Each year in Northern Ireland, the onset of autumn marks the beginning of the ‘fluke season’, with death and debility affecting sheep on farms across the province, as liver fluke burdens become established in animals that have grazed on pasture contaminated with the infective metacercarial cysts.
Liver fluke disease can occur in either the acute or chronic form. The acute form occurs in sheep and is caused by the migration of large numbers of immature flukes through the liver. Acute liver fluke is often a fatal disease and has serious welfare implications. Signs of severe infections include distended painful abdomen, anaemia and sudden death. Chronic liver fluke disease is more common than the acute form and occurs in both sheep and cattle, usually during the winter and spring although infection can persist throughout the year. The disease causes ill-thrift and under-production, and is a source of considerable unseen financial loss to the agricultural industry in Northern Ireland. Fluke infections can also predispose to metabolic conditions such as ketosis and infectious diseases such as salmonellosis and Black Disease. Recent evidence also suggests that fluke infection in cattle can interfere with the tuberculin skin test for TB.
Mild winter conditions combined with higher than average rainfall during the summer months create conditions favourable to the over-winter survival and multiplication of the snail intermediate host of the liver fluke. This situation, coupled with the on-going evolution of fluke resistance to the widely used flukicide, triclabendazole has led, in recent years, to an unusually high level of metacercariae on pasture from August onwards. Sheep die in autumn and early winter of acute fasciolosis, as large numbers of immature flukes burrow through the tissues of their liver.
While the majority of sheep and nearly all cattle survive the acute phase of infection, these animals progress into winter and spring with burdens of adult fluke established in the bile ducts, shedding eggs that exit to the environment in the hosts’ faeces. If the weather at this time is mild, large numbers of these eggs will survive to provide an early source of infection for snails as they emerge from hibernation and commence breeding in April and May.
Triclabendazole is the only commercially available flukicidal compound with the potential to destroy liver flukes from the earliest stages of infection, and so can be used successfully to treat animals affected with acute fasciolosis, as well as those harbouring chronic infections with egg-producing adult parasites. This wide-spectrum activity ensured the widespread use of triclabendazole from the time of its development in 1983. However in recent years, on premises where the drug has been used almost exclusively for a long period, local populations of fluke have evolved which are more or less resistant to the effects of triclabendazole. Surveys and field trials carried out by the Veterinary Sciences Division, AFBI, and Queen’s University, Belfast have established that tricalbendazole resistance is widespread on sheep, and probably also on cattle farms throughout NI.
The situation now facing sheep farms with suspected triclabendazole resistance is that, in the presence of high levels of fluke challenge from heavily contaminated pastures, no wholly-effective anthelmintic is available to them for prevention and treatment of acute infection. While on such farms, triclabendazole-containing products may be less effective than in previous years in treating acutely-ill animals, no other products are effective against the early immature flukes. So, triclabendazole should be considered for use in emergency situations in conjunction with a compound such as closantel that is effective against late immature and adult flukes, in an attempt to partially reduce total fluke burdens and save animals. In late summer and autumn, access to snail habitats (wet and poorly drained areas) should be restricted if possible, and sheep moved to new clean, drier pasture.
Unfortunately, on the basis of widely circulated anecdotal reports of triclabendazole resistance, many farmers have abandoned the use of triclabendazole altogether, and have opted for autumn treatment with flukicides inappropriate for acute fasciolosis. The effectiveness of flukicide treatment on individual farms should be checked by taking dung samples before treatment and 3 weeks after treatment and submitting them for laboratory examination. It is important for individual flock owners to be aware of the efficacy of all anthelmintics used in their own flocks, so that appropriate parasite control regimes can be developed in consultation with their veterinary surgeon.
Unlike the situation with acute fasciolosis, the flukicides active against adult and late immature flukes remain effective in NI, and should be used with sheep and cattle in the winter and spring months (particularly, in cattle, before turnout) to reduce contamination of spring pasture by fluke eggs. However, if the recent trend of mild winters continues, out-wintered sheep will be liable to pick up acute infection much later than expected. Treatment of cattle will depend on the timing of housing. The flukicide programme used has to be on a ‘know-your-farm’ basis and no one set of recommendations will cover all flocks or herds.
Flukicide products and anthelmintic combinations containing closantel, nitroxynil, clorsulon, albendazole, and oxyclozanide are effective against adult flukes and are licensed for use in NI. Information on the spectrum of activity of individual products, host species suitability and meat and milk withholding times is available on the respective data sheets and must be considered before development of new treatment regimes.
In recent years, due to the mild conditions and the succession of unusually wet summers, stomach (rumen) flukes, Calicophoron daubneyi, have become common in sheep and cattle in NI. The life cycle of rumen flukes is similar to that of Fasciola hepatica, and the intermediate snail host is thought to be the same. It is believed that the snails are to spread on pasture during periods of flooding. Adult rumen flukes are less damaging to sheep and cattle than liver flukes, but heavy infections of immature worms may cause death in young animals. A number of deaths due to massive burdens of immature paramphistomes have been recorded in Northern Ireland in the last few years. The presence of mature paramphistomes in ruminants can be confirmed by laboratory examination of faeces samples.
Remember that liver flukes are potentially a much more serious risk to the welfare and productivity of animals than stomach flukes. The choice of flukicides to use in autumn must reflect this. Farmers should treat first with liver fluke in mind and second for rumen fluke. At present the only products licensed for use in NI that are effective against paramphistomes (immature and adult) are those containing oxyclozanide. These products can also destroy adult liver flukes, but not the immature stages that cause acute fasciolosis.