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Liver Fluke – Fasciola Hepatica Disclaimer


Life-cycle of the liver fluke   Top
The life cycle of the Fasciola hepatica (liver fluke) is shown in Figure 1. The adult fluke, which live in the bile ducts, produce eggs which are passed in the faeces (a). When the eggs separate from the faecal material in wet areas, under optimal conditions they will hatch, releasing the larvae or miracidia (b). The miracidia invade the lymnaeid snails in which they develop and multiply as sporocyst, rediae and cercariae (c). The tadpole like cercariae leave the snails (d) and swim until they encyst on vegetation, forming metacercariae (e), which are the infective stage of the fluke.

The entire cycle of the liver fluke in the snails takes 2 - 3 months under favourable conditions in the field. If ingested by sheep, cattle or other hosts, including man (f), the metacercariae excyst in the small intestine and the released immature flukes penetrate the intestinal wall and enter the abdominal cavity. The young fluke penetrate the liver capsule and migrate through the liver tissue for 6 to 7 weeks before entering the bile ducts to become adult fluke (g). The fluke reach sexual maturity and commence egg production 8 to 10 weeks after infection.

Figure 1: Life Cycle of Fasciola Hepatica.



Epidemiology   Top
The two primary requirements for the establishment of liver fluke are a suitable snail (intermediate host) and a suitable environment. In Australia, Lymnaea tomentosa, an indigenous freshwater snail, is the most important intermediate host. Ideal situations for the environmental stages of the life cycle are slow-moving streams with marshy banks, irrigation channels, seepage areas, etc.

The eggs produced by adult fluke pass out in the host’s faeces into wet areas. They hatch when temperatures increase above 10° C (typically from mid-September to May). The larvae (miracidia) invade a suitable snail (Lymnaea spp) and develop and multiply. One miracidium hatching from a fluke egg can produce up to 4,000 infective cysts (metacercariae). These cysts attach to grass and other vegetation. The host is infected when this vegetation is ingested.

Adult fluke can survive for many years in the livers of infected sheep; the adult fluke lays between 20 000 and 50 000 eggs per day over a long period.

The rate of egg production is responsible for the degree of pasture contamination and thus greatly influences the epidemiology of the disease.

The epidemiology of the disease is also influenced by the grazing habits of the animals. Animals grazing in wet marshy areas, favoured by the fluke snail, are more likely to become infected. Typically, long, wet seasons are associated with a higher rate of infection. However, sheep are more likely to ingest large numbers of cysts during dry periods following a wet season. This is due to a reduction in available pasture, forcing the animals to graze in swampy areas or in areas where the water has receded, thus exposing them to vegetation heavily infected with metacercariae.

Fascioliasis: liver fluke disease   Top
Fascioliasis is the disease caused by liver fluke (Fasciola hepatica). Sheep and cattle are most commonly affected and sheep are more susceptible to the disease than cattle. Horses, deer and goats may also be infected by liver fluke; humans are also occasionally infected.

Acute fascioliasis   Top
Acute fascioliasis occurs as an outbreak of disease following a massive but relatively short-term intake of metacercariae. This high intake is often the result of certain seasonal and climatic conditions combined with a lack of appropriate fluke control measures. It typically occurs when stock are forced to graze in heavily contaminated wet areas as a result of over-stocking and/or drought. Animals suffering from acute fascioliasis may display no clinical signs prior to death; some may display abdominal pain and discomfort and may develop jaundice.

Death usually results from blood loss due to haemorrhage in the liver caused by the migration of the immature fluke through the liver.

Sub-acute fascioliasis   Top
Sub-acute fascioliasis is characterised by anaemia, jaundice and ill-thrift. The migrating fluke cause extensive tissue damage, haemorrhage and, in particular, liver damage. The result is severe anaemia, liver failure and death in 8-10 weeks.

Chronic fascioliasis   Top
Chronic fascioliasis is the most common clinical syndrome associated with liver fluke infection in sheep and cattle. It occurs when the parasites reach the hepatic bile ducts. The principle effects are bile duct obstruction, destruction of liver tissue, hepatic fibrosis (scarring) and anaemia.

The onset of clinical signs is slow. Animals become gradually anaemic and inappetent, as the adult fluke become active within the bile ducts; signs may include dependent oedema or swelling under the jaw (‘bottle jaw’). Affected animals are reluctant to travel. Death eventually occurs when anaemia becomes severe. Additional stress upon anaemic animals, such as droving, may lead to collapse and death. Cattle typically present with signs of weight loss, anaemia and chronic diarrhoea.

Chronic fascioliasis provides the right environment in the liver for the germination of the spores of the bacterium, Clostridium novyi Type B, which cause Black disease. Vaccines against C.novyi are available and should be given to all animals, particularly those at risk of fascioliasis.

Diagnosis   Top
Fascioliasis should be considered when there are cases of death, anaemia, chronic diarrhoea or ill thrift in sheep or cattle grazing on fluke-prone country.

In the live animal, chronic fascioliasis is indicated by the presence of fluke eggs in a faecal sample (faecal samples may be sent to a veterinary laboratory for diagnosis).

A dead animal can be quickly diagnosed by the presence of mature or immature fluke in the liver; the autopsy will also identify any other conditions that may be contributing to the problem.

Treatment   Top
The treatment recommended will depend upon the nature of disease. Some of the available anthelmintics are not effective against immature fluke and so are not recommended in acute fluke outbreaks. In addition, they are less efficient for the strategic control of fascioliasis.

At least 2 treatments annually with Flukare S or C are required on sheep and beef properties where liver fluke occurs. The initial dose should be administered in spring to remove any fluke that have been present in the animals during the winter. This is followed by a second dose in early summer to remove fluke picked up during the spring. A third treatment, in autumn, may be required on properties heavily infested with liver fluke.

Closicomb is registered for use in sheep and it is a suitable rotational product to be used with Flukare S, to delay the onset of resistance. Closicomb will kill a high proportion of fluke aged 4 weeks and is highly effective against fluke aged 6 weeks or older. The very early immature fluke will be retarded in their development and egg production will not commence for several weeks, thus reducing the level of pasture contamination.

Ask your veterinarian to develop a programme that is specific and appropriate for your property.

For further information please read the NSW Agriculture factsheet
on liver fluke disease in sheep and cattle.
 

Information supplied by Virbac (Australia) Pty Limited
For further assistance please ring 1800 242 100



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