Parasite Forecast

Issue: June

Key points:

  1. Monitor for PGE in grazing animals (sheep and cattle)
  2. April hatch for Nematodirus in England and Wales
  3. Check the NADIS blowfly alert
  4. Joint SCOPS/COWS press release on liver fluke risk for 2020

Weather report

Figure 1: Egg count data shows the most recent counts for roundworms in sheep at each location between the sample dates stated. Temperature and rainfall by region for previous months.

April was fine and settled with plenty of sunshine and warm temperature and some short spells of rain. The provisional UK mean temperature for the UK in April was 9.1oC, 1.7oC above the long-term national average (1981-2010). Regionally, temperatures were above average across all regions for April and the preceding 3 months (February – April). Total rainfall across the UK in April was only 40% of the long-term average. Again, these was seen across all regions in April, although the exceptionally wet February means rainfall for the preceding 3 months has still been above average in the longer term, particularly in England, Wales, Northern Ireland and the west of Scotland (Figure 1).


Sheep Parasitic Gastroenteritis (PGE)

The recent warm and dry weather is likely to have reduced the numbers of infective larvae that survived on pastures over winter in many instances. However, as the season progresses build-up of infective stage larvae on pastures will occur through the continuous shedding of eggs by infected individuals. With high pasture burdens this can lead to disease in lambs. Steps should be taken now to prevent this risk increasing later in the grazing season. A control plan based on a combination of diagnostics, pasture management and the strategic use of anthelmintics will reduce both risk of disease, and selection for anthelmintic resistance on your farm. Data from Parasite Watch up to the end of April indicates moderate and high counts are already being observed in the south and west of the country (Figure 1).

Advised actions include:

  • Routine worm egg count testing from early in the grazing season to monitor infection levels over time and help inform treatments (Figure 2).
    • Ideally, perform these every 2-4 weeks.
    • Egg counts can be performed on pooled faecal samples if taken from 10-12 individual animals.


Figure 2: Faecal egg counts are a useful way of evaluating various parasitic infections. This method can be used to identify PGE-causing roundworms (Str.), Nematodirus (N.b) and coccidial oocysts (arrows).

  • Monitor for signs of disease, particularly anorexia, diarrhoea, dehydration and unexplained weight loss (Figure 3).
  • Reduce pasture build-up and exposure through grazing strategy:
    • Try to graze animals on “safe” pastures, such as those not grazed by lambs the previous season where possible.
      • If safe grazing is limited at turn-out, reserve this for ewes with twins and triplets.
    • Move animals to additional safe pastures (e.g. hay or silage aftermath) later in the season as these become available.
      • Weaning is a good time to move lambs on to safe pastures.
      • Pastures grazed by last season’s lambs may be infective early in the season due to the presence of overwintered larvae, but will become safe as temperatures increase and these die off.

Figure 3: PGE commonly occurs in growing lambs and is associated with build up of infective larvae on pastures over the grazing season.

Lambs grazing permanent pastures will usually require worming to limit build-up of infective larvae. Where possible, avoid blanket treatments. Instead consider using “Targeted Selected Treatments” (TSTs) to reduce selection for anthelmintic resistance:

    • TSTSs can be directed at those animals most in need based on weight gain, body condition or worm egg counts.
    • Try to leave at least of 10% of the flock untreated. Often only 40-60% of lambs will require treatment when taking this approach.
  • Similarly, if dosing lambs ahead of moving them to safe grazing, the SCOPS group recommend reducing on-farm selection for anthelmintic resistance by:
    • Avoiding use of long-acting products such as injectable group 3-MLs.
    • Leaving animals on contaminated pastures for 2-3 days after treatment ahead of moving and/or…
    • Leaving some animals untreated by using TSTs.
  • When treating with anthelmintics, it is also worth considering post-treatment egg counts to confirm this been effective.
  • For more information and advice, please speak to your vet or SQP and see the SCOPS website.


The warm start to the year has seen forecast predictions for peak hatch of Nematodirus across much of England, Wales and Northern Ireland to occur in April, with peak hatch likely in parts of northern England and Scotland from May onwards. These forecast predictions are based on local climate data, and up-to-date risk in your area can be viewed on the SCOPS forecast. Data from the Parasite Watch is now showing positive faecal egg counts for Nematodirus in some high risk areas, confirming hatch has occurred on farms in these regions.

