Sheep Focus: Liver fluke in sheep
Liver fluke in sheep is on the increase as the UK continues to experience warmer, wetter weather conditions, says Janssen Animal Health vet Richard Sygall.
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The liver fluke is a flatworm with a complex lifecycle which relies on moisture and warm temperatures for survival, says Richard Sygall.
As well as having a detrimental effect on performance, acute infestation can cause livestock deaths if the parasite is left to multiply unchecked. Sheep of all ages are equally susceptible, with the most acute forms of infestation observed during the autumn.
“Traditionally, we have seen the majority of liver fluke cases in the west of the UK, which usually experiences heavier rainfall,” he says. “However, in recent years the problem is becoming more common in the traditionally drier eastern regions. Farms with high stocking densities are particularly vulnerable, as well as land that is prone to flooding, or fields close to a watercourse.”
The liver fluke’s lifecycle begins when the worm’s eggs pass out of sheep via their dung, where they hatch into a form known as miracidia. The parasite then uses the mudsnail - which requires temperatures above 10degC - as an intermediate host.
It is estimated that for each individual miracidium which enters the mudsnail, around 600 cercaria (next stage) will emerge. The cercaria then migrate to blades of grass, where they turn into metacercaria and can be consumed by livestock.
During the invasion of the sheep’s body, the larvae move towards the liver, through which they migrate, finally ending up in the bile ducts at the adult stage. Although the adult worm is only about 2.5cm long, one sheep may be carrying 30 plus, each consuming around 2ml of blood each day and producing eggs, which start the cycle all over again.
RECOGNISING AND CONTROLLING FLUKE
With no reliable method of testing land for liver fluke, the initial outward signs may simply be an unexpected drop in fertility levels or growth rates, says Mr Sygall.
As the adult fluke parasite feeds on blood, its presence leads to anaemia in infected sheep. Therefore, on examination, the mucous membranes and gums will be characteristically pale in colour, compared with healthy animals.
While these signs can also be symptomatic of infestation with the barber pole worm (haemonchus contortus), both fluke and the barber pole worm will respond to treatment using a closantel-based product, such as Flukiver.
Another classic sign of liver fluke is bottlejaw, says Mr Sygall: “Even if your sheep are displaying no outward symptoms, it may be worth asking your abattoir to get in touch if they spot any sign of liver damage.”
An accurate assessment, to determine the scale of the problem and plan a treatment programme, should be carried out using faecal egg counts, with samples generally sent away for laboratory analysis.
However, in the early acute stages of the disease, there may be no eggs present in the dung, as the worm has not yet reached adulthood. In this scenario, Mr Sygall suggests blood testing for raised liver enzymes to reveal the scale of the problem.
If the disease is allowed to progress, sheep will inevitably suffer liver damage, and deaths are likely to occur, despite action being taken, he says.
On farms in areas where fluke is not a major problem, livestock keepers should be alert to the possibility of the parasite being brought to the farm via newly-purchased cattle and sheep, particularly if they have come from a high-risk area, says Mr Sygall.
Stock should be dosed with an effective flukicide and isolated for at least a week.
Producers with sheep suffering from fluke should also treat cattle, he suggests, because cattle, while generally less susceptible to the disease, can act as carriers - along with rabbits and deer - and subsequently re-infect the pasture. It is not thought that bringing forages or manure on to the farm carries with it a risk of infection.
For producers who use flukicides routinely, the importance of drug rotation and avoiding under-dosing cannot be over-emphasised, says Mr Sygall: “Flukiver will disrupt fluke egg laying and reduce the viability of eggs for a 13-week period. It acts for longer than any other flukicide, and there have been no reports of drug resistance in the UK.
“In some circumstances, it may be necessary to re-treat four to eight weeks after the initial dosing, and then to treat the following spring with the aim of reducing the level of infection on the farm. With the right control programme, it should be possible to clear the disease to negligible levels within two years.”
Like many of the drugs used regularly for treating livestock, resistance build-up is an ever-present possibility.
The key to avoiding its occurrence is to rotate the types of treatment used on an annual basis, says Mr Sygall.
He also recommends producers seek expert help in planning their strategy, which should cover all the various stages of the parasite’s lifecycle.
It is thought resistance to triclabendazole is becoming more widespread, with cases reported in Scotland and South West Wales in particular.
locks in these two areas have tended to suffer from endemic fluke for many years, so perhaps this should sound a warning for producers who have been using flukicides for a shorter duration of time, says Mr Sygall. To date, there is no accurate method of assessing the level of triclabendazole resistance, he says.