Liver Fluke

Liver fluke is found on the tablelands and much of the coastal areas of eastern NSW. In wet years, the distribution of liver fluke can spread westwards. The parasite infects a range of mammal hosts and causes liver damage and blood loss. The result of the infection ranges from initially undetected production losses (reduced growth rates, wool growth and fertility) to observable clinical signs such as ill-thrift, “bottle jaw”, pale or yellow membranes in mouth and eyes, to death. Liver fluke control requires an understanding of the complex lifecycle of the liver fluke parasite, including the intermediate host snail’s habitat requirements.

The liver fluke parasite lifecycle

Liver fluke eggs develop through a complex lifecycle, via an intermediate host snail, eventually becoming infective cysts that are then eaten by the mammal host. In Australian agriculture our most important host species are cattle, sheep, goats and alpacas but a range of other animals can carry and spread the fluke parasite including deer, pigs, goats, kangaroos and wallabies, and rabbits. The broad host range which includes wild and feral animals rules out the complete eradication of liver fluke.

Liver fluke eggs are shed in the dung of the host animal onto pasture. These eggs are extremely robust and can survive on pasture for several months. Once deposited on pasture the eggs await appropriate conditions – adequate moisture and temperatures above 10◦C. They develop for several weeks with their development more rapid in warmer weather. Once hatched, larvae swim through water to find an intermediate host, a freshwater snail called a lymnaeid snail. The larvae can survive only a few hours between hatching from the egg and burrowing into a snail. Once within the snail the larvae multiply. This takes from one to three months, again developing faster in warmer weather. Now grown into a tadpole-like shape, the larvae erupt in their hundreds from the snail. They swim to vegetation and attach, then form a tough protective casing. This is the infective stage, the “infective cyst”.

A host animal eats the infective cysts as it grazes normally. Once inside the intestine of the host, the immature liver fluke hatch from their tough casing and burrow through the intestinal wall towards the liver. For six to eight weeks immature liver fluke migrate through the liver while feeding on the liver tissues and blood cells. This causes substantial damage. Production losses from this damage will be significant, even if clinical signs are not observed by the livestock manager.

Eventually the liver fluke enter the bile ducts - small tubes in the liver that carry liquid bile from the liver to the gallbladder and then to the intestine. Once in the ducts they double in size, become mature and start laying eggs that travel from the liver in the bile and are shed in the dung of the host.

The intermediate host snail

The intermediate host snail is a freshwater snail that lives in slow flowing or still waters at the edge of springs, small creeks, dam inflows and outflows, irrigation channels, poorly drained drainage channels and water trough surrounds. The snails prefer waters that are rich in nutrients and support dense herbage growth. These areas are often referred to as “flukey”.

The snails reproduce by laying about 3000 eggs per month. It takes approximately one month for an egg to develop through to an adult snail able to lay the next generation of eggs. In winter, the reproduction rate is much slower and in very cold conditions the snails hibernate. The snail can move long distances, even against the water current, meaning it can spread rapidly.

In periods of hot and dry conditions, the snails can remain dormant in dry mud and quickly return to normal activity when wet conditions return.

Talk to the Natural Resource Management team at your nearest Local Land Services office about funding opportunities for keeping stock out of waterways.

Examples of "flukey" habitat. Slow flowing or still waters, rich nutrients that support dense herbage growth. 

Factors that can be targeted for live fluke control

• Liver fluke eggs require adequate moisture to develop
• When liver fluke eggs hatch, the larvae only survive a few hours if they don't find an intermediate host snail
• When eggs hatch, larvae must swim to find the intermediate host snail
• The intermediate host snail lives in water and has a recognisable preferred habitat
• Snails hibernate in winter, and the fluke development inside them is halted during hibernation
• After erupting from the snail, tadpole-like larval live fluke must swim to vegetation in order to attach to it
• To become infected, the host animal must ingest the infective cyst by eating the herbage on which it is attached.

Factors that make live fluke difficult to control

• Live fluke eggs can survive on pasture for several months
• Wild or feral animals such as deer, pigs, kangaroos and rabbits can carry and spread live fluke
• The intermediate host snail is endemic in our area
• The intermediate host snail can reproduce quickly and in high numbers
• The intermediate host snail can move long distances and spread rapidly
• The intermediate host snail can remain dormant in mud in dry conditions and re-emerge when habitat becomes wet again
• Tadpole-like larval liver fluke erupt in high numbers (hundreds to thousands) from a single snail
• Fluke cysts on herbage can remain infective for months if conditions are cool and damp
• The microclimate of a wet and shady habitat may remain cool in warm weather allowing for infective cysts to survive
• Immature liver fluke are not susceptible to many drench chemicals
• Immature live fluke migrate through the liver for 6 - 8 weeks before reaching the adult stage that is more susceptible to drench chemicals
• clinical signs of chronic disease may only become obvious after severe liver damage has occurred
• Adult liver fluke can live for many years inside some hosts.

