Parasite Forecast

Issue: May

Weather report

Early to mid-March saw high levels of rainfall with storms Freya and Gareth bringing particularly wet and windy weather. Temperatures were mostly mild, with cold spells, sleet and snow further north. From March 18th conditions were generally mild, settled and dry.

The provisional UK mean temperature in March 2019 was 6.8 °C, 1.3 °C above the 1981-2010 long-term average. Regionally, above average temperatures were observed across all areas both in March and for the previous 3 months from January-March. Unlike most of the winter months, March also saw relatively high levels of rainfall, 140% of the long-term average. This was the case across all regions in March, although the longer-term 3 monthly average rainfall still shows below average rainfall for all regions except Northern Ireland, NW England and N Wales due to the relatively dry months of January and February.


Sheep Parasitic Gastroenteritis (PGE)
  1. Nematodirosis

Due to the unseasonably warm temperatures in February and March, this year’s NADIS Nematodirus forecast has predicted a very early hatch, with peak risk likely to have occurred across most of the country at this time. Similarly, the SCOPS Nematodirus forecast has been predicting increasing risk to “high” and “very high” in many locations throughout April.

Unlike most PGE-causing roundworms, Nematodirus battus infection passes directly from one season’s lamb crop to the next meaning pastures grazed by last season’s lambs should be considered high risk. Eggs survive on pasture over winter and develop the following spring in response to increasing temperatures. The resulting mass emergence or “peak hatch” period of infective stage larvae is what is predicted in the above forecasts. Risk of disease (nematodirosis) is generally greatest if peak hatch occurs at the same time lambs are starting to graze extensively, typically 6-12 weeks of age. Whilst the early predicted hatch this year is likely to have happened before many lambs are this age it is important to remember that in wet, cool conditions infective larvae can survive on pastures for several months. Consequently, “high risk” pastures may potentially remain a risk to grazing lambs into the summer.

Advised actions include:

  • Consulting the SCOPS and NADIS forecasts for more information on Nematodirus risk in your area.
  • Identify high risk pastures and avoid grazing the current season’s lambs here during and following peak risk periods.
    • Where no safe grazing is available, prophylactic treatment with a group 1-BZ product may be advisable. For more information please speak to your vet or SQP.
  • Continue to monitor for signs of disease including sudden onset diarrhoea, dehydration and death. Affected animals normally present with heavily soiled back ends, lack of appetite and a profound thirst (Figure 1).
  • Where disease occurs, treatment with group 1-BZ is usually effective.
    • Ensure correct dosing by weight and administration using correctly calibrated drenching equipment.
    • While worm egg counts cannot be used to identify acute (sudden onset) battus infections, they can be used to confirm treatment efficacy if taken 7-10 days after the date of treatment.


Figure 1: Nematodirus battus infection can cause sudden onset, severe diarrhoea in first season lambs often with characteristic soiling around the back end. Soiled back ends are also a common site for blowfly strike

  1. Other PGE-causing roundworms

Infected ewes will show little or no obvious signs of disease, but can have very high worm egg counts around lambing time (referred to as the periparturient rise or “PPR”). Disease generally occurs in lambs later into the grazing season as pasture contamination accumulates.

Advised actions include:

  • Regular worm egg count testing to inform treatment.
  • Monitor for signs of disease, particularly anorexia, diarrhoea, dehydration, weight loss and death.
  • When treating PPR in adult ewes, SCOPS recommend avoiding blanket treatment to minimise risk of selecting for anthelmintic resistance.
    • As a rough guide, aim to leave at least 10-20% of the flock untreated.
    • Target treatment in animals with high worm egg counts (Figure 2), low body condition score, twin/triplet or first season ewes
    • If treating with ewes with a group 3-ML seek veterinary advice first: Long-acting worming products can pass on to lambs through milk, potentially leading to unwanted selection for anthelmintic resistance.
  • To reduce disease risk in lambs later into the grazing season avoid turn-out onto pastures grazed by last season’s lambs. These may still have relatively high infectivity due to presence of overwintered larvae.
    • These pastures will become useable later in the grazing season as overwintered larvae die off.
    • Where grazing is limited, reserve safe pastures (e.g. reseeded pastures or those grazed previously by cattle) for ewes with twin lambs.



Figure 2: 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).

  1. Coccidiosis

Another parasitic disease of importance in growing lambs is coccidiosis. This is caused by protozoal (single celled) parasites and the rapid accumulation of their infective “oocsyts” in the environment leading to overwhelming infection and disease. Unlike roundworm infections, coccidiosis can affect housed animals as well as those at pasture (Figure 3).

This disease is commonly associated with high intensity husbandry systems and stocking densities as well as stress factors such as poor colostrum supply, adverse weather conditions at wet muddy paddocks previously grazed by sheep and/or extended housing periods.

