Sheep farmers across Ireland are being warned about a high risk of liver fluke infection during autumn and winter. The Agri-Food and Biosciences Institute (AFBI) and the Department of Agriculture/Met Éireann have recently released warnings based on modelling forecasts that take into account rainfall levels and average temperatures during the summer months.

AFBI forecast

The AFBI forecast highlighted rainfall volumes during the key months of importance, June and July, as being significantly higher than in 2019, while 2020 rainfall levels were some 64% above the seasonal average. In contrast, rainfall levels in May were 51% lower and September volumes were 9% lower.

The forecast reports that mean monthly temperatures were lower than average in July and September (7.4% and 0.1% below average respectively), but were slightly above average in May, June and August (10.6%, 3.5% and 2.3% above NI average temperatures respectively).

To put this in terms of actual temperatures recorded, the mean temperature for the period May to September was recorded at 13°C, significantly higher than the mean temperature of 10°C required for the intermediate host to breed and for fluke larvae to develop with snails. A minimum temperature of 10°C is also required for fluke eggs to develop and hatch.

The forecast cites poorly drained area as the highest risk, which remain wet all year round. This provides an ideal environment for the multiplication of snails, which is likely to have continued relentlessly and in turn is providing ideal conditions for liver fluke parasites to prosper.

Department warning

The Department of Agriculture forecast, carried out in collaboration with Met Éireann, is based on meteorological data collected between May and October 2020. This is used to determine Ollerenshaw Summer Index values, which highlights the risk of fluke infestation.

Following a similarly dry May, the monthly index values for June to September recorded widespread maximum values due to the wet conditions that occurred. This is driving the high nationwide values ranging from 403 to 484, as detailed in Figure 1.

Figure 1: Ollerenshaw Summer Index values 2020.

The Department forecast for a high prevalence nationwide also takes into account other sources of information. The Regional Veterinary Laboratory (RVL) liver fluke abattoir ELISA survey collates the results of blood samples collected by Department officials from a selection of lambs born in 2020 in 478 flocks across 25 counties.

The blood samples are tested for the presence of antibodies to liver fluke, to determine the level of exposure from lambs in these flocks. This is a good assessment, as older sheep are likely to have had previous exposure. Results from the survey indicated that the majority of infected flocks were located in counties on the western seaboard.

The report states that some positive and inconclusive results were found in counties outside the west and northwest during August testing and the it raises the possibility of this being linked with a high summer rainfall level.

This situation had escalated by the October testing. Along with a high frequency of positive and inconclusive tests on the western seaboard, there was also substantial evidence of fluke exposure in lambs in the border areas, midlands and parts of Leinster.

Disease threat

At this stage of the year, the threat of acute liver fluke is waning, as this is generally at its highest level from August through to October. This disease stage is best explained as the rapid ingestion of high numbers of immature fluke, which burrow through the liver leading to weakness of the animal, anaemia and sudden death. There are often few signs of any disease burden, with well-fleshed animals suffering from a high burden often collapsing when flocked due to haemorrhaging of the liver.

The threat generally transitions to subacute fluke as you progress into mid-November onwards (still some early immature fluke present, but growing numbers of immature and mature fluke). Losses due to anaemia and sudden death are still possible, but as you get closer to Christmas and the New Year, the risk switches to chronic liver fluke.

Chronic liver fluke is a result of prolonged intake of cysts and leads to the advanced loss of body condition. Typical clinical signs include anemia, weight loss, so-called bottle jaw (accumulation of fluid under the jaw) and sometimes diarrhoea or constipation.

Treatment protocols

There have been no developments in recent times regading the range of flukicides available for treating sheep. There is ongoing research being led by the National University of Galway into a liver fluke vaccine and a positive development on this front would be a major step forward for the sheep industry.

