Fasciola hepatica or Liver Fluke is a flatworm that parasitises a wide range of hosts including sheep, cattle, horses, rabbits, wallabies and even humans. The adults live in the bile duct and feed on blood, which can lead to chronic disease and impairment of performance, but greater impacts are usually seen from the damage caused by the immature flukes developing within the liver.
The Liver Fluke has an indirect life-cycle that includes a snail intermediate host in addition to the final mammalian host. Adult worms lay up to 50 000 eggs each day. The eggs travel with the bile into the intestine and are released into the environment with the host animal’s faeces. If an egg is deposited in a suitably wet area, a swimming larva (miracidium) will develop and hatch out (after about 21 days in ideal conditions). The miracidium searches for a suitable snail host (only certain species within the family Lymnaeidae). The larva penetrates the snail and a series of three asexual reproductive cycles is completed before up to 4000 larvae (cercariae) are released. These larvae swim away from the snail and encyst on vegetation just below the water surface. This encysted stage (metacercaria) is infective to livestock and can remain viable for many weeks when conditions are suitable.
Following ingestion by a suitable host, the parasite burrows through the wall of the gut and into the liver. It migrates around the liver for about seven weeks as it grows and then emerges into the bile duct where it matures, mates and starts laying eggs about eight to ten weeks after ingestion. The worms can potentially survive for about a year in adult cattle and up to eight or nine years in sheep.
Environmental conditions play a major role in determining the number of infective larvae to which stock are exposed. Throughout the colder part of the year (when minimum temperatures fall below 10°C), the snails bury into the mud and become inactive, preventing the release of fluke larvae. Very cold weather (below freezing) will also kill many of the encysted larvae that are already on the vegetation.
Conversely, snails emerging with the warm weather can release large numbers of larvae that have built up over the time that the snails were buried, resulting in massive contamination of wet areas over a short period.
Climatic conditions can strongly influence the level of fluke infection in livestock. In dry times, the snails and infective larvae are concentrated in areas where livestock congregate to consume the remaining green feed, increasing the infection rate. In very dry times, however, water sources dry out, forcing the snails to seek shelter in the damp mud below ground level and preventing the release of the infective larvae into the environment. In extended periods of drought the snail population can be reduced dramatically, suppressing the parasite population for some time even after normal seasons have returned.
The distribution of the liver fluke is largely defined by the distribution of suitable snail intermediate hosts. The dominant snail host is a native species (Austropeplea tomentosa), which is found throughout the wetter areas of South-eastern Australia. Two exotic species of snail (Austropeplea viridis and Pseudosuccinea columella) are also now established in Australia and provide potential for the parasite to spread into tropical areas.
Western Australia is presently free of liver fluke despite the presence of suitable snail species. Stringent animal import regulations are enforced by the WA Department of Agriculture to maintain the state’s fluke-free status.
The impact of Liver Fluke infections is closely linked to the number of larvae ingested by the animal and can be divided broadly into three forms of disease. Acute disease occurs when large numbers of larvae invade the liver at the same time. The resulting massive liver damage and blood loss can cause healthy looking animals to die abruptly. The subacute form results from a moderate intake of larvae. The liver damage is not sufficient to kill animals quickly, but severe illness (anaemia, weight loss, lethargy) is obvious. The chronic form of the disease occurs where animals pick up low numbers of larvae over an extended time period. The scarring and fibrosis in the liver resulting from the damage caused by the larvae and blood loss associated with the feeding of adult worms in the bile ducts leads to animals that develop chronic anaemia and protein deficiency, and perform poorly. The damage to the liver can be permanent and animals may underperform for life.
As for many parasitic diseases, more production losses result from uncontrolled subclinical disease than from clinical outbreaks. Subclinical infections can result in weight gains suppressed by 10-20%, reduced milk production and quality, reduced wool growth and quality, delayed puberty and reduced fertility, and condemnation of livers at slaughter.
Stock at risk
Sheep of all ages suffer particularly badly, often developing the acute and subacute forms of the disease. Cattle rarely suffer the acute form of the disease except when young, but often develop the chronic form of the disease. Horses quickly develop immunity and rarely show any clinical signs of infection.
Liver Fluke is best detected by post-mortem examination of the animal’s liver and bile ducts, which is easily achieved where carcase feedback can be obtained from the meatworks. Indirect methods that detect fluke eggs or antibodies can also be used even where clinical signs such as anaemia (pale gums and mucous membranes) or protein deficiency (bottle jaw, pot belly, rough coat) are absent. Egg counts for flukes are reliable indicators of infection levels in sheep (except in acute cases), but flukes in cattle shed fewer eggs, making interpretation of results more difficult. Any positive egg counts may be significant for both sheep and cattle; counts of >50 in sheep or >25 in cattle are considered high. Blood or milk samples from both sheep and cattle can be examined for the presence of antibodies to the flukes, but test results provide no indication of the number of flukes present. Bulk milk samples can provide an indication of infection rates across dairy herds. Care must be taken when interpreting results following treatment because the antibodies take about three months to decay after the worms have been removed.
It is rarely practical to eradicate liver fluke from a property because of the difficulty in controlling snail populations. Effective control of liver fluke is best achieved using an integrated approach that combines pasture management (isolation or drainage of wet areas, use of wet paddocks for less susceptible stock, strategic paddock rotation) with strategic chemical control. Treatment programmes should be designed around the level of challenge (infective stages in the environment) as well as the number of parasites in the animals. Treatments at the beginning and end of the cold season are usually required to suppress fluke populations, but in many areas additional treatments are needed to maintain optimal control.
Products that contain triclabendazole and kill all three stages of fluke offer the best treatment options.
Many products contain active ingredients that only kill adult liver flukes (e.g. clorsulon).
Contact one of our friendly team to discuss the best product to help control Liver Fluke in your stock.