Essential action to reduce the risk of Liver Fluke disease in 2010
As a result of the unusually wet summer weather in 2008 and 2009, preceded by mild winters in 2007-08 and 2008-09, conditions on Northern Irish sheep and cattle farms have recently been particularly favourable for the reproduction and spread of mud snails and thus for the propagation of liver flukes which develop within them.
Adult liver flukes escaping from the opened bile duct of an infected sheep during post-mortem examination.
During the autumn and winter just past, AFBI has discovered an unprecedented increase in the numbers of sheep dying as a result of liver fluke infection, and also a large increase in abortions of late-gestation lambs, where the livers of the dams have been so damaged by fluke activity that they are no longer able to support the demands of late pregnancy. Unusually, cattle also have been dying as a result of severe liver fluke disease, although most cases of infection in cattle result in smaller numbers of adult fluke taking up residence in the bile ducts, and remaining there over-winter to shed eggs to the pasture in the next grazing season.
This situation is only in part due to the climatically-related increase in numbers of liver fluke larvae on the pastures in later summer and autumn. An additional, and extremely worrying aspect, is the apparent rapid rise in fluke resistance to the most effective flukicidal product available to farmers, namely triclabendazole. This anthelmintic drug is the only one that is active against fluke of all ages, from the earliest stages invading the host’s liver, to the adult worms shedding eggs in the bile ducts. Now however, we are finding many farms where triclabendazole, once effective in controlling liver fluke disease, no longer removes the invading juvenile worms. They are left to wreak havoc in the hosts’ livers, and, if they do not kill the infected animals, to complete their development to egg-producing adults in the bile ducts. The uncontrolled egg output from triclabendazole-resistant adult worms means that the pasture inevitably becomes heavily contaminated with eggs that will give rise to the next generation of fully triclabendazole-resistant flukes. This has been the case throughout the autumn and winter of 2009-2010, while infected sheep or cattle have remained at pasture.
A typical habitat for mud snails, hosts for liver fluke larvae, on a farm in Northern Ireland.
Fortunately, the severe and prolonged frosts experienced during the winter and early spring of 2009-2010 are likely to have destroyed most of the fluke eggs and infective larvae remaining on the pasture since autumn. It is also likely that most of the infected snails, which went into hibernation at the beginning of the cold weather, will have died during the winter. So, at the beginning of the new grazing season this spring, pastures contaminated last year will probably be relatively free of larvae, and of fluke eggs capable of causing infection in the new generation of snails.
The main risk for fluke infection this year is likely to come from fluke eggs produced by adult triclabendazole-resistant flukes residing over-winter in the bile ducts of sheep and cattle that were exposed to infection last autumn. Because of their resistant status, these flukes will not have been removed by triclabendazole-containing drenches, and their eggs will build up on the pastures from the time of turnout, hatching from the end of May to infect young snails during the summer.
In order to help avoid severe outbreaks of triclabendazole-resistant liver fluke disease in the autumn, it is therefore essential to treat sheep and cattle prior to turnout or as soon as possible thereafter, with an anthelmintic that is capable of destroying the adult flukes in the bile ducts. Anthelmintics containing closantel, clorsulon, oxyclozanide or nitroxynil are still capable of killing adult flukes, even those that are resistant to triclabendazole.
A mud snail,
Galba truncatula, host for liver fluke larvae.
If you suspect that triclabendazole-resistant liver fluke infection might be a problem in your sheep or cattle, it would be advisable to consult your veterinary surgeon and to arrange with him to have the situation investigated by the Parasitology Section of the Veterinary Sciences Division, AFBI Stormont, Belfast BT4 3SD. Laboratory tests are available to diagnose triclabendazole-resistant fluke infection, and information on treatment and control options can be provided by AFBI specialists.
by Bob Hanna and Hillary Edgar AFBI Veterinary Sciences Division
