Parasite Forecast

Issue: March

WARNING: Due to the record temperatures reached in February, NADIS wishes to highlight the SCOPS Nematodirus forecast, which is currently predicting moderate risk in many parts of the UK. In such areas, peak risk could occur within the next 2 weeks if conditions remain favourable. NADIS produces its own Nematodirus risk forecast from mid-March onwards. Due to these unprecedented circumstances, however, it is possible that hatch may have already occurred in some areas by this time. We therefore encourage farmers, vets and SQPs to check for Nematodirus risk in their area using the SCOPS forecast to avoid being caught out by disease outbreaks earlier in the season than would usually be expected.

SCOPS press release:

Weather report


The relatively mild start to January was replaced by colder conditions in the second half of the month with occasional milder interludes. Frost and snow were widespread between the 17th and 23rd and also towards the end of the month.

The UK mean temperature in January was 3.7 °C, which is equal to the long-term monthly average (1981-2010). Regionally, expected monthly temperatures were observed across all regions except Northern Ireland, where the average monthly temperature was around 1oC above its long-term average. However, the trend of above average temperatures over the preceding three months (November – January) has continued.

January 2019 was another relatively dry month, with 52% of average rainfall overall, making it the 9th driest January in a series from 1910, and the driest since 2006. Below average rainfall for January was observed across all regions of the UK and the previous three months (November – January) with the exception of southern England and Wales.

Liver Fluke

Due to the recent cold weather, development and emergence of liver fluke on pastures is likely to have arrested. However, previously contaminated pastures will remain infective into the coming season. Grazing of such pastures at this time can cause reinfection, increasing the risk of pasture contamination with fluke later in the season.

Animals which grazed high-risk fluke pastures towards the end of last season may be affected by chronic liver fluke caused by adult flukes residing within the bile ducts of the liver (Figure 1). Such infections often present with few or no obvious signs of disease, yet can negatively affect health, welfare and productivity. Chronically infected sheep and cattle can remain infected for months or even years if untreated, making them an important source of pasture contamination for the coming season.


Figure 1: Chronic liver fluke  (fasciolosis) is caused by adult flukes residing in the bile ducts of the liver. Such infections may show no obvious signs of disease, yet can have a profound negative impact on health, welfare and productivity.

Advised actions include:

  • Monitoring for signs disease:
    • General dullness, anaemia and shortness of breath
    • Rapid weight loss, fluid accumulation (bottlejaw)
  • Try to avoid grazing “flukey” pastures (wet or boggy areas) until later into the current season to reduce likelihood of reinfection.
  • In the absence of obvious signs (i.e. chronic infection), consider diagnostic testing.
    • Faecal egg counts can be used to diagnose chronic infection in individuals, or groups of animals via pooled samples (Figure 2).
    • Abattoir feedback can provide useful information with respect to fluke infection status.

Figure 2: Chronic fluke infection can be identified through egg sedimentation using either individual or pooled faecal samples.

 Where fluke infection is identified:

    • Treatment with triclabendazole is recommended for acute disease.
    • For chronic infection, consider use of a product other than triclabendazole to reduce selection pressure for resistance.
    • Again, due to concerns over emerging drug resistance, ensure correct dosing based on bodyweight, and consider testing for treatment efficacy through pre- and post-treatment diagnostics.

For more information about how best to implement the various treatment and control options and conduct efficacy testing on your farm, please speak to your vet or SQP.


Sheep Parasitic Gastroenteritis (PGE)

In pregnant ewes, as lambing approaches the so called “periparturient rise” (PPR) in worm egg count begins to take effect. This is caused by a suppression of the ewe’s immunity around lambing that affords their gut nematodes a temporary period of increased egg production (Figure 3). If such animals are not treated ahead of turnout this can lead to heavy pasture contamination and higher risk of disease in lambs for the coming grazing season. It is important to consider both timing and choice of worming products when treating PPR, as treatments strategies must balance the reduction of pasture contamination against selection for anthelmintic resistance.

