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 - August 2017


17-08 Capture

June started largely unsettled with significant amounts of rainfall in most areas. England and Wales became more settled and dry although western Scotland remained wet at times. The middle of the month was very warm with temperatures exceeding 30 °C in many parts of the country making it the equal 5th warmest June on record. The month ended cooler, bringing plenty of rain in many areas. Rainfall was well above normal over large parts of the country, making it the 6th wettest June since 1910, with Scotland having the wettest June on record.

August Parasite Forecast/Update



Fluke Forecasts

  • Warm, wet summers create suitable conditions for mud snail (Galba) populations to rapidly increase, and for motile free-living fluke stages to complete the life cycle.
  • Under optimal conditions the entire life cycle takes 17-18 weeks.
  • Rainfall in all regions of the country was below average over the winter and spring but the wet June has increased the potential fluke risk in some regions.
  • The preliminary fluke forecast is predicting a low risk of acute fluke this summer, but a potential moderate risk of fluke infection in the autumn in Scotland, Wales, NW and SW England, although the situation may change depending on rainfall during the months July to October.
  • NADIS will be producing more localised fluke forecasts for the whole of GB later in the year.

Worm Control in Lambs post weaning

  • Sustainable worm control in lambs is about achieving acceptable growth rates whilst managing the worm burdens.
  • In wet summers, peak pasture larval infectivity occurs in July and August derived from eggs passed mainly by lambs throughout the earlier grazing season.

Safe Grazing

  • High larval challenge can be avoided by moving weaned lambs to silage or hay aftermaths not grazed by ewes and lambs this year, and preferably to pasture not grazed by lambs last year.
  • The provision of 'safe' grazing is critical to effective and sustainable parasite control on most sheep farms.
  • In general, lambs moved to aftermaths will not require anthelmintic treatment for up to 8 weeks by which time the majority will have reached 36-40 kg and been slaughtered.
  • Note: If dosing weaned lambs onto safe grazing it is important to delay the move following treatment to further reduce the pressure on selection for resistance.
  • This grazing strategy also avoids restrictions over meat withholding times after anthelmintic treatment and decisions on whether to dose or not.


17-08 PF F1

Lambs grazing silage aftermath after weaning.  This simple management practice avoids the massive larval challenge that builds up on pasture and is one of the most critical components of sustainable parasite control on sheep farms.

Contaminated pasture

  • The control options for PGE in lambs grazing contaminated pasture after weaning, and those on safe grazing after 6-8 weeks, include:

A. Targeted selective treatment based upon liveweight gain

B.  Anthelmintic treatment based upon lamb faecal worm eggs counts.

C. Whole group anthelmintic treatment at regular intervals (not sustainable and certainly not recommended)

A. Targeted selective treatments (TSTs)

  • Some lambs in the flock in good body condition and performing well can be left unwormed.  In general, only 40-60 per cent of lambs require worming.
  • Worm only those lambs that are failing to meet expected growth rates (assumed to be caused by a significant parasite burdens) by weighing lambs every 3-4 weeks.
  • Regular weighing can also identify poor growth by factors such as overstocking and trace element deficiencies. Poor growth rates should always prompt a review of management.
  • Electronic identification and automated weight recording simplifies TST.
  • TST greatly reduces the likelihood of selecting for resistant strains of worms by allowing a pool of unselected parasites to pass out eggs onto the pasture helping to maintain wormer efficiency in the longer term.
  • The term "in refugia" is often used to describe the unselected worm population present in both untreated sheep and the free-living sub-population (i.e. eggs and larval stages) not exposed to anthelmintics.
  • The bigger the in refugia population the slower resistance develops.
  • Accurate record keeping also allows selection of the best genetics in the flock with breeding stock kept based on performance not appearance.

B. Anthelmintic treatment based upon Worm Faecal Egg Counts (FEC)

  • This approach is a compromise between reduced selection pressure for anthelmintic resistance and maintaining lamb growth rates.
  • Anthelmintic treatment is generally recommended when the mean worm egg counts of faecal samples collected from 10-12 lambs are greater than 500-700 epg.

