26 February 2013

8 minute read

Liver of a sheep that died of acute fasciolosis

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.

In less severe cases poor production and growth, coupled with reduced appetite and abdominal pain are apparent. 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. Affected animals, which harbour adult, blood-sucking egg-producing flukes in the bile ducts may exhibit anaemia, ill thrift, reduced production or ‘bottle jaw’ (fluid swelling under the jaw).

Chronic fluke infection can cause a reduction of 5-15 per cent in the milk yield of dairy cows (1), loss of growth in fattening lambs and cattle, and sometimes abortion in late-pregnant ewes. It is therefore a source of considerable unseen financial loss to the agricultural industry in Northern Ireland. Fluke infections in dairy cattle can also predispose to metabolic conditions such as ketosis and infectious diseases such as salmonellosis and Black Disease (infection with Clostridium novyi). The same is likely to be true for sheep.

Cattle and sheep with access to ‘flukey’ pastures should be fully vaccinated against Black Disease. Recent evidence suggests that fluke infection in cattle can interfere with the tuberculin skin test for TB, preventing the diagnosis of reactor cattle (2).

The mild winter conditions of 2011-12, combined with higher than average rainfall during the summer months created conditions favourable to the over-winter survival and early-season multiplication of Galba (formally Lymnea) truncatula, the snail intermediate host of the liver fluke, Fasciola hepatica. This situation, coupled with the on-going evolution of fluke resistance to the widely used flukicide, triclabendazole, led to an unusually high level of metacercariae on pasture from August onwards.

Sheep are now dying of acute fasciolosis as large numbers of immature flukes burrow through the tissues of their livers, breaking down the blood sinuses, and consuming blood, ever more voraciously as they grow. Unusually high numbers of sheep carcases submitted to the VSD laboratories at Stormont and Omagh since September have been found to have died of acute fasciolosis, and it is likely that the incoming lambing season will be marked by high numbers of abortions in ewes harbouring heavy burdens of adult flukes.

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 mild winter weather ensues, 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. This may exacerbate even further the problems of acute and chronic fluke infection in 2013-14.

Mature paramphistomes in the rumen of a cow

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 (3). 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 (4). Surveys and field trials carried out by the Veterinary Sciences Division, AFBI, and Queen’s University, Belfast in 2010 and 2011 have established that tricalbendazole resistance is widespread on sheep, and probably also on cattle farms throughout NI.

This is particularly the case amongst well-managed farms with a long history of triclabendazole use, but also on premises where stock has been bought-in from farms with a pre-existing triclabendazole-resistance problem. By the time a stockholder notices that triclabendazole-based flukicides no longer seem to be working effectively for his animals, resistance is well-established on the farm, and even if the flukicide is changed, it is unlikely that triclabendazole will ever be useful on that farm again.

The situation facing sheep farms with suspected triclabendazole resistance this year is that, in the presence of almost unprecedented 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. Use of such a regime on out-wintered sheep once or twice in autumn and again in January, coupled with a treatment effective against adult flukes in early spring, should significantly reduce the fluke burden on individual farms.

However, 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 that are inappropriate for the treatment of acute fasciolosis.

The effectiveness of flukicide treatment on individual farms should be checked by taking dung samples before treatment and 3-4 weeks after treatment and submitting them for laboratory examination. This is best done in spring, when adult flukes producing eggs are likely to be present in the bile ducts. It is important for individual flock owners to be aware of the efficacy of triclabendazole (and indeed all anthelmintics likely to be used) in their own flocks, so that appropriate parasite control regimes can be developed in consultation with their veterinary surgeon.

Galba truncatula, intermediate host of liver flukes, on wet pasture

Unlike the situation with acute fasciolosis, the flukicides active against adult and late immature flukes (i.e. those present in the bile ducts) 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.

Use of a flukicide with activity mainly against adult flukes may be sufficient in these circumstances, but 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 and the effectiveness of particular flukicides against the fluke present on individual farms.

However 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. Flukicide treatment of ewes in late pregnancy should be avoided if possible.

Planorbid snails, intermediate host of rumen flakes

In recent years, due to a succession of unusually wet summers, stomach (rumen) flukes, Paramphistomum cervi, have become common in sheep and cattle in NI. The life cycle of rumen flukes is similar to that of Fasciola hepatica (5), but the intermediate snail host is aquatic, and is believed to spread unto pasture during periods of flooding, where infective metacercaria larvae may be deposited. Adult rumen flukes are less damaging to sheep and cattle than liver flukes, but heavy infections of immature worms may cause diarrhoea , ill-thrift and, exceptionally, death in young animals.

A number of deaths in young sheep and cattle due to massive burdens of immature paramphistomes migrating in the gastrointestinal tract have been recorded in Northern Ireland, in the last two years. The presence of mature paramphistomes in ruminants can be confirmed by laboratory examination of faeces samples.

The eggs of paramphistomes are similar in size and density to the eggs of Fasciola hepatica, but can be distinguished from them by colour. Remember that liver flukes, particularly in acute infections, 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. If need be, different treatments can be used for each parasite. At present the only products licensed for use in NI that are effective against paramphistomes (immature and adult) are those containing oxyclozanide (15mg/kg bwt has been reported as having 85 to 100 % efficacy against paraphistomes in the intestine and 100 % efficacy against paraphistomes in the rumen (6)). These products can also destroy adult liver flukes, but not the immature stages that cause acute fasciolosis.

The AFBI veterinary laboratories at Stormont and Omagh can assist veterinary surgeons by testing dung samples from livestock for evidence of liver and stomach fluke infection and can carry out blood tests to reveal liver damage associated with fluke infection. Further information on liver fluke disease in cattle and sheep may be found at: http://www.afbini.gov.uk/index/services/diagnostic/adds/adds-articles-diseases.h

References:

• Schweizer G, Braun U, Deplazes P, Torgerson PR. (2005), Estimating the financial losses due to bovine fasciolosis in Switzerland. Veterinary Record 157, 188-193
• Flynn RJ, Mannion C, Golden O, Hacariz O, Mulcahy G. (2007), Experimental Fasciola hepatica infection alters responses to tests used for diagnosis of bovine tuberculosis. Infection and Immunity 75, 1373-1381
• Boray JC, Crowfoot PD, Strong MB, Allison JR, Schellenbaum M, Van Orelli M, Sarasin G. (1983), Treatment of immature and mature Fasciola hepatica infections in sheep with triclabendazole. Veterinary Record 113, 315-317
• Sargison ND, Scott PR. (2011), Diagnosis and economic consequences of triclabendazole resistance in Fasciola hepatica in a sheep flock in south-east Scotland. Veterinary Record 168, 159
• De Waal T. (2010), Paramphistomum – a brief review. Irish Veterinary Journal 63, 313-315
• Rolfe PF, Boray JC. (1987), Chemotherapy of paramphistomosis in cattle. Australian Veterinary Journal 64, 328-332

December 2012