Welcome to NADIS. Are you a...

Vet SQP Farmer/Animal Keeper Industry Professional Researcher Not In Animal Health Industry

You will only be asked this once.

Editorial Information

Professor Mike Taylor BVMS PhD MRCVS DipEVPC DipECSRHM CBiol MRSB

Published 2017

Parasite Forecast - May 2017

17-05 PF Capture

The weather in March was mild with mean maximum temperatures 1.5 - 2.5°C above average across most of the country. Rainfall was below average in the south-east and in parts of Scotland, but above average in Wales, north-west England and the Borders.

May Parasite Forecast/Update



Parasitic Gastroenteritis (PGE)

  • Ewes grazing heavily infected pastures and treated with a short-acting wormer at turnout, may require further worming treatments to limit pasture contamination during the 'peri-parturient rise" (PPR).
    • Aim to leave a proportion (~10%) of the ewes untreated by targeting treatments based either on body condition or faecal egg counts (FEC).
    • Ewes treated with a persistent anthelmintic before turnout should require no further treatment.
  • Ewes on safe grazing shouldn't need worming again.
  • Lambs on safe grazing shouldn't need worming until after they are weaned.
  • Ideally wean the lambs onto aftermaths, with decisions to treat based on either clinical signs, or on faecal egg counts.
  • Lambs on contaminated pastures should be wormed from 6 weeks of age onwards, based on the Nematodirus risk (see below), and for other worm species by monitoring clinically, or by FEC.


  • Severe outbreaks of disease due to the parasite, Nematodirus battus may occur in May depending on several factors:
    • A cold late spring followed by a sudden rise in temperature can trigger a synchronised, mass hatch of infective larvae on heavily contaminated pastures grazed by last year's lamb crop.
    • If this coincides with the presence of susceptible, 6-12 week-old lambs then scouring, severe production losses, and sudden deaths may follow.
  • March was mild with mean temperatures 1.5 - 2.5°C above average across most of the UK continuing the trend for above average temperatures over the last few months.
  • Consequently, the NADIS and SCOPS Nematodirus forecasts by mid-April, were predicting an early spring hatch spreading northwards with moderate to high risks for early lambing flocks with susceptible grazing lambs.
  • Continue to monitor the NADIS ( ) and SCOPS ( websites for updates on the situation as temperatures rise in the north.
  • White drench (1-BZ) wormers are still generally recommended to control Nematodirus and in severe outbreaks it is important to treat all lambs in the group immediately.
  • The FEC of several lambs should be checked 10 days later for the presence of other worm species, which if present, would require treatment with a wormer from another group due to 1-BZ resistance.


  • Later born lambs, particularly twins grazing contaminated pastures, may be affected by coccidiosis around 4-8 weeks of age.
  • Adverse weather conditions, poor colostrum supply, overcrowding, wet muddy paddocks previously grazed by sheep, and/or extended housing periods all predispose.
  • Avoid heavily contaminated pastures, particularly those grazed by earlier born lambs and those grazed by last year lamb crops.
  • Batch rear lambs of similar ages and consider creep feed especially during periods of inclement weather.
  • Disease prevention can also include strategic dosing lambs on contaminated pastures with diclazuril or toltrazuril at around 3-4 weeks of age, or providing medicated creep feed containing decoquinate for 28 days.
17-05 PFF1
Lambs on safe grazing should not require anthelmintic treatment until after weaning. Treatments can be further avoided by moving weaned lambs to aftermaths.

17-05 PFF2

Scour in lambs grazing contaminated pastures may be caused by nematodirosis or coccidiosis. For advice on diagnosis and correct treatment consult your veterinary surgeon.

Liver fluke

  • A high-risk of liver fluke disease was predicted for Scotland, NW England, and North Wales for last autumn/winter.
  • Sheep on farms with a known fluke problem in these regions should be dosed in the spring with a flukicide active against adult flukes to limit pasture contamination with fluke eggs.
  • Avoid the use of triclabendazole as it may hasten the selection for resistant strains of liver fluke.
  • High rainfall in May and June favours proliferation and infection of the snail intermediate hosts, which may lead to increased risk of severe fluke infections later in the season.
  • NADIS will be producing more localised fluke forecasts later this year - watch out for further details.

