Among flukes of veterinary importance the closest relatives of F. hepatica are F. gigantica of cattle and sheep in several parts of world (but not Canada), Fascioloides magna of free-ranging ruminants and domestic animals in North America and Europe, and Fasciolosis buski of people and pigs in Asia. The other liver fluke of ruminants in North America, and other parts of the world, is Dicrocoelium dendriticum.
Eggs of F. hepatica are oval, measure approximately 140 µ by 75 µ, and have a thin, smooth shell and an operculum (lid) at one end. When passed, each egg contains a clump of cells. It is fairly difficult to distinguish the eggs of F. hepatica from those of F. magna, but both are very different from those of D. dendriticum.
Host range and geographic distribution
Life cycle - indirect
The hermaphrodite adult flukes live in the biliary system. Eggs are passed in the feces. In an aquatic or semi-aquatic environment, a miracidium (first larval stage) develops in each egg, which then hatches. The miracidium penetrates a suitable snail intermediate host (most commonly a species of the genus Lymnaea), in which a sporocyst, then rediae, then cercariae develop. The cercariae leave the snail and encyst on vegetation as metacercaria. The development outside the mammalian host takes four to seven weeks, depending on environmental conditions, and means that F. hepatica can increase its numbers in both the definitive and intermediate hosts.
Infection of the definitive host is by ingestion of vegetation with metacercariae. The immature flukes penetrate the intestinal wall and cross the peritoneal cavity to the liver, enter the liver and migrate through the parenchyma to the bile ducts. The pre-patent period is approximately eight weeks. Rarely the flukes are located in unusual sites, particularly the lungs.
Life Cycle: Fasciola hepatica
Pathology and clinical signs
Treatment and control
The only drug approved in this country for Fasciola hepatica in cattle, but not sheep, is albendazole (VALBAZEN).Additional information on the product mentioned above is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.
Another product, triclabendazole (FASINEX), is available for extra-label use by Emergency Drug Release from the Veterinary Drugs Directorate of Health Canada.
There are no products approved in Canada for F. hepatica in sheep and, as the parasite apparently does not occur here, no demand for treatments
Other than in areas of Quebec, control of F. hepatica in cattle in Canada is not an issue. In other parts of the world where the parasite is a significant threat to animal and sometimes human health, treatment of the definitive hosts, grazing management and control of the intermediate hosts are the major means by which control is attempted.