Parasite Forecast

Issue: September

Weather report

Figure 1: Egg count data shows the most recent counts for roundworms in sheep at each location between the sample dates stated.

July started out cool with showers, becoming dry and bright for most of the first half of the month, particularly in England and south Wales. The second half of the month was generally more unsettled with a hot spell from the 22nd to 26th which saw record-breaking temperatures in many parts of the country, including a new record for the UK as a whole before giving way to widespread thunderstorms.

The provisional UK mean temperature in July was 16.4 °C, 1.2 °C above the 1981-2010 long-term average. This equated to above average temperatures for July and and the previous 3 months (May to July) across all regions of the UK.

Rainfall was 114% of the long-term monthly average, but varied widely across the country, with below-average rainfall over most of Wales and southwest England and very high rainfall in other areas, notably western Scotland. Over the previous 3 months above average rainfall was present in all areas of the UK, particularly in Scotland, northern England and the midlands, with the exception of south Wales and southwest England where rainfall was below average for May to July.

Summer fluke forecast

The summer fluke forecast was first published in August of this year and is based on rainfall and temperature for the months of August-October 2018, and May-June 2019. This indicates risk from liver fluke emerging onto pastures early in the season from snails infected the previous season. This forecast is predicting high risk in western Scotland and moderate risk across the rest of Scotland, the whole of Wales and Northern Ireland and the west of England (Figure 2). It should also be noted ahead of the autumn risk period which that the warm and generally wet conditions experienced over the 2019 grazing season to date will likely result in an autumn fluke risk that is considerably higher than that experienced in 2018.

Figure 2: Summer fluke forecast 2019 by UK region.

It is therefore important to be on the lookout for signs of disease, particularly if you have a known history of fluke infection, and/or if you have animals grazing “flukey” pastures. Both sheep and cattle are susceptible to infection with liver fluke, although acute outbreaks are more common in sheep, with signs including:

  • Sudden death in heavy infections
  • General dullness, anaemia and shortness of breath
  • Rapid weight loss, fluid accumulation (e.g. bottle jaw)

 Advised actions include:

  • Monitoring for signs of disease.
  • Routine diagnostic testing to give a greater insight into the current infection:
    • Post-mortem in acute outbreaks allows for a definitive diagnosis.
    • Worm egg counts can be used to diagnose infection in individuals, or groups of animals when using a composite sample.
      • Egg counts cannot detect pre-patent infection and should not be relied upon for the diagnosis of acute disease.
    • Where acute disease occurs, treatment with triclabendazole is recommended as this is the only product effective against both adult and immature stages of the parasite.
      • Due to growing concerns over drug resistance it is also advised such treatments are accompanied by resistance testing at 21 days post-treatment to monitor efficacy.
      • Where drug failure is present and resistance suspected, please seek veterinary advice.
    • Risk of infection can be reduced by identifying high risk “flukey” pastures and avoiding grazing them during peak risk periods.
      • Mud snails are generally found in damp, muddy areas such as the borders of permanent water bodies, wet flushes (often identifiable through presence of rushes and other water loving plant species), ditches, boggy areas etc.
      • Pastures previously grazed by fluke infected sheep should be considered a risk to cattle and vice versa.

Quarantine treatments

When buying in new stock, it is important to remember that any animals coming onto your farm could be a potential source of new parasites and/or drug resistance. As a consequence, quarantine measures should be undertaken.

As with all anthelmintic dosing, it is important to check you are dosing correctly by weighing animals, calibrating your dosing and weighing equipment, and administering treatments following the manufacturer’s recommendations (Figure 3).

Figure 3: Use of weigh crates/ crushes are useful in monitoring of weight-gain for targeted selective treatments, and as a way of ensuring you are administering the correct dose to each individual animal. Under-dosing can contribute to selection for anthelmintic resistance, whilst overdosing adds unnecessary cost and risk too your treatments.

For sheep, current ‘gold standard’ practice advised by Sustainable Control of Parasites in Sheep (SCOPS) is as follows:

  1. To prevent introduction of resistant roundworms, administer sequential treatments of both 4-AD (monepantel) AND 5-SI (derquantel in combination with abamectin) products.
  2. Where introduction of sheep scab is of concern, treat either with injectable moxidectin, or an organophosphate dip.
  3. Hold purchased animals away from pasture for 24-48 hours post treatment, then turn out to “dirty” pasture previously grazed by sheep.
  4. Where introduction of triclabendazole-resistant liver fluke is a concern:
    1. Treat animals with either 2 doses of closantel, 6 weeks apart, or 2 doses of notroxynil, 7 weeks apart. Alternative strategies using a combination of products may help prevent introducing resistance to any one particular flukicide (eg. closantel). For more advice on this, please seek veterinary advice.
    2. Hold purchased animals away from pasture, or on grazing free of mud snails (dry, well drained pastures) until at least 4 weeks after treatment.
  5. Maintain purchased stock separately for at least 30 days to monitor for disease before mixing.
  6. For additional details on these and alternative treatment options see the SCOPS quarantine guidelines, and seek veterinary advice

For cattle, Control of Worms Sustainably (COWS) guidelines highlight the importance of knowing disease status of purchased animals or their farm of origin.