Blowfly strike

  • Fly strike caused by the larval stages (maggots) of the blowfly Lucilia sericata (greenbottles) affects around 80% of UK sheep flocks each year.
  • The severity of fly strike is highly variable depending on several factors including the weather.
  • Failure to treat promptly is a welfare issue and can lead to reduced performance, secondary infections and death.
  • Even very small fly strike lesions cause disrupted grazing and rapid weight loss.
  • Female flies are attracted by the odour of decomposing matter such as wounds, soiled fleece or dead animals.
  • Footrot lesions, dermatophilosis (lumpy wool), and urine scalding around the prepuce also attract egg-laying adult flies.
  • Preventing diarrhoea caused by parasitic worms will greatly reduce the risk of blowfly strike on the breech.
  • The blowfly season usually extends from May to September but with changing climate the season can be from March through to December in some regions.
  • Parasite control plans should include blowfly protection during the fly-risk period and fit with the need to control other parasites.
  • Topical preparations containing the insect growth regulators (IGRs) cyromazine and dicyclanil, which prevent blowfly strike, should be applied before the identified risk period.
    • Cyromazine provides protection against blowfly strike for up to 10 weeks.
    • Products containing dicyclanil afford 8-19 weeks' protection against blowfly strike depending on product choice.
  • Repeat treatments may be required, depending on the product used, and season length, necessitating careful planning when treating lambs due to the long meat withdrawal periods.
  • Pour-on preparations containing cypermethrin provide protection against fly strike for up to 6 to 8 weeks; alpha-cypermethrin products provide protection for 8-10 weeks.
  • These products can also be used for the treatment of active maggot infestations.
  • Deltamethrin spot-on products are used for treatment of blowfly strike only and provide no protection.
  • Diazinon dips treat active maggot infestations and provide good protection against blowfly strike for up to 6 weeks.

17-05 PFF3

Flystrike problem waiting to happen - this lamb must be "dagged" immediately and a suitable preparation applied topically to prevent myiasis. A review of PGE control measures on the farm should be undertaken with the farmer's veterinary surgeon.

17-5 PFF4

Blowfly strike attracted by neglected footrot lesion.

Cattle PGE

  • Weaned dairy or beef calves in their first grazing season that are turned out onto "safe" pasture (fields grazed by sheep the previous year or previously arable) should not require worming.
  • In contrast, calves turned out onto "high-risk" pasture (fields grazed by calves the previous year) should be wormed at, or around, turnout to limit acquired worm burdens and reduce further pasture contamination.
    • With these calves, the aim is to minimise pasture contamination through further strategic treatments where required, up to mid-July by which time any over-wintering larvae should have declined to insignificant levels.
    • Strategic treatments include administration of either a pulse or continuous release bolus at turnout or administration of pour-on, or injectable, macrocyclic lactones (MLs) at defined intervals, as recommended by the manufacturers.
  • With both strategies calves should remain set-stocked, or moved to safe pastures (aftermaths) when these become available.


  • There are many common species of flies which feed on grazing cattle.
  • Biting flies, which include stable flies (Stomoxys), horn flies (Haematobia), head flies (Hydrotaea), horse flies (tabanids), midges (Culicoides) and blackflies (Simulium) feed by puncturing the skin directly and may act as vectors of several bacterial and viral diseases.
  • Nuisance flies, such as face flies and sweat flies, scavenge the surface of the skin, wounds, or body orifices feeding on sweat, skin secretions, tears, or saliva.
  • Face flies are often the most numerous nuisance flies causing serious annoyance to grazing cattle and are linked to the transmission of diseases such as summer mastitis, New Forest Disease ("pinkeye"), and possibly BVD virus.
  • Midges transmit bluetongue virus (BTV) and Schmallenberg virus (SBV)
  • Insecticide impregnated ear tags should be applied at the start of the grazing season to the whole herd to provide season long protection.

