A series of wet seasons has led to increased liver fluke incidence. Chloe Palmer speaks to experts to learn how to tackle the parasite.
Liver fluke was once confined to the wetter western corners of the British Isles and was a rare occurrence elsewhere.
Now it can crop up almost anywhere and. in areas traditionally prone to the parasite, infection levels are reaching almost epidemic proportions.
Fluke infection is estimated to cost UK agriculture about £300 million a year due to poor performance, mortality and treatment costs. Liver condemnations alone cost the industry £3.2m in 2010.
Fluke is notoriously problematic to diagnose in live animals and its complex life cycle makes it difficult to treat effectively.
Dr Philip Skuce, senior research scientist at Moredun Research Institute, has worked on fluke for many years but believes the situation is now becoming critical.
He says: “I became aware of the scale of the problem in 2012, after a wet summer, when we were receiving reports of fluke on farms in the east of Scotland where farmers had not seen it before.
“Diagnostic testing for fluke is not an exact science because all available tests have their limitations. Fewer than 20 per cent of farmers are testing for fluke, so many cases go undetected.”
The scientific name for the fluke trematode is fasciola hepatica and its life cycle includes an intermediate host, the mud snail, also known as galba truncatula. In line with its name, the snail thrives in wet areas and it is here where infection risk to animals is greatest.
The problem of fluke in cattle is often overlooked because the acute form of the disease is rarely seen in the species in the UK.
A study led by the University of Liverpool between 2006 and 2007, involving more than 3,000 dairy herds, showed infection levels running at 75 per cent nationally and as high as 95 per cent in the North West.
More recent results suggest prevalence of infection is similar in beef and dairy herds.
Prof Diana Williams, head of the infection biology department at the university, says the results of many recent studies concur with these infection rates in dairy herds.
“Many dairy farmers are now testing regularly for fluke because milk recording companies are offering a bulk tank test. For dairy cows, treatment options are limited because of withdrawal periods, so the best option is to treat cows at the start of the drying off period.”
Knowledge of the fluke life cycle is imperative when planning management and treatment.
Prof Williams says: “Fluke is seasonal and the greatest challenge is in autumn when infection risk is highest, so avoid grazing wet pastures and other high risk areas.
“Ideally, animals should be treated before turnout, but obviously this is not possible with cows in-milk. Treating in this way may help reduce the level of infection reaching the fluke intermediate snail host.”
There is less evidence of resistance to triclabendazole in fluke populations in cattle, but Prof Williams suggests this may be because there have been fewer studies in cattle and may also be due to reduced selection pressure.
Future research aimed at preventing or reducing fluke infection includes work on a vaccine, but the picture is complicated, Prof Williams says.
“Sheep and cattle do not develop protective immunity to fluke, so a vaccine is unlikely to offer full protection, although it may help control numbers of viable eggs contaminating the pasture.
“We are dealing with a clever parasite which affects the immune response and suppresses the diagnostic test for bovine TB. We are also studying a possible interaction between fluke and Johne’s disease.”
Fluke eggs are passed out in the dung of infected sheep and cattle. Eggs can take up to one month to develop before hatching, when the microscopic miracidium is released.
The miracidium only lives for a few hours and requires water to swim through to reach a snail. It will then burrow through the snail’s foot and into the body cavity. Here the fluke grows and multiplies for a period of about six weeks.
After six weeks, the next stage of fluke, the cercaria, is released from the snail.
A snail infected with a single miracidium can produce hundreds of cercariae.
These swim through water and reach grass and vegetation around the habitat where snails are found.
Here, they form infective cysts, the metacercariae.
These can remain viable on pasture for several months, depending on weather.
When eaten by a grazing animal, cysts are swallowed, then they hatch and burrow through the gut and into the liver.
It takes about eight weeks for flukes to complete their migration through the liver and reach the bile ducts. Eggs can be detected in the dung about 10-12 weeks after infection.
Dr Fiona Lovatt runs a sheep veterinary consultancy and has witnessed an increase in incidence of fluke in northern England as it has spread eastwards. She points to several signs which can indicate the presence of a fluke problem.
She says: “A loss of body condition is a common sign and ‘bottle jaw’ or ‘poke’ may also be seen, where a swelling forms beneath the jaw caused by a loss of protein because of liver damage. Farmers should also consider chronic fluke if scanning percentages are disappointing.”
Chronic fluke infection occurs when there are adult fluke in the bile duct and is usually detected in winter or spring. It can have ‘massive knock-on effects’, according to Dr Lovatt, as lambing percentages can be dramatically reduced and it will affect the quantity of colostrum and milk produced and lambs will not thrive.
In contrast, the acute form of fluke infestation can cause sudden death in extreme cases, and it is usually detected in early winter when large numbers of immature fluke migrate through the liver, causing haemorrhaging and severe acute liver damage.
Dr Lovatt says an understanding of the fluke life cycle is essential for farmers if they are to treat animals successfully.
She says: “A faecal egg count taken in January will indicate whether adult fluke are present in the flock and, if so, they should be treated using a flukicide which is effective against adult stages.
“Sheep which have been housed for more than 10 weeks will not be carrying the immature fluke stages.”
Detecting the immature fluke stages in sheep is more difficult as there are no adults laying eggs.
It is possible to take blood tests based on antibodies, but these will show fluke present historically and are not definitive proof of current infection.
Dr Lovatt says: “Blood testing lambs through late summer will indicate when they pick up fluke for the first time. This is probably the best way to detect the start of fluke risk each year.”
Treatment for fluke in sheep is made more difficult because of rising resistance to triclabendazole in the national flock.
Responsible use of flukicide products is therefore vital, according to Dr Lovatt.
She says: “Every farmer should think like an organic farmer when tackling fluke and be aware of the cycle and the environment to enable them to plan grazing to minimise the risk of fluke infection.
“Finishing lambs should not be grazed on high risk pastures. Similarly, do not turn out bought-in ewes onto snail ground if there is any possibility they have fluke.”
Problems are at their greatest where resistance to triclabendazole is undetected.
Dr Lovatt says: “If you do not have a triclabendazole resistance on your farm, you do not want it. Either test post-treatment or treat new arrivals with closantel or nitroxynil to kill older immatures and adult fluke.
“Ideally, put them on pasture free of snails and always treat them again after six weeks to kill young immature stages.
“Currently, there is no reported resistance to closantel and nitroxynil in the UK, but we must be careful to use all products responsibly.”
Dr Lovatt believes some reports of resistance with triclabendazole may be due to incorrect dosing or re-infection of sheep after treatment because they are turned out onto infected pastures.
Dr Philip Skuce agrees there is no room for complacency when tackling fluke in sheep flocks.
He says: “We cannot simply rely on chemicals and treat our way out of trouble because even where there is no known resistance to drugs. History tells us this approach is not sustainable.
“Fluke risk has increased recently, but there is still a significant difference in infection levels between farms, so there are a lot of on-farm fluke risk factors.
“Farmers must consider how they can protect their farm from fluke and, wherever possible, fence off wet areas within fields. Treating fluke is a real challenge because of the massive replication of the cercaria parasite within the snail and there are no new fluke drugs coming onto the market any time soon.”
If a fluke problem is suspected on-farm, working with a vet to plan ahead and implement management control options, treatments and monitoring is essential to avoid losses.
Source: Adapted from Control of Worms Sustainably and Sustainable Control of Parasites in Sheep websites