What is liver fluke?
The common liver fluke (Fasciola hepatica) is a parasitic trematode (flat worm) that infects sheep and cattle in the UK. Liver fluke is also a zoonotic disease which means it can infect people, although this is more of a problem in developing countries. Adult flukes have a flat leaf-like body which measures 2-3cm in length and are found in the bile ducts of the liver. They have an oral sucker and a ventral sucker, as well as spines on their surface, to help them attach to the tissue of their host. They are also hermaphrodites and contain both male and female reproductive organs in the one body.
Life cycle of liver fluke
Liver fluke has a complex two stage life cycle. It can affect a wide range of mammalian hosts, but in the UK is a particular risk to sheep and cattle. Infective cysts are ingested during grazing and develop into juvenile fluke which then migrate from the gut through the liver to the bile ducts where they mature. Adult fluke lay eggs which are passed in dung where they develop and hatch before infecting an intermediate host. In the UK this is the mud snail (Galba truncatula), which is found in wet muddy conditions and areas with poor drainage. The fluke develops in the snail, taking 6-8 weeks before the infective stages are shed and encyst on the pasture. The life cycle of both parasite and snail are dependent on temperatures above 10 degrees centigrade and water (rainfall). The incidence of fluke disease has been increasing over the past ten years. The reasons for this are multifactorial but recent wetter summers probably play a role, providing ideal conditions for the snails and the parasite to reproduce.
Disease caused by liver fluke
Disease due to liver fluke is known as fasciolosis. Disease can be acute or chronic.
Acute fasciolosis, normally seen in sheep, is caused by large numbers of juvenile fluke migrating through the liver. These cause extensive haemorrhage and damage to the liver parenchyma. Animals are typically weak, and anaemic, often with palpably large livers, abdominal pain, ascites and sudden death is common.
Chronic fasciolosis occurs in both sheep and cattle and occurs several months after moderate intake of infective cysts. Chronic disease is associated with adult fluke in the bile ducts. Anaemia, loss of appetite and gradual weight loss are common clinical signs. Infection also has an impact on fertility, growth rates and milk production.
Diagnosis and treatment
There are various laboratory tests available for the diagnosis of fasciolosis but there are currently no tests specifically for acute fluke infection in sheep.
The main method of treatment is the use of anthelmintic drugs, with triclabendazole being the most widely used in sheep because it is effective against the juvenile parasites and thus acute disease. However, due to the lack of diagnostic tests for acute infection in sheep, farmers have no choice but to repeatedly treat their animals with triclabendazole to prevent losses which has contributed to the development of drug resistance. Relying solely on drug treatment to control of liver fluke is not sustainable in the long term.
A vaccine would be the most effective way to control fasciolosis however there is currently no licensed vaccine available against Fasciola hepatica.
The impact and growing problem of fluke
Liver fluke infections are on the increase in the UK and are responsible for considerable economic losses, due to direct production losses, poor reproductive performance and infected livers condemned at slaughter. Conservative estimates of the cost to the cattle industry is currently £300 million per annum. Perhaps more importantly, liver fluke is also a welfare problem so it is important that measures are taken to protect sheep and cattle from disease.