Local farm conditions may vary so consult your veterinary surgeon. Parasite control should be part of your veterinary health plan.


Worming ewes at lambing time

· With lambing now well under way, attention should be focusing on controlling parasite burdens in ewes.

· The principle aim of worming ewes is to minimise the future contamination of pastures by worm egg output during the 'peri-parturient rise" (PPR).

· Eggs passed in the faeces of infected ewes hatch and develop into infective larvae, which can cause disease in lambs later in the season.

· Worm faecal egg output is much reduced in well fed ewes in good condition.

· Provision of 'safe grazing' for ewes and lambs available at turnout will also help avoid the risk of worm infections in lambs later in the season.

o 'Safe' pastures should not have been grazed by lambs the previous year and include fields grazed by cattle last year; and re-seeded pastures.

· If only limited safe grazing is available, then this should be reserved for ewes with twin lambs whilst those with single lambs can graze the more contaminated pastures.

· As wormer resistance is becoming more common, advice on worming ewes is changing, influencing both the choice and frequency of treatment post-lambing.

· The timing and choice of wormer are both important in controlling the PPR, as the ewes can quickly become re-infected, particularly when grazing heavily infected pastures post turnout.

· Current worming advice recommends leaving a proportion of the ewes untreated by targeting treatments to include;

o Gimmers and young ewes

o Ewes nursing twins and triplets

o Ewes in low body condition

· Ewes with single lambs or those in good body condition can be left undosed unless there is a risk from fluke or haemonchosis.

· Persistent, or long-acting wormers, which provide a prolonged period of protection if given later in the lactation period before ewes become re-infected from the in refugia population can be highly selective for resistance.

· Worms in refugia include the population of worms present in untreated sheep and the free-living population of eggs and larvae not exposed to wormers.

· The recommendation for long-acting formulations of moxidectin, is to use these products prior to lambing, or at turnout.

Further details can be found on the SCOPS website at

Ewe worming treatments should aim to reduce pasture contamination during the periparturient rise whilst at the same time not selecting for anthelmintic-resistant strains of parasites.

Some ewes nursing singles could be left untreated; seek veterinary advice for your farm.


· Severe outbreaks of nematodirosis can occur in 6 to 12-week-old lambs usually from April to June in some years, depending on prevailing weather conditions.

· Cold late springs followed by sudden changes in temperature can trigger a mass synchronised hatch of infective larvae leading to severe production losses and even death in lambs grazing contaminated pastures.

· Monitor the SCOPS website ( for regular updates on risk of disease in your area.

· As weather conditions during March and April can significantly alter early season predictions of nematodirosis for flocks lambing during March/April, an updated disease risk will be included in the NADIS May parasite forecast.

Nematodirus control

· Control is best achieved by grazing lambs on pasture not grazed by lambs the previous year ('safe pasture').

· Where this is not possible, and local weather conditions are such that an early hatch occurs, then late January/February-born lambs may need prophylactic anthelmintic drenching before the end of March. Consult the SCOPS website regularly for disease risk in your area.

· Late March/April-born lambs may require prophylactic anthelmintic drenching in May if prolonged cold weather during April delays hatching.

· While incidents of wormer resistance have been reported with Nematodirus, white drench (1-BZ) wormers are still generally recommended to control this parasite.

· As disease is primarily caused by developing larvae, faecal egg count (FEC) monitoring is unreliable in determining risk and the need to treat.

· When a white drench (1-BZ) wormer is used in outbreaks of nematodirosis, the FEC of several lambs should be checked 10 days later for the presence of other worm species, which if present, would require treatment with a wormer from another group.


Nematodirus infection in lambs.  These lambs suffered a serious and costly check in growth rate.


· Outbreaks of coccidiosis may be encountered during April in lambs between 4-8 weeks of age, particularly in twin lambs grazing contaminated pastures.

