March 2003 Flies may help curb one of Agriculture's greatest scourges

It is a very serious problem for most farmers in the west of Ireland. Urban dwellers are familiar with it from radio advertisements promoting products for its eradication. What is it? It's liver fluke – often regarded as one of the world's greatest agricultural pests. However, scientists in NUI Galway, with the help of Mother Nature, are well on the way to arresting the scourge of liver fluke, thus saving the Irish and global economy millions of Euro each year.

Scientists led by Dr. Mike Gormally at the Applied Ecology Unit (AEU) at the National University of Ireland, Galway have been investigating marsh fly biology. Research over the last three years by Rory Mc Donnell (PhD student) and Collette Mulkeen (Honours Environmental Science student), have produced encouraging results. It is possible that a humble group of insects called marsh flies might indeed prove to be the liver fluke's Nemesis.

Liver fluke parasites live in the bile ducts of sheep, cattle, deer, rabbits and even humans. The eggs pass out of the host animal with its dung, hatch into a larvae and must then find a special type of snail, called a mud snail, in order to complete its development. After reproducing up to 600 times within the snail, new larvae emerge, crawl up blades of grass and form weather-resistant cysts, which are then ingested inadvertently by passing grazing animals. The immature flukes then penetrate the gut wall and make their way to the bile ducts causing extensive liver damage along the way.

Enter the marsh fly! Dr. Mike Gormally and his team at NUI Galway have discovered that several Irish marsh fly species attack, kill and feed on the mud snail, which is so crucial to liver fluke development. Marsh flies, which are generally no bigger than a common house-fly, are yellowish-brown in colour and are found on all continents except the Antarctic. "In Ireland, we have 52 different kinds and they are usually found in marshy areas," explains Dr. Gormally. " If mud snail numbers can be reduced in an area by releasing these insects, then the incidences of liver fluke in livestock is also likely to decline."

The NUI Galway research is aimed at gaining an understanding of the growth patterns, feeding behaviour and habitat requirements of these snail-killing flies. "This information is essential before we can release these insects into fluke-prone areas and expect them to do their job," says Rory McDonnell, who is currently finishing his Ph.D thesis. "We need to know the conditions they prefer, how long they feed on snails, how many snails they kill and which kinds they like most," he says. Results to date show that these insects are voracious predators that are easily reared under laboratory conditions for release into problem areas.

The next step in the NUI Galway research is the release of marsh flies into areas where liver fluke is a problem and assessing their efficacy in the wild. This is a crucial stage as the marsh flies will have to deal with factors such as predation, competition, diseases and adverse weather conditions which they were not faced with in laboratory testing.

It is perhaps difficult to see how such a small organism as liver fluke can be such a scourge to world agriculture but the statistics speak for themselves:

• · Liver fluke costs the global economy US$2,000 million (€1,850 million) annually.
• · 600 million animals are now infected worldwide.
• · 2.4 million people are now parasitised by liver fluke (the chief avenue of human infection is by eating watercress contaminated by liver fluke cysts).
• · In Ireland (where the disease is common in wet pastures), liver fluke cost our agricultural sector €25 million in 2001.

"Traditional methods of keeping fluke at bay, such as land drainage, are no longer an option in most areas, now that many Irish wetlands are a priority habitat for conservation," says Collette Mulkeen. Modern control methods using drugs which target adult and immature flukes in livestock, were initially very successful but the development of resistance by flukes to many of these chemicals has now raised considerable concern. McDonnell points out that; "If the global economic loss due to liver fluke is reduced by a meagre 0.5% by using marsh flies, then the world will be US$10 million better off and it will be a lot less worrying having to eat a watercress salad"!

Information from: Máire Mhic Uidhir, Press Officer, NUI Galway. Tel. 091-750418; 087-2986592