''Dicrocoelium dendriticum'', the lancet liver fluke, is a
parasite
Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
fluke that tends to live in
cattle
Cattle (''Bos taurus'') are large, domesticated, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...
or other grazing mammals.
History of discovery
Much of what is presently known about ''Dicrocoelium 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 ''
Cochlicopa lubrica
''Cochlicopa lubrica'' is a species of small air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Cochlicopidae.MolluscaBase eds. (2020). MolluscaBase. Cochlicopa lubrica (O. F. Müller, 1774). Accessed through: World ...
'' (synonym: ''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
''Formica fusca'' is a black-colored ant commonly found throughout Europe as well as parts of southern Asia and Africa. It has the common names silky ant or dusky ant. The range within the palaearctic region extends from Portugal in the west to J ...
'' was found to be the second intermediate host by which sheep were infected.
Their work is the foundation of modern understanding of the parasite.
Clinical presentation in humans
''Dicrocoelium dendriticum'' along with ''
Dicrocoelium hospes
''Dicrocoelium hospes'' is a species of flatworms
Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, un ...
'' are part of a group of flukes 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 ''Dicrocoelium 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
Hepatomegaly is enlargement of the liver. It is a non-specific sign (medicine), medical sign, having many causes, which can broadly be broken down into infection, hepatic tumours, and metabolic disorder. Often, hepatomegaly presents as an abdomin ...
) or inflammation of the liver (
cirrhosis
Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, chronic liver failure or chronic hepatic failure and end-stage liver disease, is a chronic condition of the liver in which the normal functioning tissue, or parenchyma, is replaced ...
).
In one unique case, an infection with ''Dicrocoelium dendriticum'' was associated with a skin rash
urticaria
Hives, also known as urticaria, is a kind of skin rash with red or flesh-colored, raised, itchy bumps. Hives may burn or sting. The patches of rash may appear on different body parts, with variable duration from minutes to days, and typically ...
.
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 other herbivorous mammals and humans can also serve as definitive hosts through the ingestion of infected ants. One definitive case involved a man who ingested bottled water contaminated by infected ants.
Reservoirs
The main reservoirs for ''Dicrocoelium dendriticum'' are sheep, cows, land snails and ants. However, ''Dicrocoelium dendriticum'' has also been found in goats, pigs and even
llama
The llama (; or ) (''Lama glama'') is a domesticated South American camelid, widely used as a List of meat animals, meat and pack animal by Inca empire, Andean cultures since the pre-Columbian era.
Llamas are social animals and live with ...
s and
alpaca
The alpaca (''Lama pacos'') is a species of South American camelid mammal. Traditionally, alpacas were kept in herds that grazed on the level heights of the Andes of Southern Peru, Western Bolivia, Ecuador, and Northern Chile. More recentl ...
s.
In ruminants
Ruminants are the main definitive host of this fluke but other herbivorous animals, carnivores, and humans can be accidental definitive host.
Most infections, especially in cows, are asymptomatic but the effect on the liver depends on the number of flukes and the length of infection.
Since the fluke migrates up the biliary duct — but does not penetrate the gut wall or liver tissue — long infections may cause hypertrophy of the bile duct and liver lesion, even in the absence of symptoms. While infections with ''D. dendriticum'' are usually symptom free, some animals may show anemia, edema, emaciation, and liver cirrhosis. However, many of the symptoms of dicroceliosis are similar to those of other gastro-, intestinal-, and lung-nematode infections.
The diagnosis of ''D. dendriticum'' flukes is mainly from the recovery of adults in liver during necropsy or detecting eggs in animal feces.
There is some evidence connecting decreased liver function from the trematode infection with pregnancy toxaemia and mastitis in ewes when combined with other risk factors.
Treatment can be difficult due to the fluke's complex life-cycle. Various antihelminths, especially Netobimin, have been shown to be effective treatment when an entire herd is infected. Animal husbandry practices can decrease the incidence of infection. This includes the avoidance of animal grazing early in the day or late in the evening, when ants are more likely to climb to the top of the grass blade.
Incubation period
The incubation period for ''Dicrocoelium dendriticum'' is currently unknown.
Morphology
''Dicrocoelium dendriticum'' has a similar morphology to ''Clonorchis sinensis
''Clonorchis sinensis'', the Chinese liver fluke, is a liver fluke belonging to the class trematode, Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, ...
'', the Chinese liver fluke. ''Dicrocoelium dendriticum'' is distinguished by lobed testes in the anterior of the body, as opposed to ''Clonorchis 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 midsection lie the vitelline glands that are involved in egg formation.
