Liver fluke – diagnosis, treatment and control

Introduction

Liver fluke (Fasciola hepatica)affects a range of livestock and other species; it is the most important trematode of domestic ruminants. The trematode group of flatworms live in several hosts during their life cycles. Suitable hosts for F. hepatica are alpacas, cattle, deer, goats, horses, pigs and sheep, as well as a range of other species including kangaroos, rabbits, wallabies and humans.

Penny has worked extensively in private mixed practice, and in animal biosecurity and welfarefor both government and not-for-profit organisations. She manages a 212 hectare Dorper sheep breeding property in NSW,and is co-Director ofYour Hobby FarmSuccess, helping hobby farmers enjoy their lifestyle, and reap the benefitsof having healthy and happy animals.

Large numbers of sheep and cattle inAustralia graze on pastures where liver fluke is endemic; infection costs millions of dollars each year due to reduced animal productivity, stock deaths and costs of treatment and prevention. Much of the economic cost can be associated with production losses from infections that may not be apparent. Fasciolosis also results in condemnation of infected livers in abattoirs.1

The eggs hatch in wet areas on pasture when the mean daily temperature is over 10oC. The small larvae (miracidia) swim around until they find a small water snail (the intermediate host), which are lymnaeid snails. In Australia, the most common is the indigenous freswater snail L. tomentosa;it is about 6to 12 mm long and the shell has a clockwise or ‘right hand’ thread when viewed from the point to the base of the snail.

An introduced North American snail (L. columella)and an introduced snail from the Pacific area (L.viridis), which have been found in defined locations on the NSW coast, have also been identified as intermediate hosts.2

The larvae (single: miracidium) enter the snails where they develop and multiply as sporocycts, rediae and cercariae. One single miracidium

After grazing animals ingest metacercariae, the immature liver flukes are released into the small intestine, penetrating the intestinal wall as they make their way to the liver. Migration through the hepatic tissues occurs for 6to 7 weeks before the larvae enter the bile ducts to become adults. Egg production starts about 8 to 10 weeks after infection. Adult liver flukes can live for several years in the liver of infected sheep and produce between 20,000 and 50,000 eggs per day.4 In cattle, egg production declines as natural resistance to chronic infection develops.

The epidemiology of fasciolosis is influenced by the grazing habits of animals. Cattle often graze in wet marshy areas inhabited by the liver fluke snail, while sheep and goats prefer to graze away from such areas if food is available elsewhere. Long wet seasons are usually associated with

Distribution

Liver fluke is limited to higher rainfall (>600 mm per year) areas of New South Wales (NSW); especially in the south-east (the tablelands and nearby coastal regions to the east, and slopes to the west), Victoria and Tasmania, and to small areas in Queensland and South Australia.

It occurs in areas on properties where there is suitable habitat for the eggs, the intermediate host snails (Lymnea tomentosa)and the larval liver fluke, such as springs, slow-moving creeks, swampy areas, water courses, irrigated pastures and seepages (e.g. leaking water troughs). Affected properties may have a historyof liver fluke.

Epidemiology2, 3, 4 hatching from a liver fluke egg can produce up to 4000 infective cysts (metacercariae).

The adult liver fluke is pale brown, flat and leaf-shaped. It is 2 to 3 cm long and has tiny sharp spines that irritate the hepatic tissue of the final host. The adult liver fluke residing in the bile ducts produces eggs which pass into the intestine and are excreted in the faeces. The eggs are a golden brown colour, oval and operculated (130–150 x 65–90 µm).

The tiny tadpole-shaped cercariae then leave the snails, swimming until they attach to vegetation (usually grass) where they encyst again into a resistant cyst stage (metacercariae), which is the infective stage of liver fluke. When adequate moisture is present, the metacercariae will remain alive for many weeks; they can survive longer at or below 20oC. The inter-mediate host snails produce eggs throughout the year and these eggs hatch when the temperature is suitable; usually spring to late autumn. Snails can produce 3000 eggs a month and one generation of snails from egg to egg takes about one month under optimum conditions. L. tomentosa survives in drymud for at least one year, and tolerates low temperatures; it can move with and against water currents.

The larval stages of liver fluke (sporocysts, rediae) also survive in snails for long periods, and will resume development when climatic conditions improve.2 ahigher infection rate but sheep are more likely to ingest large numbers of cysts during dry periods after a wet season, when they are forced to graze in wet areas.2,3, 4

Clinical signs and diagnosis

2,3, 4 Infection with liver fluke can produce sub-acute, acute and chronic fasciolosis depending on the size of the infection and how quickly it is acquired. Disease is due to haemorrhage and hepatic damage from migrating immature liver fluke, and from damage to bile ducts and blood loss from the adult liver fluke. In NSW, acute fasciolosis is most commonly seen in sheep and usually occurs from late summer to late autumn, while chronic disease is more common in cattle mostly occurring from autumn to spring. Bottle jaw, pale gums and eye membranes, weight loss, ill-thrift and diarrhea may be reported.

The clinical signs for acute fasciolosis range from no obvious signs to death, with or without sudden weakness, colic, jaundice and anaemia.

The history may indicate the possibility of a very high but relatively short-term intake of metacercariae.