Dicrocoelium dendriticum

Description and significance
Dicrocoelium dendriticum a small liver fluke, is a parasitic organism. That is, it benefits from its relationship to the host while contributing nothing to the survival of that host such that it in most cases may lower host fitness. Theory and mechanisms on parasite manipulation of host fitness is a current topic of much controversy (see ‘Parasitic Manipulation’ section below).Dicrocoeliosis is a globally present parasitic infection caused by Dicroceolium, which infect the bile ducts and gall bladder of wild and domestic animals. Since it is not as pathogenic as other flukes very little is known about this parasite. Moreover, since it has two intermediate hosts Dicrocoeliosis is difficult to reproduce under experimental conditions.

Morphology
Dorso-ventrally flattned (lance shaped), adult lancet flukes are semi transparent ~8-14mm in length and ~2-3mm in width, oval shaped with the anterior slightly more narrow in shape compared to its posterior, both ends being slightly tapered (Citation 1, 2 ). At its tip the anterior contains an oral sucker. A hermaphrodite, the adult lancet fluke contains two lobed testes, in its anterior region, juxtaposed to its ovary, and the uterus lies below in its midsection. Vitellaria glands flank its reproductive organs, and are important in egg production. The digestive components (gut and bladder) lie in the posterior portion of its body

Parasitic Reservoirs: inside and outside the Definitive Host
Dicrocoelium dendriticum’s  habitat includes lowland or mountain pastures, of dry and alkaline consistency, providing appropriate conditions for its definitive and intermediate hosts. The adult lancet fluke necessarily inhabits the liver of its definitive host, specifically the bile ducts and gall bladder of domesticated and wild ruminating animals (sheep, goat and cattle). Although, they can also be found in the liver of dogs, rabbits, horses, humans and some rodents. Endogenous host and environmental conditions are critical for its survival, whereby each intermediate host must be favored in the collective biotope, making it difficult to model similar conditions in the laboratory. Since its continuance into the liver of its definitive host for sexual reproduction is dependent on prior passage through two other distinct host organisms, conditions must favor each part of this biological community. Once an adult, it uses its hermaphroditic body plan to propagate itself several orders of magnitude, but to get their the fluke must be ingested by specific intermediate hosts, which allow it to carry out the different stages of its life cycle.

Life Cycle: A Definitive Host and Two Intermediate Hosts
A trifecta of successful host:parasite adaptation, the small liver flukes’ life cycle and mode of transmission have been well described by the work of Krull and Mapes. Collectively their work showed that infection of the definitive host is initiated by ingestion of infected ants and cannot be bypassed by eating the slimeballs of infected snails, the first intermediate host. Adult flukes in the bile ducts, lay dark brown eggs each containing a miracidium, which are expelled with the bile into the intestine and later incorporated into the feces. These eggs are viable embryos, which are only hatched upon ingestion by the appropriate species of snail, Cionella lubrica in North America (There are many other possible host snail species depending on geographical location i.e. In France, Cochlicella acuta). The hatching mericidia penetrate the glandular intestinal epithelium and then undergo several rounds of asexual replication into daughter sporocysts. These daughter sporocysts mature into motile larva, called mature cercariae and travel to the snails’ respiratory chambers, a process that can take 5 months depending on season or age of the snail. The cercariae contain glands which are speculated to be involved in the formation of slimeballs. Several ~500-5000 cercariae are collected in these slime accumulations but little is known about the mechanism of their formation. Slimeballs stick to nearby plants and debris, each snail usually produces one but can produce more. These slimeballs are ingested by a specific species of ant, in North America Formica fusca (Again, several species are capable of being intermediate host, differences being dependent on location). The larva then transform into metacercariae which grow in the abdomen of the ant. After a period of about a month, at the expense of its own survival, a single metacercariae will become localized in the subesophageal ganglion, which results in changes in normal behavior of the ant, when temperatures are low. In this parasite adaptive state the infected ant climbs to the tips on blades of grass where they are exposed to grazing mammals such as sheep, goats and cows. Finally, the metacercariae have arrived at their destination in the definitive host, where they are encysted in the duodenum. The larva excyst and migrate to the bile ducts and then the gall bladder. In the bile ducts they develop into cross fertilizing and hermaphroditic adult flukes, capable of releasing new eggs into the environment, in the host excrement.

Pathology
Overall, the small liver fluke has little identifiable clinical manifestations in infected hosts, compared other liver flukes i.e. Fasciola. Even in heavily infected hosts they are assymptomatic and often suffering from anaemia, oedema or emaciation. Also, the pathogenic effects of D. dendriticum can become confounded by other simultaneous infections with other nematodes. Still some have been able to detect its effect on the liver. In this case sheep liver was examined post mortum which appeared to have heavy scarring on its surface, hardening due to calcification and fibrosis, and displayed disruption of the bile ducts as a result of irritation caused by the presence of the small liver flukes.

Links
1.	http://www.fao.org/wairdocs/ILRI/x5492E/x5492e04.htm

2.	http://www.weichtiere.at/Mollusks/Schnecken/parasitismus/dicrocoelium.html