Parasite Forecast

Issue: July

Weather report


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

May began with showery periods, frosts and wintry conditions in northern and eastern regions giving way to warmer, sunny weather for most of the month, with generally cooler changeable conditions towards the end of the month and some warm weather in the south and east.

The provisional UK mean temperature in May was 10oC, 0.3oC below the 1981-2010 long-term average. Regionally, mean temperatures were at, or slightly below the monthly average. However, the regional average for the previous 3 months (March-May) was still above average across all regions.

Rainfall was 93% of the long-term monthly average for May. Regionally this varied, with very dry conditions observed in Wales and southern England, and above average rainfall in north-east Scotland.


Sheep Parasitic Gastroenteritis (PGE)

PGE is caused by roundworms present within the digestive tract. Heavy worm burdens can cause serious 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 lamb performance, particularly weight gain.

Eggs shed in the faeces of already infected individuals and develop into infective stage larvae which are then ingested through grazing (Figure 2). Early in the grazing season, pregnant or recently lambed ewes are an important source of pasture contamination due to the increased egg output associated with the peri-parturient rise. These larval burdens then continue to accumulate on pastures where ewes and their lambs continue to graze as the season progresses.

Figure 2: Pasture contamination results from the development of eggs passed in faeces to infective stage larvae. Without appropriate control, large numbers of eggs and warmer temperatures later in the season can lead to high levels of pasture contamination and disease.

Weaning lambs are at greatest risk from PGE during the summer months due to the increased levels of pasture contamination combined with their increasing grazing activity, and since they have yet to develop a protective immunity.

Temperature plays an important role in development and survival of eggs and larvae at pasture, with faster rates of development at higher temperatures. The above average temperatures observed over the preceding 3 months (March – May) could therefore increase the level of pasture contamination expected at this time of year. Wet periods of weather may also help to liberate infective larvae onto pasture from faeces in relatively large numbers. Egg count data from Parasite Watch show medium and high egg counts observed in groups of sheep across Great Britain between April and May (Figure 1).

Advised actions include:

  • Monitoring for signs of PGE in “at risk” animals: Weaned lambs grazing “dirty” pastures, such as those grazed by ewes earlier in the season or lambs over the previous season.
  • Monitor performance using indicators such as weight gain, ideally every 3-4 weeks, and faecal egg counts for evidence of infection.
    • To evaluate levels of infection within groups of animals, egg counts can be performed on a pooled faecal sample taken from 10-12 individual animals.
    • For performance monitoring to be effective, accurate record keeping is essential.
  • Where treatment is required, consider the SCOPS guidelines to reduce selection for anthelmintic resistance:
    • Targeted selected treatments (TST) can help to limit the number of treatments required on farm, reducing both overall cost of treatment and selection for anthelmintic resistance. Generally, only 40-60% of lambs require worming based on performance indicators such as weight gain.
    • Ensure correct dosing by weight, and check the calibration of dosing guns/syringes prior to use by using a measuring jug or similar to check the expected dose is being administered.
    • Rotate worming products, but avoid using those against which resistance is already established. Where resistance against one or more anthelmintic groups is present, group 4-AD and 5-SI wormers may be included as part of a structured rotational worming programme planned and implemented under veterinary direction.
    • Perform post-treatment egg counts to test treatment efficacy. Different products require post-treatment testing to be carried out at different times ranging from 7-14 days. For more information please speak to your vet or SQP.
  • Where available, move animals to “safe” grazing (e.g. hay or silage aftermath).
    • 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.
    • If treating AND moving animals to safe pasture, it is recommended to:
      • Avoid the use of long-acting products such as group 3-MLs
      • Delay movement for a short period (2-3 days) post-treatment to allow lambs to become re-infected at low levels. This will prevent only resistant parasites being moved to the new clean pastures and/ or…
      • Leave some animals untreated, ideally not less than 10% of the flock.
    • For more information and advice, please speak to your vet or SQP and see the SCOPS website.


Haemonchus contortus or the barber’s pole worm is another type of PGE-causing roundworm. Unlike other PGE-causing roundworms, Haemonchus feeds on blood. A single worm may consume up to 0.05ml of blood per day, meaning in heavy infections large volumes of blood loss can occur rapidly. Consequently, whilst infection in the UK is less common than PGE, disease onset can be sudden and severe.

  • Acute onset disease is characterised by:
    • Anaemia, observable as pallor of the tissues around the eyes (Figure 3).
    • Oedema or fluid accumulation, including “bottle jaw” resulting from blood feeding.
    • Sudden death in heavy infections.
  • Chronic infections may also occur, characterised by progressive weight loss, anaemia and loss of appetite.
  • Unlike PGE, Nematodirus and other intestinal parasitic infections, haemonchosis does not usually present with diarrhoea.
  • Also unlike PGE and Nematodirus, both lambs and ewes are considered at risk.
  • Females are prolific egg producers (up to 10,000 eggs per day), meaning that under optimal conditions heavy pasture contamination can occur in a very short space of time.

