Dicrocoelium dendriticum
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| Lancet liver fluke | |
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| Species: | D. dendriticum
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| Dicrocoelium dendriticum (Rudolphi, 1819)
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The Lancet liver fluke (Dicrocoelium dendriticum) is a parasite fluke that tends to live in cattle or other grazing mammals.[1]
History of discovery
Much of what is presently known about D. dendriticum is the result of the work of the naturalist Wendell Krull.[2] 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.[3] Next came the discovery that the slime balls coughed up by the snails could be a potential method of transfer of the parasite.[3] Shortly thereafter, the ant, Formica fusca was found to be the second intermediate host by which sheep were infected.[3] 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, causing an enlarged liver (hepatomegaly) or inflammation of the liver (cirrhosis) [citation needed]. In one unique case, an infection with D. dendriticum was associated with a skin rash urticaria [citation needed].
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.[4]
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
D. 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 Miracidium) 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[5]), 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.[6] There, the fluke takes control of the ant's actions by manipulating these nerves.[citation needed] 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.[7][unreliable source?][8][unreliable source?][9]
Diagnostic tests
Traditionally, diagnosis for dicrocoeliasis infection involves the identification of D. dendriticum eggs in the faeces 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[citation needed].
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[citation needed].
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 [citation needed]. 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[citation needed]. Treatment with triclobendazole has also shown to be effective[citation needed]. One treatment that has proven to be successful in both humans, sheep and goats was 2 capsules of Mirazid (300 mg) for six days[citation needed].
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.
References
- ^ "The Merckt Veterinary Manual". Merck & Co., Inc. 2008. Retrieved 2008-07-03.
- ^ Esch, Gerald. Parasites and Infectious Disease Discovery by Serendipity and Otherwise (Parasites and Infectious Disease). New York: Cambridge UP, 2007.
- ^ a b c Krull, W. H., and C. R. Mapes. "Studies on the biology of Dicrocoelium dendriticum (Rudolphi, 1819) Looss, 1899 (Trematoda: Dicrocoeliidae), including its relation to the intermediate host, Cionella lubrica (Müller). II. Collection of the snail, Cionella lubrica, and its maintenance in the laboratory." Cornell Vet 41 (1951): 433-44. Cite error: The named reference "Krull" was defined multiple times with different content (see the help page).
- ^ Drabick et al. Dicroceliasis (lancet fluke disease) in an HIV seropositive man. JAMA 1988; 259(4):567-8.
- ^ "Dicrocoelium dendriticum is a bile duct fluke of ruminants such as sheep". California University of Pennsylvania. Retrieved 2009-01-09.
- ^ "University of Alberta Parasites Lab". University of Alberta. Retrieved 2008-11-07.
- ^ "Dicrocoelium Dendriticum, the Liver Fluke". The Exile. December 12, 2003. Retrieved 2008-07-03.
- ^ Callahan, Gerald N. (November 24, 2002). "Infectious Madness: Disease with a Past and a Purpose: Mental illness may not be just craziness, but have a parasitic, fungal, or viral etiology". Emergency Medicine News. Retrieved 2008-07-03.
- ^ Holmes, Bob (November 6, 1993). "Evolution's neglected superstars: There is nothing glamorous about fleas, flukes or intestinal worms. So why are they suddenly attracting so much attention?". NewScientist. Retrieved 2008-07-03.