Nematodirosis is most common in lambs aged 6-12 weeks of age as they begin to graze more extensively. Whilst hatch may have occurred early in many areas, it is important to bear in mind the larvae can remain infective on the pasture for several weeks afterwards under optimal conditions. The dry weather experienced in April is likely to have increased death of these larvae on pastures, but it is still important to monitor at risk animals and take appropriate action where this is suspected.

Advised actions include:

  • Continue to monitor for signs of disease, including sudden onset diarrhoea, dehydration and death.
  • Where an outbreak of nematodirosis is identified, treatment with group 1-BZ is usually effective.
    • In acute cases of disease worm egg counts cannot be used reliably for diagnosis, but can be used to confirm treatment efficacy if taken 7-10 days after the date of treatment.
  • Routine worm egg counts for PGE will also help you to monitor Nematodirus infection in growing lambs as the season progresses (Figure 2).
  • For more information please speak to your vet or SQP.

Blowfly strike and ticks

The NADIS blowfly alert is based on daily temperature and rainfall data and is updated every 2 weeks over the course of the grazing season. This was predicting medium across most of England and south Wales by the 29th of May.

Female blowflies will lay their eggs in sites with dead or decaying material. The resulting maggots can cause extensive and severe disease and considerable discomfort for affected animals, with soiled back ends resulting from PGE foot rot lesions being particularly common sites for this condition to occur (Figure 4). Failure to treat even very small lesions promptly is a welfare issue and can lead to disrupted grazing, loss of condition, secondary infections and death. Disease severity depends upon a variety of factors including weather, with warmer, humid conditions favourable.

The lack of rain means that most lambs should be dry, which helps to increase the mortality of fly eggs and larvae, reducing strike risk.  Shearing will also significantly reduce the risk for ewes.  Of concern is that any rain in the next few weeks will start to increase the risk substantially, particularly for lambs. Continuation of the warm weather will gradually see the strike risk increase in more northerly areas over the next few weeks.


Figure 4: Wounds and footrot lesions are a common site for blowfly strike.

Ticks are another important consideration in sheep during the grazing season. These blood feeding parasites are very common in some areas, particularly where grazing is mixed with heath and woodland environments. In addition to the effects of their blood feeding behaviour, these parasites are important because of their role in transmitting a number of important diseases of both animals and people, including louping-ill in sheep. The conditions so far this year appear to have favoured ticks, with reports of high burdens on some farms. It is therefore advisable to be on the lookout for these parasites.

Advised actions include:

  • Consult the NADIS blowfly alert for up-to-date disease risk in your area.
  • Inspect stock daily for evidence of fly strike.
  • Management of fly populations early in the season through fly traps and prompt removal of dead stock can significantly reduce their numbers.
  • Prevention of diarrhoea through good parasite control will greatly reduce the risk of strike.
  • Dagging, crutching, shearing and treating lame sheep promptly can help to limit the effects of flystrike.
  • A number of chemical formulations can be used to aid in the prevention of blowfly strike.
    • These should be used in conjunction with the management points listed above.
    • Many of these products can also be used to treat blowfly strike where it occurs.
    • Some synthetic pyrethroid products also carry a licence for use against common tick species in sheep.
  • For more information and advice, please speak to your vet or SQP.


Cattle Parasitic Gastroenteritis (PGE)

Calves and youngstock in their first or second grazing season are at greatest risk of PGE (Figure 5). The relatively warm and dry weather experienced in April is likely to have reduced numbers of overwintered larvae on pasture at this time. However, appropriate measures should be taken to avoid significant build-up on pastures and risk of disease in grazing calves and youngstock later into the season. To achieve this, the COWS group typically recommend either set stocking with strategic anthelmintic dosing or close monitoring and therapeutic dosing as effective strategies.

Figure 5: 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).