Figure 3: Pale mucous membranes of the eyes are an indication of severe anaemia.

Given the presenting signs described above, Haemonchosis can appear similar to Fasciolosis. Diagnosis to further distinguish can be achieved through:

  • Faecal egg counts, which will typically be very high with patent Haemonchus
    • Additional testing is available to distinguish Haemonchus eggs from those of other PGE-causing roundworms
  • In acute cases with sudden death, post-mortem is a valuable way of making a definitive diagnosis. Large (2-3cm), robust barber’s pole worms are visible on the lining of the abomasum (true stomach), which may appear damaged and haemorrhagic.

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.

Monezia tapeworms

Adult tapeworm segments may be seen in the faeces of lambs during summer months (Figure 4). These are acquired through consumption of infected intermediate hosts (orbatid mites) living on the pasture.

Monezia species tape worms are generally not considered as a cause of disease in livestock. Roundworm treatment with a white drench (1-BZ) is generally effective against these tapeworms.

Figure 4: Tapeworm segments (Moniezia) are commonly seen in faeces passed by lambs during the summer months. These are generally considered non-pathogenic.

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 (Figure 5).

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 moderate risk for most of England, Wales and Northern Ireland by the end of May.


Figure 5: Blowfly strike is a major welfare concern, often resulting in death if left untreated.

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 Parasitic Gastroenteritis (PGE)

The situation with PGE in cattle is similar to that described for sheep, although the worms which cause disease are different. Of these, the brown stomach worm (Ostertagia ostertagi) is the most important, although co-infection with other roundworms such as Cooperia can contribute to and further exacerbate disease, characterised by a loss of appetite, weight and body condition accompanied by profuse diarrhoea (Figure 6).

Calves and young stock are most at risk, particularly autumn and winter-born weaned calves entering their first grazing season and spring-born beef suckler calves entering their second grazing season, with PGE reducing growth rates by up to 30%. Additionally, adult cattle may also experience performance limiting disease if grazing heavily contaminated pastures; PGE may reduce a lactating dairy cow’s daily milk yield by up to 1 kg per day.

Temperature plays an important role in development and survival of eggs and larvae at pasture, with faster rates of development at higher temperatures. The above average temperatures observed over the preceding 3 months (March – May) could therefore increase the level of pasture expected at this time of year.

Figure 6: Clinical cases of PGE are typically characterised by loss of appetite accompanied by a profuse, green diarrhoea affecting large numbers of animals.

To control PGE in cattle:

  1. Set stocking with strategic dosing from early in the season is designed to protect young stock against PGE by limiting infection with over-wintered larvae present on pastures, and prevent subsequent build-up of pasture contamination later in the season.
  • Such animals should remain set stocked and moved to “safe” pastures (hay or silage aftermaths) as they become available later in the grazing season.
  • Bolus wormers at turnout, or repeated administration of shorter duration group 3-ML products at 6-8 week intervals until mid- to late summer are appropriate forms of treatment.
  1. Where strategic dosing is not implemented, risk of disease peaks during the summer months due to increasing pasture contamination with infective larvae and rising temperatures shortening development time from eggs.
  • Consider turning spring born calves out onto “safe” grazing such as hay or silage aftermath to minimise their exposure.
  • In lactating dairy cattle a bulk-tank antibody ELISA is available to monitor infection levels and inform treatment decisions.

Advised actions include:

  • Monitor for signs of PGE in at risk groups:
    • ­­­­Young stock (first and second season grazing).
    • Animals grazing “dirty” pastures, especially those where no strategic dosing has been given.
  • Where dosing with anthelmintics is indicated:
    • In the event of an outbreak of clinical disease treat all animals in the affected group.
    • 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
    • Consider faecal egg counts to check for effective anthelmintic dosing. Discuss this with your vet or SQP, or see the COWS group guidelines.


Lungworm infection (or “husk”) can occur from June onwards. Outbreaks are difficult to predict, but may be associated with wetter summers and following periods of wet weather. In areas where lungworm is known to be present, it is recommended first season calves are vaccinated prior to turn-out.

Unvaccinated calves and those not part of strategic dosing programmes should be considered at risk. Older cattle may also be at risk if they have not developed an effective immune response previously. This can be due to a lack of previous exposure, notably bought-in cattle on farms with a known history of lungworm. Effective worm control in previous seasons not allowing immunity to develop - vaccinated animals require subsequent natural exposure to lungworm to develop full immunity.

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.
    • 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 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
      • In patent infections larvae can be detected in saliva or faecal samples (Figure 7)
      • 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 7: Lungworm larvae passed in the faeces of infected animals can be used to diagnose infection


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


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