Figure 3: The “periparturient rise” in ewes is the increase in daily egg output in pregnant animals that occurs around lambing. Whilst this has little to no impact on the health of ewes themselves, the increased egg output is an important source of pasture contamination and subsequent risk to their lambs.

It is important to consider nematodirosis (disease caused by Nematodirus battus) ahead of the current grazing season. Unlike other gut nematodes, Nematodirus infection passes directly from one season’s lamb crop to the next. Pastures may become highly infective in a short space of time due to favourable conditions leading to mass hatching and emergence of infective stage larvae. If this mass emergence or “peak hatch” occurs at a time when lambs are starting to graze extensively, typically around 6-12 weeks of age, this can lead to widespread and severe disease characterised by sudden onset diarrhoea, dehydration and death. Affected animals normally present with heavily soiled back ends, lack of appetite and a profound thirst (Figure 4). It is therefore important to identify potentially contaminated pastures and avoid grazing lambs on these during peak risk periods for disease. Both NADIS and SCOPS produce risk forecasts for Nematodirus based on local climatic conditions to help predict when “peak hatch” periods are likely to occur. Due to the record temperatures experienced in February 2019, the SCOPS forecast is already predicting moderate risk for nematodirosis in many parts of the UK, with some cases already confirmed. The NADIS forecast will be available from the end of March. It is strongly advised you consult these forecasts and plan your grazing and parasite control strategies accordingly.

Figure 4: Nematodirus battus infection can cause sudden onset, severe diarrhoea in first season lambs often with characteristic soiling around the back end.

Advised actions include:

  • When treating PPR in ewes, SCOPS currently recommend one of the following options as a way of reducing pasture contamination and risk to lambs whilst minimising risk of selecting for anthelmintic resistance:
    • Leave a proportion of the ewes untreated.
      • Targeted treatment of specific groups of animals can be an effective way of achieving this. For example, animals with high worm egg counts (Figure 5), low body condition score, twin/triplet or first season ewes
      • As a rough guide, leaving around 10% of the flock untreated is suggested to dilute any selection for anthelmintic resistance.
    • Treat early in the post-lambing phase so that ewes can become re-infected with unselected parasites through grazing before their immunity is fully restored.
  • For nematodirosis in lambs:
    • Identify high risk pastures, namely those grazed by the previous season’s lambs, and avoid grazing the current season’s lambs here during peak risk periods.
    • Consult the SCOPS and NADIS Nematodirus forecasts as these become available to determine when the peak risk period is likely to be in your area and monitor for signs of disease.
    • Where disease occurs treatment with group 1-BZ is usually effective. However, reports of treatment failures have been reported in the UK.
      • It is therefore essential to ensure correct dosing by weight and administration using correctly calibrated drenching equipment.
      • Consider egg counts 7-10 days post-treatment to confirm efficacy (Figure 5).
      • Where treatment failure is suspected, please seek veterinary advice.



Figure 5: Faecal egg counts are a useful way of evaluating various parasitic infections. This method can be used to identify Nematodirus eggs (N.b) and those of other PGE-causing roundworms (Str.) as well as coccidial oocysts (arrows).


Another parasitic disease of importance in growing lambs is coccidiosis. This disease is caused by protozoal parasites (Eimeria spp.) causing gut damage. Signs of disease typically occur around 4-8 weeks of age and are characterised by anorexia, weight loss, diarrhoea (with or without blood) and death in severe cases. Coccidiosis results from a rapid accumulation of infective “oocysts” in the environment, which can occur both indoors and at pasture. This is commonly associated with high intensity husbandry systems and stress factors such as poor colostrum supply, high stocking densities, adverse weather conditions at wet muddy paddocks previously grazed by sheep and/or extended housing periods.

