The liver fluke, Fasciola hepatica, would be financially devastating to the livestock industries if it became established in Western Australia, as eradication would be almost impossible. The need to control fluke infections would significantly increase the cost and complexity of routine worm control on a large proportion of the state’s sheep, cattle, goat and camelid properties.
Fascioliasis is a disease caused by liver fluke in the liver and bile ducts of sheep and cattle, although sheep are more susceptible to the disease than cattle. Horses, deer and goats may also harbour liver fluke and humans can potentially be infected.
The liver fluke is a flat, leaf-like parasite with a complex lifecycle. A liver fluke burden can result in reduced animal production, illthrift, anaemia and jaundice, and in extreme cases, stock deaths. If flukes are detected in livers at the abattoir, the livers are condemned as unsuitable for human consumption.
Adult flukes are pale brown or greyish-brown in colour and when mature vary from 15 to 40 millimetres (mm) in length and up to 12mm in width. The flukes live in the bile ducts of the host where they lay large numbers of eggs. The eggs are then passed down the bile ducts and enter the intestine to eventually be excreted with the faeces.
Under favourable circumstances — temperature conditions and moisture — the eggs hatch into larvae (miracidia) which invade an intermediate host (an aquatic snail). Miracida must reach the host snail within 24 to 30 hours, and need temperatures above 5°C to survive. The larval stages multiply within the snail and after five to eight weeks (depending on the temperature), minute tadpole-like larvae (cercaria) emerge from the snail. These form cysts (metacercaria) on herbage, where they can survive for some months providing there is moisture and temperatures are not excessive. Metacercaria are unlikely to survive through typical WA summer conditions.
After ingestion of infested pasture by a host animal, the young flukes penetrate the animal’s intestinal wall and enter the peritoneal cavity. From there they penetrate the liver where they mature and cause damage as they move about. The damage may be sufficiently severe to cause haemorrhage and liver function failure, which results in death within a few weeks to a few months (acute fascioliasis). The adult flukes may also cause a more chronic form of the disease due to lower-grade liver damage and blockage of the bile ducts, leading to anaemia, jaundice, ‘bottlejaw’ and significantly impaired animal production. Liver damage due to fluke infections may also make host animals susceptible to the acute and fatal Black Disease, caused by the bacterium Clostridium novyi. Liver fluke are extremely long-lived, often for the length of the host’s lifetime.
Liver fluke is common throughout the world, including in Australia (other than in WA, the Northern Territory and parts of South Australia and Queensland). However, even on infected properties, transmission between stock is confined to areas of moisture where the snails occur and metacercaria develop and survive. Control measures include preventing access to fluke-risk areas.
Sheep and goats are more susceptible than cattle to fascioliasis, while horses and pigs may become infected with flukes but rarely show disease. Wildlife such as kangaroos and rabbits can also carry the flukes, making eradication very difficult.
Tests to diagnose liver fluke infection include the demonstration of the eggs in faecal samples from host animals, or antibody tests on blood or faecal samples. Several anthelmintics (drenches) are available for the treatment of liver fluke, of varying effectiveness against the adult and immature stages. Resistance has developed to the main flukicide, triclabendazole, but so far this appears to be restricted to a small number of properties.
The intermediate snail host
The snail, Pseudosuccinea columella is one of the several possible intermediate hosts for the liver fluke and the only one widespread in Western Australia. It is an aquatic species, found only in fresh water. P. columella has colonised waterways as far south as the Vasse River in Busselton and can be found in most irrigation channels in the South-West, which has been designated a ‘restricted area’. This encompasses coastal shires north to Dandaragan, east to Esperance, and inland to Northam and Manjimup.
P. columella is a small snail: fully grown it is about 11mm long with a dark grey body. When viewed from the top, it has a clockwise coil. When active, the snail projects through an aperture which is more than half the length of the shell, which is plugged with the soft foot withdrawn (there is no hard operculum). The tentacles on the head are fleshy, flat and triangular-shaped rather than filamentous, as with native aquatic snails.
The snails are found in and beside lakes, ponds, streams, drainage ditches and seepage areas. It prefers sunny, open, shallow pools where there is an abundance of aquatic plant growth and algae on which to feed. On cool, sunny days the snail can be found on the waterline or even a few centimetres out of the water, as well as on the bank. However, it also thrives in deeper waters with grassy edges. The snails can survive Australian winter temperatures, and also survive through drought conditions, beneath the mud. This snail has an extremely high reproductive rate.
The snail is believed to have colonised waterways in WA relatively recently, and was first detected in the early 1970s in Perth. Fortunately, measures to ensure imported carrier animals are certified fluke-free were introduced before transmission in domestic livestock began.
Quarantine restrictions on imported animals
Restrictions to prevent the introduction of liver fluke into WA apply to all potential carrier animals sent to liver fluke restricted areas (all ruminants, camelids and horses, other than those sent for slaughter or from areas declared exempt). These must be tested to show they are free of liver fluke prior to consignment and receive a treatment on entry to WA, and are then subject to a quarantine order until further treatment and negative testing is confirmed.
The stock owner or agent must seek and receive approval from a Biosecurity Inspector of the Department of Primary Industries and Regional Development (DPIRD), ensuring the appropriate protocols are followed and importation forms accompany the consignment. Current details of interstate livestock movement requirements can be provided by Quarantine WA on email at QWALivestockImports@agric.wa.gov.au or by phone +61 (0)8 9334 1800.
(Restricted areas include the cities, shires and localities of Albany, Armadale, Augusta-Margaret River, Bridgetown-Greenbushes, Bunbury, Busselton, Capel, Chittering, Collie, Dandaragan, Dardanup, Denmark, Donnybrook-Balingup, Esperance, Gingin, Harvey, Jerramungup, Kalamunda, Mandurah, Manjimup, Mundaring, Murray, Nannup, Northam, Perth Metropolitan Area, Ravensthorpe, Rockingham, Serpentine-Jarrahdale, Swan, Toodyay, Wanneroo, Waroona.)