Live fluke is found in environments where the conditions support the survival of fluke eggs, the host snail, and fluke larvae. If larvae hatching from eggs deposited on pasture are unable to swim to the snail, and livestock never eat the herbage on which infective cysts are attached, the lifecycle of the parasite is broken. Therefore, the most effective method of control is to exclude livestock from grazing flukey habitat. In other words, the best way to manage liver fluke is with a stockproof fence.

Other strategies

In practice, fencing out flukey habitat takes time and money and will be part of a longer-term whole farm plan. Also, the feed available in flukey habitat may be needed, especially where it provides high quality feed when there is not much else on the farm. So other management strategies must come into play.

Grazing with less susceptible stock

Minimise grazing of flukey habitat by the most vulnerable livestock: young cattle, sheep, goats and alpacas. After being infected by liver fluke, adult cattle develop a resistance to subsequent infection due to liver scarring (fibrosis) that forms a physical barrier to fluke migration. The cattle survive and do not show overt disease. Their livers are large, and they retain enough undamaged liver to function. Note that liver function may be permanently reduced in some cattle with long term reduced productivity.

Sheep, goats and alpacas don’t develop resistance to liver fluke so can suffer at any age. Alpacas are thought to be particularly susceptible to liver fluke, perhaps as their liver is small and therefore damage is more rapidly overwhelming. Alpacas are also prone to spread of bacteria, such as e. coli, from the damaged liver to the heart, which can cause endocarditis and heart failure.

Chemical drenches to reduce infection levels

Strategic chemical treatments are used to reduce the liver fluke population, but they cannot be used alone to eliminate liver fluke in areas where it is common. Chemical drenches are used once to three times per year depending on how badly properties are affected and how susceptible the livestock are. In south eastern Australia, the treatment given in early winter (April/May) is the most important as this represents the time of highest liver fluke levels. An early spring treatment and a summer treatment can also be given. Speak to an advisor or your local District Veterinarian to discuss requirements for chemical treatments on your farm.

Grazing rotation with chemical drenches

If flukey areas must be grazed, consider giving an effective fluke drench two weeks before moving stock into that area. This prevents stock from shedding fluke eggs while grazing there. Try to restrict this grazing period to less than six weeks, moving stock back to non-flukey country before they are shedding eggs in their dung again.

Quarantine drenching

When buying in livestock, treat them with a combination drench effective against all life stages of liver fluke, then hold them in non-flukey paddocks for 21 days before testing for liver fluke to ensure the treatment was effective. Speak to your advisor or local District Veterinarian about testing options.

Chemical drenches

At the early winter drench in April/May, animals that have grazed flukey areas through spring and summer will be host to early immature, immature and adult-aged liver flukes. A drench that eliminates all these life stages should be used.

Table 1: Efficiency of drugs available in Australia for the treatment of liver fluke in sheep and cattle (modified from Boray 2007).

Triclabendazole is the chemical of choice for the early winter drench for sheep and goats. Either triclabendazole, or a nitroxynil and clorsulon combination is the drench of choice for the early winter drench for cattle.

Overuse of chemical drenches should be avoided as resistance is likely to develop. To slow the development of resistance a different chemical should be used for the spring and summer treatments if they are required.

Drenching alpacas

While alpacas are particularly susceptible to liver fluke, and require chemical drenching just as other livestock do, no chemical products are currently registered for alpacas for treatment of liver fluke. Consult with your private veterinarian about options for an off-label authority. An off-label authority allows a registered veterinary chemical to be used in a manner that is not specified on the product label. Alpacas require different doses to those of sheep and cattle for the chemical to be effective. Private veterinarians are legally required to examine the alpacas once or twice per year to supply the off-label authority.

Recognising the disease

Liver fluke disease is the result of tissue damage, thickening of the bile ducts and blood loss. If very large numbers of immature liver fluke migrate at once, the tissue damage and blood loss will quickly overwhelm the host animal, causing rapid death.

This acute form of disease may be accompanied by signs of pain such as tucking up in the abdomen and reluctance to move, anaemia which is seen as pale membranes of the mouth and eyes, or jaundice which may cause the membranes or the skin to appear yellow.

Sudden death may also be the result of Black Disease in animals that are not up to date with clostridial vaccinations. The damage to the liver caused by migrating immature liver fluke can activate a bacterium called Clostridium novyi, which produces a fatal toxin.

If the immature liver fluke numbers are somewhat less, the clinical signs include anaemia, jaundice and failure to thrive, with animals potentially dying after several weeks.

In the chronic form of disease, clinical signs come on slowly, and a “bottle jaw” develops. This form of the disease is most typical in autumn and winter when animals have been eating infective cysts over the course of spring and summer. In sheep it can be difficult to distinguish chronic liver fluke disease from barber’s pole worm infection.

Bottle jaw in a Suffolk ewe (dashed line). Note that when pressed with the thumb, a temporary indentation remains in the spongey swelling (arrow).