  1. For roundworms, particularly 3-ML resistant Cooperia and/or lungworm:
    1. House animals on arrival and implement control measures ahead of turn out in spring.
    2. Treat with an effective anthelmintic. For example, for 3-ML resistant Cooperia treatment with either a 1-BZ or 2-LV product is advised.
    3. Test efficacy of treatments.
  2. For ectoparasites such as mites and lice, treat with a 3-ML or permethrin product to prevent spread during winter housing
  3. For liver fluke, specific guidelines have been developed for bought in cattle based on the three basic principles of HOUSE, TREAT and TEST.


Sheep Parasitic Gastroenteritis (PGE)

Heavy roundworm burdens can cause severe disease in lambs, with signs including loss of appetite, diarrhoea, dehydration, weight loss and death in heavy infections. Lower levels of infection may have no obvious signs but can still impact performance, particularly weight gain.

The relatively warm and wet conditions experienced so far this season may have allowed survival of the infective larvae on pastures later into the season than usual, with egg count data from Parasite Watch showing medium and high egg counts in sheep flocks across Great Britain between May and July (Figure 1). It should also be noted that on some farms high egg counts for Nematodirus battus are still being observed.

Consequently, it is important to stay alert for signs of PGE, particularly in groups of lambs grazing “dirty” pastures, namely those grazed by ewes earlier in the season, or lambs over the previous season.

Advised actions include:

  • Monitoring for signs of disease.
  • Implementation of targeted selective treatments (TSTs) by monitoring performance indicators such as weight gain (Figure 3), or worm egg counts for evidence of infection.
    • TSTs based on performance indicators require routine monitoring, ideally every 3-4 weeks, and accurate record keeping.
      • Generally only 40-60% of lambs should require treatment based on weight gain. You should aim to leave at least 10% of the flock untreated to reduce selection for anthelmintic resistance.
    • Egg counts can be performed on a pooled faecal samples taken from 10-12 individual animals to monitor infection in the group as a whole.
      • Treatment based on worm egg counts may be indicated where average worm egg counts are greater than 500-700 epg. If you are unsure how to interpret your egg count results, please speak to your vet or SQP.
    • For lambs grazing dirty pasture, consider dose and moving to safe pastures (e.g. silage aftermath) as these become available. When performing dose and move, to reduce selection for anthelmintic resistance:
      • Leave animals on dirty pasture for 2-3 days post-treatment prior to moving.
      • Avoid dosing with long-acting group 3-ML products
      • Leave at least 10% of the flock untreated
    • Where anthelmintic treatments are administered, it is advised to check efficacy at 7-14 days post treatment depending upon the product used.
      • If anthelmintic resistance is suspected, strategic use of group 4-AD or 5-SI wormers may be indicated under veterinary guidance. For more information see the SCOPS guidelines, and seek veterinary advice.


Haemonchus contortus or the barber’s pole worm is another type of PGE-causing roundworm. Unlike other PGE-causing roundworms, haemonchosis affects both lambs and ewes. Whilst it is less common than other roundworms it is important to consider, especially given the recent weather. Heavy pasture burdens can result in sudden and severe disease with little to no prior warning. A number of cases have been seen in recent months (Figure 4)


Figure 4: Haemonchosis has been identified a number of times this season. Left: a ewe in Dorset at the end of July displaying signs of bottle jaw and a worm egg count of 2750epg (Photo credit: Emily Gascoine, Synergy Farm Health; @Em_the_SheepVet). Right: Adult worms recovered from the stomach of a severely anaemic lamb at post-mortem in Berkshire in July (Photo credit: JP Crilley, Larkmead Veterinary Group; @flock_health)

Advised actions include:

  • Monitoring for signs of disease:
    • Generally acute onset with anaemia and general fatigue.
    • Oedema or fluid accumulation (including bottle jaw; figure 4).
    • Sudden death in heavy infections.
    • Haemonchosis does not usually present with diarrhoea.
    • Chronic infections may also occur, characterised by progressive weight loss, anaemia and loss of appetite.
  • Given the presenting signs described, haemonchosis can appear similar to fasciolosis. Diagnosis to further distinguish can be achieved through:
    • Post-mortem an identification of adult worms in cases of sudden death associated with severe outbreaks (Figure 4).
    • Worm egg counts. These are generally very high and can be differentiated from other species of intestinal roundworm through specialist techniques. For more information, please speak to your vet.