If practicing set stocking with strategic anthelmintic dosing:

  • Animals grazing “high-risk” pasture (fields grazed by calves the previous year) should be wormed within 3 weeks of turnout to prevent burdens getting too high and limit pasture contamination.
  • Suitable strategic treatments include administration of a bolus wormer at turnout or repeated administration of shorter duration group 3-ML products at 6-8 week intervals until mid- to late summer when either safe grazing in the form of aftermaths comes available, or levels of over-wintered larvae have declined to insignificant levels.
  • Animals turned out and set stocked on “safe” pastures (fields grazed by sheep the previous year or previously arable) are unlikely to require worming until later into the grazing season.
  • Whilst this is a robust method for PGE control, strategic has a relatively high selection pressure for anthelmintic resistance. Regular performance testing through weight gain and diagnostics e.g. worm egg counts (including post-treatment efficacy testing) are therefore advised as a way to check your parasite control plan and anthelmintic doses are working as expected.
  • For farms taking this approach which have vaccinated youngstock for lungworm infection ahead of turnout it is important to dose at intervals which allow these animals exposure to infection if they are to develop a fully protective immunity (Figure 6).

If practicing therapeutic treatments:

  • Continue to monitor faecal egg counts, growth rates and/or body condition score
  • Be alert for signs of disease, specifically loss of appetite, weight and body condition and diarrhoea (Figure 5).
  • When taking this approach, it is also important to consider the risk posed by lungworm infection in unvaccinated animals (Figure 6).
  • Whilst this approach aims to minimise use of anthelmintics on farm, it is less effective at preventing sub-clinical production losses and will not prevent build-up of pasture larval burdens through the grazing season. 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.
  • A one-off mid-season tactical treatment may also be advisable in at-risk groups of animals where high pasture burdens are anticipated.

For more information on treatment and control options for PGE in cattle, please speak to your vet or SQP or visit the COWS group website.

Figure 6: Lungworm infection can be a very serious problem for grazing youngstock, particularly in the latter part of the season. Vaccination is a hugely valuable in reducing its impact, but planning of anthelmintic use over the course of the grazing season is equally important to allow immunity to develop. Without appropriate control, unvaccinated animals may still be at risk of sudden and severe disease.

Ectoparasites: Fly and tick control

As for cattle, but also lead to additional health concerns including transsion of several bacterial and viral diseases.

  • Nuisance flies, particularly face flies (Musca autumnalis), scavenge the surface of the skin, wounds, sweat, secretions, tears etc. and in the process can transmit diseases such as summer mastitis, New Forest Disease (“pinkeye”; Figure 7), and possibly BVD virus.
  • Ticks are common to many parts of the UK. This parasite can transmit both babesiosis (“redwater fever”) and tick-borne fever to cattle.

Figure 7: New Forest Disease (“pink eye” or infectious bovine keratoconjunctivitis) is a potentially serious bacterial infection of cattle which can be spread by nuisance flies.

 Control of ectoparasites, in particular ticks, can be problematic. However, options are available:

  • Some insecticide-impregnated ear tags and tail bands provide season-long protection against biting and nuisance flies if applied to the whole herd at the start of the grazing season.
  • Pour-on, spot-on and spray-on synthetic pyrethroids are also available for flies and can be used during periods of high fly activity to limit their impact.
  • Some synthetic pyrethroid and group 3-ML products may provide protection against ticks, although these products do not carry a licence for this purpose.
  • Additional environmental practices such as traps to reduce fly numbers and pasture improvement and management to reduce tick numbers may be helpful, but require sustained effort.

For more information on ectoparasite control please speak to your vet or SQP and see the COWS group website.

Joint SCOPS/COWS press release

A recent press release by SCOPS and COWS has warned farmers not to be complacent when it comes to fluke infection this year. Whilst infection levels last year appear to have been low and conditions to date this year have not particularly favoured fluke development it is advised this important parasite is given due consideration and not overlooked, as this could result in considerably higher levels of infection than might otherwise occur. In particular, farmers should consider testing for chronic infections and treating infected animals appropriately to reduce levels egg contamination on pastures later in the year when conditions may be more favourable for fluke development. You can read the full press release here.

John Graham-Brown BVSc (hons) MSc (hons) PhD MRCVS
June 2020


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