Life cycle
''Dicrocoelium dendriticum'' spends its adult life
Life, also known as biota, refers to matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes. It is defined descriptively by the capacity for homeostasis, Structure#Biological, organisation, met ...
inside the liver
The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
of its host
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
* Host Island, in the Wilhelm Archipelago, Antarctica
People
* ...
. After mating, the eggs are excreted in the feces
Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...
.
The first intermediate host, the terrestrial snail
A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...
(''Cochlicopa lubrica
''Cochlicopa lubrica'' is a species of small air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Cochlicopidae.MolluscaBase eds. (2020). MolluscaBase. Cochlicopa lubrica (O. F. Müller, 1774). Accessed through: World ...
'' in the United States), consumes the feces, and becomes infected by the larva
A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
l parasites. The larvae (or miracidium
The miracidium is the second stage in the life cycle of trematodes. When trematode eggs are laid and come into contact with fresh water, they hatch and release miracidium. In this phase, miracidia are ciliated and free-swimming. This stage is comp ...
) drill through the wall of the gut and settle in its digestive tract, where they develop into a juvenile stage. The snail attempts to defend itself by walling the parasites off in cysts, which it then excretes and leaves behind in the grass or substrate.
The second intermediate host, an ant
Ants are Eusociality, eusocial insects of the Family (biology), family Formicidae and, along with the related wasps and bees, belong to the Taxonomy (biology), order Hymenoptera. Ants evolved from Vespoidea, vespoid wasp ancestors in the Cre ...
(''Formica fusca
''Formica fusca'' is a black-colored ant commonly found throughout Europe as well as parts of southern Asia and Africa. It has the common names silky ant or dusky ant. The range within the palaearctic region extends from Portugal in the west to J ...
'' in the United States), uses the trail of snail slime
Snail slime is a kind of mucus (an external bodily secretion) produced by snails, which are gastropod mollusks. Land snails and slugs both produce mucus, as does every other kind of gastropod, from marine, freshwater, and terrestrial habitats. T ...
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
Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two Sucker (zoology), suckers: one ventral and the other Mouth, oral. Trematodes are covered by a Tegument (helminth), tegument, that protects the o ...
, but one moves to the sub-esophageal ganglion
A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there are ...
(a cluster of nerve cell
A neuron (American English), neurone (British English), or nerve cell, is an excitable cell that fires electric signals called action potentials across a neural network in the nervous system. They are located in the nervous system and help to ...
s underneath the esophagus
The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...
). There, the fluke takes control of the ant's actions by manipulating these nerves. As evening approaches and the air cools, the infected 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 mandible
In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone i ...
s 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. 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.
Diagnostic tests
Traditionally, diagnosis for dicrocoeliasis infection involves the identification of ''Dicrocoelium dendriticum'' eggs in the faeces
Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...
of a human or other 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 combination with a liver-free diet.
In animals, diagnosis has traditionally involved stool examination or post-mortem examination of the liver. Recently, an ELISA
The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay is a solid-phase type of enzyme immunoassay (EIA) to detect the presence of ...
using a ''Dicrocoelium 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 ''Dicrocoelium dendriticum'' are so rare, there are multiple suggestions for treatment. The standard treatment is an anthelmintic
Anthelmintics or antihelminthics are a group of antiparasitic drugs that expel parasitic worms (helminths) and other internal parasites from the body by either stunning or killing them without causing significant damage to the host. They may also ...
such as Praziquantel
Praziquantel, sold under the brandname Biltricide among others, is a medication used to treat a number of types of parasitic worm infections in mammals, birds, amphibians, reptiles, and fish. In humans specifically, it is used to treat schist ...
, Triclabendazole
Triclabendazole, sold under the brand name Egaten among others, is a medication used to treat fascioliasis and paragonimiasis. It is very effective for both conditions. Treatment in hospital may be required. It is taken by mouth with typicall ...
, or Mirazid.
Epidemiology
Dicrocoeliasis is believed to be endemic or potentially endemic in 30 countries. ''Dicrocoelium 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.
In addition, a study completed in Sweden combining data about the ''Dicrocoelium dendriticum'' prevalence and landscape data to discover in which landscape the parasite thrives. It was found that grazing land near forest areas (good for mollusks) and dry pastures with little other biodiversity (good for ants) both increased parasite prevalence.
References
External links
Lancet Fluke (''Dicrocoelium lanceolatum'') at The Living World of Mollusks
Cartoon illustration of the cycle of ''Dicrocoelium dendriticum''.
{{Taxonbar, from=Q21053
Animals described in 1819
Plagiorchiida
Parasites of mammals
Bovine diseases
Suicide-inducing parasitism
Veterinary helminthology
Taxa named by Karl Rudolphi