morphology
D. dendriticum has a similar morphology to Chlonorchis sinensis, the Chinese liver fluke. D. dendriticum is distinguished by lobed testes in the anterior of the body, as opposed to C. sinensis whose testes are located in the posterior. They both are flat and have a characteristic taper at the anterior and posterior ends. The anterior is distinguished by an oral sucker at the point, an acetabulum and the testes. The posterior is where the uterus lies. In the parasite’s mid-section lie the vitelline glands that are involved in egg formation.
Life cycle
Dicrocoelium dendriticum spends its adult life inside the liver of its host. After mating, the eggs are excreted in the feces.
The first intermediate host, the terrestrial snail (Cionella lubrica in the United States), eats the feces, and becomes infected by the larval parasites. The larvae (or cercariae) drill through the wall of the gut and settle in its digestive tract, where they develop into a juvenile stage. The snail tries to defend itself by walling the parasites off in cysts, which it then excretes and leaves behind in the grass.
The second intermediate host, an ant (Formica fusca in the United States), uses the trail of slime as a source of moisture. The ant then swallows a cyst loaded with hundreds of juvenile lancet flukes. The parasites enter the gut and then drift through its body. Most of the cercariae encyst in the haemocoel of the ant and mature into metacercariae, but one moves to the sub-esophageal ganglion (a cluster of nerve cells underneath the esophagus). Infected ants may contain 100 metacercariae, and a high percentage of ants may be infected. Typical infections in cattle may be in the tens of thousands of adult worms. There, the fluke takes control of the ant's actions by manipulating these nerves. As evening approaches and the air cools, the infested ant is drawn away from other members of the colony and upward to the top of a blade of grass. Once there, it clamps its mandibles onto the top of the blade and stays there until dawn. Afterward, it goes back to its normal activity at the ant colony. If the host ant were to be subjected to the heat of the direct sun, it would die along with the parasite. Night after night, the ant goes back to the top of a blade of grass until a grazing animal comes along and eats the blade, ingesting the ant along with it, thus putting lancet flukes back inside their host.
They live out their adult lives inside the animal, reproducing so that the cycle begins again.
Diagnostic tests
Traditionally, diagnosis for dicrocoeliasis infection involves the identification of D. dendriticum eggs in the feces of a human or animal. However, in humans, eggs in the stool may be a result of ingesting raw infected animal liver and may not in fact indicate dicrocoeliasis. Therefore, examining bile or duodenal fluid for eggs is a more accurate diagnostic technique.
In animals, diagnosis has traditionally involved stool examination or post-mortem examination of the liver. Recently, an ELISA using a D. dendriticum antigen was able to identify cases of dicrocoeliasis in sheep in Italy 28 days earlier than traditional methods.
Management and therapy
Because human infections with D. dendriticum are so rare, there are multiple suggestions for treatment. The standard treatment is with Praziquantel at 25 milligrams per kilogram three times a day for one day. In one German patient 600 mg of Praziquantel administered three times a day for three days also proved to be successful in eliminating the parasite from the body. Treatment with triclobendazole has also shown to be effective. One treatment that has proven to be successful in both humans, sheep and goats was 2 capsules of Mirazid (300 mg) for six days.
Epidemiology
Dicrocoeliasis is believed to be endemic or potentially endemic in 30 countries. D. dendriticum is found throughout Europe (former U.S.S.R., Switzerland, Italy, Germany, Spain, Turkey), the Middle East (Iran), Asia (China, Japan, Vietnam), Africa (Ghana, Nigeria, Sierra Leone) and in North and South America and Australia. The parasite tends to be found in areas that favor the intermediate hosts, such as fields with dry, chalky and alkaline soils.
Public health prevention strategies
Current public health prevention strategies have involved the condemnation of contaminated livers so as to eliminate any possibility for food-borne infection.
In addition, in 2007 the World Health Organization included Dicrocoelium dendriticum on its list of organisms to target with its Foodborne Disease Burden Epidemiology Reference Group.
