Dicrocoelium dendriticum

All unapproved Citizendium articles may contain errors of fact, bias, grammar etc. A version of an article is unapproved unless it is marked as citable with a dedicated green template at the top of the page, as in this version of the 'Biology' article. Citable articles are intended to be of reasonably high quality. The participants in the Citizendium project make no representations about the reliability of Citizendium articles or, generally, their suitability for any purpose. This article is currently being developed as part of an Eduzendium student project in the framework of a course entitled Microbiology 201 at Queens College, CUNY. The course homepage can be found at CZ:Biol 201: General Microbiology. For the course duration, the article is closed to outside editing. Of course you can always leave comments on the discussion page. The anticipated date of course completion is May 21, 2009. One month after that date at the latest, this notice shall be removed. Besides, many other Citizendium articles welcome your collaboration!

Dicrocoelium dendriticum
Scientific classification
Kingdom: Animalia Phylum: Platyhelminthes Class: Trematoda Order: Plagiorchiida Family: Dicrocoeliidae Genus: Dicrocoelium Species: D. dendriticum
Binomial name
Dicrocoelium dendriticum Rudolphi, 1819

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 ‘Selfish Memes’ section below).

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 (Citation 3).

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 (Citations 1, 2, and 3). 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 Citation 2. 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 (Citation 2).

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 second intermediate hosts (Citation 3). Adult flukes in the bile ducts, lay dark brown eggs each containing a miracidium, which are expelled with the bile into the intestine and incorporated in the feces. These are viable embryos 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) Citation 2. The hatching mericidia penetrate the glandular intestinal epithelium and then undergo several rounds of asexual replication into daughter sporocycts Citation 1 and 2. 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 Citation 2, and 3. 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 Citation 1, 2, and 3. Slimeballs stick to nearby plants and debris, each snail usually produces one but can produce more Citation 3. 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 (Citation 2) one or more may become localized in the subesophageal ganglion, which alters the normal neurological processes resulting in changes in normal behavior of the ant, when temperatures are low (Citation 1 ). In this parasite mediated hijacked state the ant climbs to the tips on blades of grass where they exposed to grazing mammals such as sheep, goats and cows Citation. Finally, the metacercariae are in the gut of the now infected definitive host where they are encysted in the duodenum (citation 2). The larva excyst and migrate to the bile ducts and then the gall bladder. In the bile ducts is where they will develop into cross fertilizing and hermaphroditic adult flukes, capable of releasing new eggs into the environment, in the host excrement Citations 1,2 and 3.

References

References 1. Otranto, Domenic Dicrocoeliosis of ruminants: a little known fluke disease 2. Desmond, James introduction to animal parisitology p. 214 Figure 14.14 3. 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 II. Collection of the snail, Cionella lubrica, and its maintenance in the laboratory." Cornell Vet 41 (1951): 433-44.

Links

1. http://www.fao.org/wairdocs/ILRI/x5492E/x5492e04.htm 2. http://www.weichtiere.at/Mollusks/Schnecken/parasitismus/dicrocoelium.html