Liver fluke – A growing threat to UK livestock production

The liver fluke, Fasciola hepatica, is becoming increasingly common in cattle and sheep in the UK. The suitability of different methods for detection of liver fluke infection depends on the type of animal, the purpose of the investigation (individual or herd level) and the stage of the liver fluke life-cycle. Treatment options are also influenced by the type of animal (beef, dairy, sheep) and the fluke’s life-cycle and by regulatory issues that determine availability of effective products. To some extent, forecasting of fluke risk at regional or herd level is possible based on weather conditions and farm management. Official guidelines for evaluation of treatment efficacy do not exist, but several methods have been proposed based on reduction in faecal egg counts or coproantigen secretion. Detection of liver fluke infection and treatment efficacy may be compromised by the presence of rumen fluke. The predominant rumen fluke species in the UK has been shown to be Calicophoron daubneyi. This rumen fluke shares an intermediate snail host, Galba truncatula, with the liver fluke, which contributes to the risk of mixed infections. In this paper, we review the respective flukes’ lifecycles, their impact on production and health, and diagnostic and control options in cattle and sheep. FASCIOLOSIS AS AN EMERGING DISEASE An emerging disease is one that has appeared in a population for the first time, or that may have existed previously but is rapidly increasing in incidence or geographic range. Based on this World Health Organisation (WHO) definition, liver fluke disease or fasciolosis would be classed as an emerging disease in the UK. Liver fluke has been present in the UK for decades, but the incidence and geographic range of this trematode parasite has increased rapidly in the last 10 years, with a disastrous liver fluke season in winter 2012/2013. An important factor in the emergence of liver fluke is its life-cycle (Figure 1). Unlike roundworms, which have direct life-cycles (host-pasture-host), most flatworms have life-cycles that involve intermediate snail hosts. In the case of the liver fluke in the UK, the preferred snail host is a tiny mud snail, Galba truncatula (formerly Lymnaea truncatula), although other snails serve as the main intermediate host in other countries. It is worth understanding the liver fluke’s complicated life-cycle because it has implications for the increase in disease occurrence, for diagnostics, treatment options and herd or flock management. Adult liver fluke live in the host’s bile duct system where they produce massive numbers of eggs, up to 50,000 each a day, which are excreted via the host’s faeces. In water, the eggs become embryonated and miracidia hatch in 2 to 4 weeks depending on prevailing temperature and rainfall. The miracidia are motile and have a “battery life” of about 3 hours, long enough to allow them to find a host snail in wet environmental conditions. The miracidia penetrate the snail where they go through several life stages, which may lead to shedding of an average of ca. 240 cercariae per snail over a period of approximately 4 months (Dreyfuss and Rondelaud 1994). The cercariae have a tail, which enables them to move through wet environments until they have found an appropriate plant, where they attach, lose their tail, and encyst. The cyst stage is very hardy and can survive in dry and cold conditions. There are anectodal reports of the presence of viable cysts on hay and in silage, but the biggest risk factor by far would be the ingestion of fresh herbage with viable liver fluke cysts. After ingestion, juvenile liver fluke excyst in the duodenum and start their migration through the gut wall and peritoneal cavity towards the liver, where they work their way through the parenchyma to the bile ducts. Here, the liver fluke mature and several months after ingestion of the infective cysts, they start to lay eggs and the life-cycle resumes. Because a large part of the liver fluke’s life cycle takes place in the outside environment, it is highly dependent on temperature and moisture. This explains why the recent spate of wet summers and relatively mild winters have supported the expansion of the host