· Coccidiosis is a disease of intensive husbandry with stress a major factor in triggering outbreaks of disease.

· Adverse weather conditions, poor colostrum supply, overcrowding, wet muddy paddocks previously grazed by sheep, and/or extended housing periods all predispose.

· Reduction of stocking densities, batch rearing of lambs, creep feeding and avoidance of heavily contaminated pastures/premises are measures that can be taken to reduce the risk of disease outbreaks.

· Disease prevention can also include strategic dosing lambs on contaminated pastures with diclazuril or toltrazuril at around 3-4 weeks of age, or administration of medicated creep feed containing decoquinate for 28 days.

Liver fluke

· Sheep on premises with known fluke populations, or in high risk areas, should already have been dosed in the autumn and early winter and may need to be dosed again this spring.

· Chronic liver fluke may still be encountered in sheep flocks and can be confirmed by checking for the presence of fluke eggs in faeces.

· On positive farms, the presence of fluke eggs in faecal samples reflects inadequate control of fasciolosis and control should be reviewed.

· Limiting pasture contamination with fluke eggs from patent infections will help reduce subsequent fluke challenge later in the year.

· Flukicides containing closantel, nitroxynil, oxyclozanide or albendazole (at the fluke dose rate), are all effective against adult flukes present during the spring and should be used to reduce reliance on triclabendazole.

· Sheep should always be moved to clean pastures after treatment; and supplementary feeding may be necessary to maintain condition.




· Housed, yearling cattle not dosed in the autumn, may be at risk from type II ostertagiosis towards the end of the housing period.

· Prevalence of clinical disease is usually comparatively low and only a proportion of animals in a group may be affected.


· The disease presents as intermittent diarrhoea with loss of appetite and rapid loss of body weight.

· Mortality in affected cattle can be high unless early treatment with a wormer effective against both arrested and developing larval stages is given.

PGE Control

· Decisions should have been made on the parasite control plan for the forthcoming grazing season.

· Prevention of PGE in growing cattle on a sustainable basis is best achieved by annual rotational grazing (cattle/sheep/crops) but this is not often possible on many farms.

· Parasite control plans based on anthelmintic use may be strategic (early season dosing) or "wait-see" (monitor/treat in the latter part of the grazing season).

· To be effective, strategic worm dosing needs to be initiated at, or around turnout, to limit pasture contamination up to mid-July by which time the over-wintered larval population should have declined to insignificant levels.

o Strategic treatments include administration of a bolus at turnout. or administration of pour-on, or injectable macrocyclic lactones (MLs) at defined intervals.

· Cattle treated strategically should remain set-stocked, or moved to safe pastures (aftermaths) when these become available.

· If "wait and see", then ensure that effective, regular monitoring and diagnostic procedures are in place to act quickly if required.

· Where lungworm is a problem, there is still time to discuss control, including vaccination, with your veterinary surgeon before turnout in most areas.

· Vaccination of calves over two months-old requires two doses of lungworm vaccine four weeks apart with a second dose at least two weeks before turnout.

· For more information see the COWS ( website.


Unless safe grazing is available, dairy calves and suckled calves born during the previous autumn require preventive treatment in their first full grazing season to control PGE

Parasite Forecast Related Health Information
Nematodirus in Sheep
Nematodirus is a parasitic infection which causes profuse scour in lambs...
PGE and Lungworm in cattle
Parasitic gastroenteritis usually affects growing cattle during their fi...
Gastrointestinal Nematodes in Sheep
Parasitic gut worms have the potential to cause significant losses in sh...
Liver Fluke Control in Cattle
In cattle, infection is more commonly encountered in beef cows grazing p...
Liver Fluke Control in Sheep
Liver fluke has always been a problem during the late autumn and winter ...
Case Study
Consider targeted parasite control in beef and sheep throughout grazing ...
Case Study
Don’t ignore the threat of lungworm...
Case Study
Prepare for sheep parasite control in the season ahead...
sponsor logosponsor logosponsor logoImproving Sheep & Cattle Health