Haemonchosis can be treated with most anthelmintic products, although some evidence of resistance to white drenches (1-BZ) has been reported previously in the UK. Some flukicidal products, such as nitroxynil and closantel are also effective against Haemonchus contortus and should be considered in certain cases.

Blowfly strike

Failure to treat fly strike 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 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 high risk in Northern Ireland, northern and eastern Scotland, and severe risk across the rest of the UK at the end of July.

Advised actions include:

  • Inspect stock daily for evidence of strike, particularly during high-risk periods.
  • On-farm disease risk can be reduced significantly by:
    • Management of fly populations from early in the season
    • Prevention of diarrhoea through good parasite control
    • Dagging, crutching, shearing and treating lame sheep promptly
  • 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.
  • For more information and advice, please speak to your vet or SQP.


Cattle PGE

The wetter than usual conditions experienced so far this grazing season may have prolonged survival of infective larvae on pastures to some extent, especially Ostertagia ostertagi, which can remain on pastures in large numbers until the end of the grazing season under normal conditions. Young stock in their first grazing season may therefore still be at risk of type-1 ostertagiosis, particularly autumn and winter-born weaned calves entering their first grazing season and spring-born beef suckler calves entering their second grazing season. Set stocking on permanent pasture also increases risk due to the continual shedding of eggs by grazing animals over the season. It is therefore important to continue monitoring for signs of disease and/or poor performance.

Advised actions include:

  • Monitoring for signs of PGE in at-risk groups:
    • Loss of appetite
    • Loss of weight and body condition
    • Profuse diarrhoea
  • Animals which have been set stocked on the same pasture and received strategic dosing up to this point should be moved to “safe” pastures (hay or silage aftermaths) as they become available.
    • Due to strategic dosing earlier in the season these animals should have relatively low larval burdens. If there is any doubt, worm egg counts can be used to check level of infection and effectiveness of your control program:
      • Perform counts on faecal samples (10g) taken from 10-15 calves selected for treatment.
      • Repeat sampling of animals with a previously high egg count (e.g. more than 200 eggs per gram) at 14-17 days post treatment (7-10 days for 2-LV group products) to assess efficacy.
    • Where strategic dosing is not implemented, risk of disease peaks during the summer months, but could remain high until the end of the season, particularly given the recent weather.
      • It is therefore essential to monitor for signs of disease in such animals.
      • Monitoring liveweight gain and/or worm egg counts will help to identify infections before clinical signs occur and reduce production losses.
    • Where dosing with anthelmintics is indicated:
      • Treat all animals in groups where there is an outbreak of clinical disease.
      • Considering the COWS group’s “5 Rs” to ensure your worming strategy is both effective and sustainable. These include:
        • Considering the type of wormer used – ongoing repeated use of the same active/ wormer group or use of certain wormers at inappropriate times may increase selection for resistance.
        • Dosing appropriately by weight of each animal and ensure correct calibration of dosing guns/syringes/applicators
      • Use worm egg counts to check for effective anthelmintic dosing as described.
    • For more information, discuss this with your vet or SQP, and see the COWS group guidelines.


Lungworm infection (or “husk”) occurs in animals at pasture over the summer and autumn months. Outbreaks are difficult to predict, but may be associated with wetter summers and following periods of wet weather.

On farms with a history of lungworm, unvaccinated calves that have not been part of strategic dosing programmes should be considered at risk of disease, as should older bought-in cattle from farms without a history of lungworm.

Calves that have been vaccinated ahead of turnout will be protected against clinical disease, although these animals require some natural infection to develop a fully protective immunity for subsequent grazing seasons. Individuals that did not receive sufficient natural challenge following vaccination may still be at risk of disease the following year.

Advised actions include:

  • Monitor for infection. Early signs include:
    • Widespread coughing in the group, initially after exercise then at rest.
    • Increased respiratory rate and difficulty breathing (Figure 5).
    • Rapid loss of weight and body condition
    • Milk drop in lactating cattle
    • Death in heavy infections
  • Where infection is suspected:
    • Treat all animals within the affected group
      • Most roundworm products are effective.
      • Severely affected animals may require additional treatments (eg. anti-inflammatories and antibiotics)
      • Consider withdrawal periods in lactating animals.
    • Affected cattle should be removed from contaminated to “safe” pasture (e.g. aftermath) or housed in a well-ventilated building.
    • Infection can be confirmed by:
      • Post-mortem of dead animals
      • Observation of larvae in saliva or faecal samples in patent infections
      • Serum and milk sample antibody ELISAs are also available.
    • For more information, please speak to your vet or SQP, see “COWS” group guidelines and see our recent NADIS lungworm webinar.

Figure 5. Early signs of lungworm infection include widespread coughing and elevated respiratory rates and difficulty breathing. Depending on their history, both youngstock and (in some cases) adult cattle can be at risk of infection.


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


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