Due to the similar presenting signs and age ranges of affected animals, in grazing lambs it is important to determine whether Eimeria or Nematodirus infection is present and causing disease. Concurrent infection with both parasites is not uncommon, potentially leading to greater disease severity.

Advised actions include:

  • Monitoring for signs of disease
  • Routine worm egg counts can also be used to evaluate oocyst numbers (Figure 5).
    • When considering treatment, further testing to determine whether oocysts present are from a pathogenic Eimeria is also advised.
  • Reduction of stocking densities, batch rearing of lambs by age and avoidance of heavily contaminated pastures/premises can reduce risk of disease outbreaks.
  • A number of anticoccidial products including feed medication are available for both prevention and treatment of coccidiosis. For more information on these, please speak to your vet or SQP.


Cattle Parasitic Gastroenteritis (PGE)

Calves and youngstock entering their first or second grazing season are at greatest risk of PGE (Figure 6). It is therefore important to plan around these animals when devising an effective, sustainable parasite control plan for your farm. The COWS group currently recommend one of two options. Choice of strategy is largely dependent upon individual farm objectives and the feasibility of their implementation:

  1. Set stocking with strategic anthelmintic treatments. This option is particularly useful on farms where grazing choice is limited. The aim is to minimise pasture contamination up to mid-July, by which time the over-wintered larval populations should have declined to insignificant levels. Cattle treated strategically should therefore remain set-stocked and only moved to safe pastures (e.g. hay or silage aftermaths) later in the grazing season as these become available. To be effective, this strategy needs to be initiated early in the grazing season, within 3 weeks of turnout, to avoid build-up of worm eggs on pasture. Strategic treatments include administration of a bolus wormer at turnout or repeated administration of shorter duration group 3-ML products at a 6-8 week interval.
  2. A “wait and see” approach aiming to avoid use of anthelmintics unless needed. Whilst reducing selection pressure for anthelmintic resistance, this approach is heavily reliant on monitoring and diagnostic testing to prevent disease and significant production losses. Where available, this approach can make use of pasture rotation to prevent animals being exposed to a significant build-up of infective larvae later in the grazing season, although this needs to be planned well in advance to ensure safe grazing options are available when needed. Targeted selective treatments based on weight gain and/or body condition score and overall health status can be used to minimise and direct anthelmintic treatment to where it is most needed. Similarly, treatments informed by worm egg counts can reduce pasture contamination without applying a significant selection pressure on the parasite population with respect to anthelmintic resistance.

Figure 6: PGE in cattle causes diarrhoea and up to a 30% reduction in the growth rates of youngstock. Commonly affected animals include growing dairy heifers in their first grazing season (left) and weaned autumn-born suckler calves in their second grazing season (right).

Irrespective of which approach is taken, over the course of the grazing season regular performance testing through weight gain, diagnostics (e.g. worm egg counts) and post-treatment efficacy testing are hugely valuable and important tools to ensure your chosen control plan is working. Incorrectly controlled PGE can severely impair performance and productivity in both calves and adult cattle. For more information, please speak to your vet or SQP and see the COWS website (


On farms with a history of lungworm infection, vaccination offers a valuable tool for protection against disease in calves (Figure 7). Since the lungworm vaccine is live, it must be purchased fresh ahead of each grazing season. Planning and ordering the number of doses required for your farm well in advance is therefore advisable.

  • All calves over 8 weeks old entering their first grazing season should be given two doses of lungworm vaccine four weeks apart, with the second dose being given at least two weeks before turnout.
  • In some instances, such as where anthelmintic regimes may have prevented full immunity being acquired over the previous grazing season, a further one off vaccination may be recommended.
  • For more information, please speak to your vet or SQP and see “COWS” group guidelines.

 Figure 7: Lungworm infection can be a very serious problem for youngstock. On farms with a history of disease vaccination can be hugely valuable in reducing disease incidence and severity, but must be ordered and planned well in advance.

John Graham-Brown BVSc (hons) MSc (hons) PhD MRCVS
March 2019


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