Summary

The trematode Dicrocoelium dendriticum occurs in cattle, sheep and other ungulates in many parts of the world, including western Canada. The adult parasites live in the gall bladder and bile ducts. The life cycle is indirect involving a snail first intermediate host, in whch cercariae develop in accumulations of mucus (slime balls) which are extruded by the snail. For the life cycle to continue the slime balls must be ingested by an ant second intermediate host, in which the infective metacercariae develop. Infection of the definitive host is by ingestion of infected ants. Significant pathology associated with D. dendriticum in sheep and cattle is believed to occur only very rarely and is usually associated with very large numbers of parasites. Dicrocoelium dendriticum has very rarely been reported in people co-infected with HIV.

Taxonomy

Phylum: Platyhelminthes
Class: Trematoda
Subclass: Digenea
Order: Plagiorchiformes
Family: Dicrocoeliidae

Other genera within the Family include the liver flukes Platynosomum (in cats in several areas of the world but not Canada), and Eurytrema (in ruminants in Asia and South America).

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.

Morphology

Adult D. dendriticum are small and lancet-shaped and measure up to approximately 10 mm by 2.5 mm. The oral and ventral suckers, and many of the internal structures, particularly elements of the alimentary and reproductive systems, can easily be seen microscopically in fixed, stained specimens.

Eggs of D. dendriticum are approximately 40 µm in length, and not quite oval with a thick, yellow-brown, smooth shell and an operculum. When passed each contains a miracidium. Eggs of D. dendriticum are very different from those of the other flukes of ruminants in North America, F. hepatica and F. magna, which are very similar to each other.

Host range and geographic distribution

Dicrocoelium dendriticum occurs in a wide range of domestic and free-ranging mammals in various parts of the world. In Canada, the parasite has been reported from cattle and sheep in Alberta, especially in the Cypress Hills where it also infects elk, and in British Columbia.

Life cycle - indirect

The hermaphrodite adult D. dendriticum live in the biliary system. Eggs are passed in the feces and are swallowed by a suitable snail first intermediate host. Egg hatching occurs in the gut of the snail, and the miracidium released from each egg penetrates into the tissues of the snail and develops through two generations of sporocysts (but not rediae) and then cercariae. Cercarial development requires approximately three months.

The cercariae emerge from the snail in "slime balls" (each containing 200-400 cercariae) released from the respiratory pore of the snail and are eaten by an ant second intermediate host, in which the metacercariae develop and encyst. This phase of the life cycle requires up to two months. The metacercaria(e) in the ant’s nerve ganglion alter its behaviour – using their mouthparts, they attach to the upper portions of pasture grasses and other vegetation during the night and are then available to grazing animals early in the day, thereby enhancing transmission to the definitive host. Following ingestion of the ants by a definitive host, the metacercariae develop into immature flukes which migrate directly from the intestinal lumen into the biliary system. The pre-patent period is approximately eight weeks. Thus Dicrocoelium dendriticum can increase its numbers in its mammalian and molluscan, but not arthropod, hosts.

Life Cycle: Dicrocoelium dendriticum

Epidemiology

The key features of the epidemiology of D. dendriticum are the long periods required for parasite development in the two intermediate hosts, and the long survival periods – several months - for the eggs, even in freezing temperatures. Also, in the Cypress Hills of southwestern Saskatchewan and southeastern Alberta, D. dendriticum is common in wapiti (elk), which can act as reservoir hosts for sympatric cattle, in which the fluke also occurs.

Pathology and clinical signs

Generally, D. dendriticum is considered non-pathogenic, but non-specific clinical signs (anemia, oedema and emaciation) have been reported. There are reports from southern Saskatchewan and southern Alberta of liver lesions in cattle associated with D. dendriticum, but their clinical and production significance is unclear. Sheep with large parasite burdens and long-standing infections may show distension of the bile ducts and hepatic fibrosis with marked distortion of the liver surface.

Diagnosis

Detection of eggs in feces using a flotation technique is helpful (specific gravity 1.30 to 1.45), or a post mortem examination. Sedimentation techniques appear to have a very low sensitivity for D. dendriticum. The eggs of D. dendriticum are very different from those of F. hepatica and F. magna.

Treatment and control

There are no products approved in Canada for the treatment of Dicrocoelium dendriticum and, because of the usually low pathogenicity of the parasite, treatment and control are not usually an issue.

Public health significance

Human infections with D. dendriticum are reported occasionally, particularly in people who are immunosuppressed, for example as a result of HIV/AIDS.

References

Goater C et al. (2007) Epidemiological characteristics of an invading parasite: Dicrocoelium dendriticum in sympatric wapiti and beef cattle in Southren Alberta, Canada. Journal of Parasitology 93: 491-494.

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