Figure 1. Young adult liver fluke (marked x) on cut surface of liver. Image courtesy of Bruce Watt Central Tablelands LLS
Figure 2. Adult liver fluke showing mouth at top with ventral attachment sucker below (damage to specimen visible at lower left). Image courtesy of Mukund Madhav
Figure 3. Pickled liver flukes. Image courtesy of Mukund Madhav
Liver fluke (Fasciola hepatica) is a parasite affecting a range of livestock and often goes undiagnosed because it shows no obvious signs or symptoms while having a hidden impact.
Final hosts in which it can develop to sexual maturity include livestock such as sheep, cattle, horses, pigs, goats, alpacas and deer. Other species include kangaroos, wallabies, rabbits, and humans. Millions of sheep, goats and cattle graze pastures where liver fluke is endemic, mainly in south-eastern Australia.
Liver fluke disease in cattle is not as acute as it is in sheep although it can become very important in some districts such as the irrigation districts that provide good habitat for fluke snails. Flukes are leaf shaped parasites with a flattened body, 20 to 30 mm long and seven to 14 mm broad.
The economic importance of this parasite is partly due to the downgrading of livers affected by flukes during meat inspection procedures. Failure of young stock to thrive, and significantly decreased milk production in dairy cattle are unrealised costs in endemic areas where infestations may be heavy.
Liver fluke is a zoonotic – it infects man!
Humans are infected by eating contaminated watercress and other salad greens harvested from waterways and creek banks. The metacercarial cysts of the liver fluke are very tiny and not readily visible to the naked eye. Human infection is very rare, and treatable.
Liver fluke occurs in regions and areas on individual farms where watery environments, such as springs, slow-moving streams with marshy banks, irrigation channels and seepages, suit hatching of fluke eggs into free-swimming larvae, and most importantly, also provide suitable habitat for freshwater ‘fluke’ snails.
Liver fluke is limited to the higher rainfall (>600 mm per year) areas of NSW (typically the tablelands in the eastern part of the state, and nearby coastal areas to the east and slopes to the west), Victoria and Tasmania, to small areas of coastal and subcoastal south east Queensland, and small areas of South Australia. It does not occur in Western Australia (quarantine requirements are in place to prevent its introduction).
Adult liver flukes are located in the bile ducts of the liver of cattle, and sometimes in the gall bladder. Immature (pre-adult) flukes are found tunnelling (migrating) through the liver.
These trematodes have a two-host indirect life cycle with adult flukes in cattle, and the intermediate stages in freshwater lymnaeid ‘fluke’ snails.
Adult fluke in bile ducts produce eggs that move with the bile into the intestines and then the dung. Eggs released onto wet areas, once free of dung, hatch when mean daily temperatures are over 10°C. The released free-swimming miracidium has 24-30 hours to locate the freshwater fluke snail otherwise it dies.
Once inside the snail, the miracidium transforms and multiplies through several more stages. After 4-7 weeks, tadpole-like cercariae leave the snail, swim around, and drop their tails as they encyst on vegetation to produce the metacercarial cysts infective to sheep, goats, cattle, other species. Infection of a snail with one miracidium can produce over 600 metacercariae.
Grazing animals ingest the cysts with the vegetation. Immature flukes once released in the small intestine penetrate the intestinal wall, make their way to the liver, and then migrate through the liver tissue for 6–7 weeks before entering the bile ducts to become adults. Adult fluke can live for several years and produce over 20,000 eggs per day. The pre-patent period is 8–10 weeks after infection.
The ‘fluke’ snail most commonly found to transmit liver fluke in Australia is the indigenous Austropeplea (Lymnaea) tomentosa (Figure 4), which has characteristic broad triangular shaped, ear-like tentacles. Introduced snails Pseudosuccinea columella, and Radix viridis are commonly found in warmer coastal areas and have been identified as potential fluke hosts.
Cattle often feed in wet areas that suit both the snail intermediate host and the infective stage of the fluke. Metacercariae are very resistant to drying out and can remain a source of infection for many months during dry weather. Cattle become infected when they graze any remaining green pick after other pastures have died off. Snails escape the dehydrating effects of dry weather by burying into muddy patches and becoming dormant until rains return.
Figure 4. The liver fluke intermediate snail host, Astropeplea tomentosa, has a shell that spirals clockwise and flat, triangular shaped tentacles. Image courtesy of Mark Blacket
Liver fluke disease
Infections have been associated with three types of liver disease in domestic animals:
Sub-acute disease causes jaundice, ill thrift, anaemia, and possibly death after several weeks. Trauma to the liver by the migrating immature fluke predisposes to black disease infection (a bacteria that produces toxins in the liver).
Chronic disease is the most common form of the disease in cattle and is the result of ingestion of metacercariae, usually in small numbers over many months. It is associated with adult fluke in the bile duct. Typically, there is a gradual loss of condition with progressive signs such as weakness, anaemia and lethargy, loss of appetite, and occasionally ‘bottle jaw’ (submandibular oedema) becoming apparent. Cattle develop distended and thickened bile ducts which later calcify to produce what looks like a branching system of clay pipes and referred to as ‘pipe stem livers’ which are thought to indicate protection against reinfection. Some flukes can become encapsulated in the liver tissues and develop cysts, 2-3 cm in diameter. Small adult flukes are not infrequently found in the lungs in semi-caseous nodular lesions.
In beef cattle, clinical signs are seen in yearlings when sufficient numbers of adult fluke have established, or when infections of immature fluke continue in the presence of a substantial burden of adult fluke in the bile ducts.
In dairy cattle, the most common symptom is a loss of body condition after calving and a significant fall in milk production. Affected cattle have rough coats which become pale in breeds, such as Herefords, that have red coats.
Black disease is an acute and fatal liver disease of ruminants such as sheep, goats and cattle, which is caused by the bacterium Clostridium novyi, usually in association with traumatic injury caused by young fluke migrating through the liver. It is preventable by vaccination with a multi-valent clostridial vaccine (e.g. ‘5 in 1’).
Treatment and prevention
Flukicide drenches i.e. anthelmintics effective against liver fluke, are used to treat infection.
Control is by a combination of strategic flukicide treatment and pasture management. As ‘flukey’ areas are confined to certain parts of a farm, grazing these areas can be managed or even precluded. For example, grazing affected areas by the most vulnerable stock (sheep, goats and young cattle) can be minimised.
Strategic treatments can help to reduce liver fluke populations. One to three treatments may be needed per year, depending on the severity of the problem. The most important treatment is the April–May treatment, and a highly effective flukicide (one based on triclabendazole or clorsulon) should be used. The annual program section provides liver fluke treatment advice for different regions around Australia.
Regular monitoring should be undertaken. Testing options include:
Tropical liver fluke
Fasciola gigantica is closely related to Fasciola hepatica and is one of the most important infections of ruminants including buffalo, in restricted areas of southern Asia, south east Asia and Africa. The adult fluke is similar in shape to adult F. hepatica but is longer and narrower. The life cycle is also similar to that of F. hepatica except that most stages take longer to complete. Radix auricularia is the intermediate snail host. The pre-patent period is 9–16 weeks.
The tropical liver fluke is zoonotic – it infects man!
Humans are infected accidentally by ingesting contaminated watercress or water containing encysted infective metacercariae.
This parasite has not been found in Australia!
Figure 5. Tropical liver fluke, Fasciola gigantica, adult. Image courtesy of Constantin Constantinoiu