There are six different flukicide options with products licensed for treating sheep at present – albendazole, closantel, nitroxynil, oxyclozanide, rafoxanide and trichlabendazole. These exist as a single ingredient flukicide or combination product, the majority of which also offer treatment for worms. Given the rate at which anthelmintic resistance to wormers is developing, the latest advice is to refrain from treating healthy mature ewes with combination products. This eliminates albendazole-based ingredients from the list, as there are only combination products on the market.

Table 1 captures the treatment properties of commonly used flukicide-only products. In terms of treatment products used at present should target at least immature and mature liver fluke parasites. The interval between treatments depends on land type and previous history on the farm. The general recommendation where the risk of fluke is seen as moderate is to treat outwintered sheep in November, January and April.

Where there is a higher risk, the treatment interval is reduced to six to eight weeks, while in the highest risk situations, it may be necessary to reduce this interval even further. The advice in these circumstances is to prioritise grazing on drier lands and limit grazing on heavily infested areas.

There is viewed to be a lower risk of liver fluke where grazing peat hills, but it is important to take account of periods where sheep are grazing on bye or lowland areas. Housing sheep opens up the opportunity of alternating product use and selecting a product that targets only mature liver fluke parasites once sheep have been housed for eight weeks

Rumen fluke

Rumen fluke has become more prevalent on the island of Ireland in recent years, but it is regularly considered to be much more widespread than it actually is, with sheep often treated unnecessarily. This is a risky prospect, as with no licensed products on the market to treat rumen fluke and oxyclozanide-based products the only flukicide to offer treatment properties, there is a need to protect its efficacy and only treat sheep where necessary.

The presence of a low positive or low count for rumen fluke eggs should not convey a need to treat animals that are thriving satisfactorily, as adult rumen fluke parasites can frequently be found in the rumen of animals without being associated with clinical signs of disease.

Therefore, treatment should be based on clinical signs such as rapid weight loss/diarrhoea, a history of issues on the farm, a combination of these and the presence of a high faecal egg count.

Strategy on programme farms - Senan White, programme adviser

The significant level of variation in land type and volume of rainfall typically received between the programme farms means a blanket approach to liver fluke is neither possible nor wise. As such, each farm has addressed the threat and control options for liver fluke when developing their animal health plan.

The history of issues was taken into account as was land type and the grazing/housing plan in late autumn and winter. On farms with more marginal land type, such as our participants in Fermanagh for example, the liver fluke treatment programme starts in August.

In a high risk year, some of these farms are treating sheep in September/October, usually prior to breeding and possibly in mid-November (targeting immature fluke parasites). Most farms are carrying out strategic treatment indoors, with high risk farms treating sheep two to three weeks post-housing (December/January) and also using faecal egg counts to determine the need for further treatments.

Some are also treating outwintered sheep in late April or carrying out a spring treatment prior to turnout. Indoor treatments open up the potential of using products that just target mature fluke and which are not appropriate to use earlier in the season. There is also more planning around the autumn/winter grazing plan and strategic grazing of high risk areas.

This varies significantly among drier farms in the programme. For example, for our Co Down farmer, liver fluke treatment does not start until November or December depending on the year and in a good year can be limited to a couple of treatments. For hill sheep farms, treatment coincides with grazing on improved vegetation or green ground, with peat soils posing less of a risk.

Information sources

There has been a big focus in the programme on making maximum use of all potential sources of information to inform the need to treat sheep. The health status of livers from sheep sent for slaughter provides valuable advice, while there is also a focus on carrying out post mortem examination of any unexplained incidence of mortality.

Faecal egg counts are limited in their role in determining the need to treat sheep for acute or sub-acute liver fluke in particular, but they have an important role in establishing product efficacy, with some farms confirming triclabendazole resistance. There is also more attention placed on product choice, ensuring that the active ingredient selected targets the stage of liver fluke present. Farms are keeping a good record of when treatment was administered and what product was used, and this is being used to alternate product choice and hopefully safeguard its efficacy in the flock.