Summary

The trematode Fasciola hepatica (the common liver fluke of ruminants) infects cattle and sheep, and less commonly a range of domestic and wildlife species, around the world. Currently in Canada the parasite occurs only in cattle and is restricted to dairy cattle around Quebec City and an isolated and inexplicable focus in central Alberta. The life cycle is indirect and requires a snail intermediate host in which the cercariae develop. These leave the snail and encyst as infective metacercariae on aquatic vegetation. Infection of the definitive host is by ingestion of this vegetation. While some animals with this liver fluke show few if any ill effects, in many parts of the world F. hepatica is a major pathogen of cattle and sheep. In cattle disease is associated primarily with the adult flukes in the biliary system, where they cause chronic inflammation which eventually interferes with hepatic function, as well as blood loss. Sheep also develop a similar chronic disease, but more important in this host is acute fascioliasis caused by immature pre-adult flukes migrating through the liver parenchyma. Acute fluke disease can be associated with notable morbidity and mortality. In some cases the fluke infection, especially in sheep, is complicated by the bacterium Clostridium novyi, the causative agent of "black disease" In some areas of the world where fascioliasis is endemic in ruminants, wildlife are significant reservoirs for the parasite. Fasciola hepatica infection is increasingly recognized as an important zoonosis in many parts of the world, not including Canada, with major impacts on human health..

Taxonomy

Phylum: Platyhelminthes
Class: Trematoda
Subclass: Digenea
Order: Echinostomatiformes
Family: Fasciolidae

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.

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 F. hepatica measure up to approximately 30 mm by 13 mm and have distinct "shoulders" immediately behind the cone-shaped anterior part of the body. The tegument of the fluke is covered with sharp spines. 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. As with all trematodes of veterinary importance in Canada, adult F. hepatica are hermaphrodite.

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

Fasciola hepatica occurs in cattle and sheep, and occasionally in other mammalian hosts, world-wide. In Canada, the fluke is apparently restricted to cattle around Quebec City, and in a small focus in central Alberta. The situation in cattle in British Columbia and Manitoba, and in sheep across the country, is uncertain. Fasciola hepatica is widespread in the southern and western US.

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

Epidemiology

The epidemiology of F. hepatica in cattle and sheep is strongly influenced by the effects of the environment, particularly temperature, on the development of the fluke in the molluscan intermediate hosts and on the ecology of the molluscs. In the UK, the more common infection of snails during the summer can produce disease in the fall and winter, while winter infection of snails can produce losses in the late summer and fall. Also in the UK, the likely severity of fluke disease can be forecast using meteorological data.

Pathology and clinical signs

Although low levels of infection may not be an apparent problem, in endemic areas of the world F. hepatica is a major cause of economic loss to cattle producers, both beef and dairy, and to the sheep industry. In cattle the disease is usually chronic – associated with adult flukes in the biliary system and the resulting hepatic fibrosis - and causes production losses as well as clinical disease. In sheep, liver fluke disease can be either acute – associated with the damage caused by the immature flukes migrating through the liver parenchyma – or chronic. In Canada, F. hepatica is a problem in areas of Quebec, but in Alberta seems to be an incidental finding at post mortem. Acute F. hepatica infection may pre-dispose sheep to clostridial infection, particularly "black disease" (Clostridium novyi).

Diagnosis

History and clinical signs may be useful for the diagnosis of F. heptica. Egg detection in feces, for which a sedimentation technique rather than a flotation is used, has sub-optimal sensitivity. In North America, a simple sedimentation kit (FlukeFinder) was available. Also, in some endemic areas, a serological test is used to establish a herd diagnosis. This test is not available in Canada.

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.

Public health significance

In areas endemic for F. hepatica in animals, human infection occurs from time to time. Most human cases follow ingestion of watercress or other aquatic vegetables with metacercariae attached, harvested from a water body contaminated with cattle and/or sheep feces and harbouring suitable snails. A recently published retrospective study detected 616 human cases over a 28 year period in three departments of the Limousin region of France, where human F. hepatica is not notifiable. For 98% of the cases, watercress was the source of infection.

References

Rondelaud D et al. (2000) Changes in human fasciolosis in a temperate area: about some observations over a 28-year period in central France. Parasitology Research 86: 753-757.

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