Coccidiosis typically affects later born lambs aged 4-8 weeks. Signs include anorexia, weight loss, diarrhoea (with or without blood) and death in severe cases. Due the similarities in presenting signs and ages of affected animals, in grazing lambs it is important to determine whether coccidia or Nematodirus infection is present and causing disease. Coccidial oocysts can be detected in faecal egg counts (Figure 3). Further testing to determine if these are a disease-causing species is also advised when deciding whether or not to treat.

Figure 3: Unlike PGE caused by roundworms, coccidiosis can affect both housed and grazing lambs.

To reduce risk of disease outbreaks:

  • Reduce stocking densities and batch rear lambs by age.
  • Avoid putting younger at risk animals out to pastures or areas heavily used by older lambs earlier in the year, or towards the end of last year.
  • Ensure adequate provision of creep feed, particularly during periods of inclement weather.
  • 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.

Blowfly strike

As new adult flies emerge in the spring and become active it is important to consider blowfly strike and plan accordingly. Fly strike caused by blowfly maggots, commonly greenbottles (Lucilia sericata; Figure 4) is a hugely important disease in terms of its animal welfare and economic impact. Blowfly strike affects around 80% of UK sheep flocks each year, with an estimated cost of £2.2 million per annum to the UK sheep industry through loss in productivity, fleece damage, treatment costs and death in severe cases.

Figure 4: Blowfly strike is caused by adult female greenbottles laying eggs in wounds and soiled fleece. Maggots then hatch and cause extensive damage.

Female flies are attracted to odours produced by decomposing matter. Soiled back ends resulting from PGE (Figures 1 & 3), foot rot lesions (Figure 5), dermatophilosis (lumpy wool), urine scalding around the prepuce and shearing injuries are all common sites for fly strike. 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. As was the case last year, NADIS produces a blowfly alert based on daily temperature and rainfall data and updated every 2 weeks over the course of the grazing season.

Figure 5: Wounds and footrot lesions are a common site for blowfly strike

Advised actions include:

  • Consult the NADIS blowfly alert for up-to-date disease risk in your area and further advice on treatment and control strategies.
  • Inspect stock daily for evidence of strike, particularly during high-risk periods. Early signs include irritation, nibbling of the tailhead and increased tail swishing, rubbing and further signs of discomfort in lame animals.
  • Prevention of diarrhoea through good worm and coccidial control will greatly reduce the risk of blowfly strike.
  • Dagging, crutching, shearing and treating lame sheep promptly can help to limit the effects of flystrike.
  • Include blowfly protection, including monitoring during risk periods and the need to control other parasites in your parasite control plans.


Cattle Parasitic Gastroenteritis (PGE)

If practicing set stocking with strategic anthelmintic dosing to control PGE in grazing calves and young stock:

  • 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 to 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.
  • Regular performance testing through weight gain and diagnostics e.g. worm egg counts (including post-treatment efficacy testing) are a useful way to check your parasite control plan is working.

If taking a “wait and see” approach to PGE control in youngstock:

  • 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 6).
  • Perform targeted selective treatments in animals where needed.

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: Common signs of PGE in young stock include loss of appetite, body condition and diarrhoea.


On farms with a history of lungworm infection, vaccination offers a valuable tool for protecting against disease in calves.

  • 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.
  • Where possible, try to turn vaccinated calves out onto lungworm-contaminated pastures in order to allow their immunity to develop fully.
  • 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 the following season may be recommended.
  • For more information, please speak to your vet or SQP, see “COWS” group guidelines and see our recent NADIS lungworm webinar.

Ectoparasites: Fly and tick control

As temperatures increase, fly and tick activity will resume. These pests can cause their own problems and irritations for cattle, but also lead to additional health concerns:

  • Biting flies including stable flies (Stomoxys), horn flies (Haematobia), head flies (Hydrotaea), horse flies (tabanids), midges (Culicoides) and blackflies (Simulium) which puncture the skin and feed on blood may transmit 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, in particular the species Ixodes ricinus, 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.

Chronic fluke infections

Due to the mild winter, development of liver fluke on pastures may have continued later and longer than usual. Consequently, treatments given during the “usual” risk period in autumn may not have been effective in controlling disease, with deaths caused by chronic fluke infection currently being reported in some areas of the UK (Figure 8).

Advised action include:

  • Monitoring for signs of disease. Chronic infection is characterised by weight loss, anaemia and fluid accumulation (e.g. “bottlejaw).
  • If there is doubt, consider diagnostic testing such as fluke egg counts
  • Abattoir feedback and post-mortems are another useful way of monitoring fluke infection in your herd or flock.
  • Where treatment is indicated, consider use of an alternative product to triclabendazole to treat chronic fluke infections.

Figure 8: Chronic liver fluke in a shearling ewe at lambing. Presenting signs were poor condition; this was the 3rd death in a group of 200. April 2019, North York Moors (Photo credit: Ben Strugnell, Farm post mortems Ltd.)

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


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