C. Whole group anthelmintic treatment at regular intervals

  • Whole group anthelmintic treatment at regular intervals is NOT recommended because this approach selects for anthelmintic-resistant strains of worms especially if lambs are grazing relatively safe pastures (pasture containing low numbers of eggs and developing larvae "in refugia").

Selecting the most appropriate anthelmintic at weaning

  • Testing for the presence of wormer resistance is an increasingly important part of maintaining an effective worm control strategy.
  • Where AR is an issue, the Sustained Control of Parasites of Sheep (SCOPS) group advises that products containing group 4-AD or group 5-SI wormers, should be used strategically and where necessary, and they should not be left in reserve for when all other groups have failed on a farm.
  • With the recent re-licensing of the 4-AD monepantel to POM-VPS, vets, SQPS and their clients need to work much more closely on parasite control.
  • Follow SCOPS recommendations by leaving some lambs untreated and monitor treatment efficacy by performing a drench test post-treatment.

Flocks with multiple resistance grazing safe pastures

  • Where triple resistance has previously been confirmed on the farm, products containing Group 4-AD and group 5-SI wormers could be especially useful in weaned fattening lambs before moving to safe pasture.
  • Around 10% of the strongest lambs could be left untreated to ensure that some susceptible worms are carried over onto the new pasture to reduce selection pressure on these new wormer groups.

Flocks with multiple resistance grazing contaminated pastures

  • There are no proven strategies on how best to use 4-AD and group 5-SI wormers in weaned lambs grazing contaminated pasture but leaving some stronger lambs untreated is likely to be beneficial in reducing selection pressure and by adding to the "in refugia" population in peak summer.
  • Computer simulation studies have indicated that new wormers may be best used as a "break" treatment in mid-summer.

17-08 PF F2

Fattening lambs weaned onto safe grazing on your farm.  Consult your veterinary adviser about the best worm control strategy which may include group 4-AD and 5-SI anthelmintics.

Ewes and Rams

  • Once lambs have been weaned, the ewes should not require any further worming treatments until late pregnancy.
  • As rams are generally much more susceptible to worm infections than ewes, remember to check a pooled faecal egg count of your rams as this group of sheep are often neglected at this time of year.

17-08 PF F3

Rams are generally much more susceptible to PGE than ewes.  It will take 6-8 weeks for these rams to gain one unit of condition score ahead of the mating period (Target 3.5; scale 1-5).

Blowfly strike

  • Blowfly strike continues to be a major risk during the summer months.
  • Active maggot infestations can be treated using pour-on products containing (alpha) cypermethrin; spot-on products containing deltamethrin; or by dipping in diazinon dip baths.
  • Pour-on preparations containing cypermethrin also provide protection against fly strike for up to 6 to 8 weeks; with alpha-cypermethrin products providing protection for 8-10 weeks.
  • Diazinon (dimpylate) dips provide protection for up to six weeks.
  • Deltamethrin spot-on products are used for treatment of blowfly strike only and provide no protection.
  • Repeat applications of the insect growth regulators (IGRs) cyromazine and dicyclanil, which prevent blowfly strike, may be required to provide season long protection depending on the product used earlier in the season and persistence.
    • 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.
  • Be aware of withdrawal periods for all products and consult the product labels or data sheets.


  • Cattle can be infected with several species of gut nematodes, with Ostertagia ostertagi the main parasite associated with disease, and Cooperia spp. the most commonly encountered in young cattle in their first grazing season.
  • Peak pasture larval infectivity occurs from mid-July onwards and is derived from eggs passed by calves throughout the earlier grazing season.
  • High pasture larval challenge can be avoided by moving calves to safe grazing that includes silage or hay aftermaths not grazed by calves this year, and preferably to pasture not grazed by calves last year.
  • On permanent pasture, type 1 ostertagiosis in cattle usually peaks during August and September depending on summer rainfall.
  • Outbreaks of ostertagiosis can be prevented by targeted anthelmintic treatments based upon liveweight gain, or anthelmintic treatment based upon faecal worm eggs counts.
  • If clinical disease does occur, there is acute onset profuse diarrhoea that quickly affects most cattle in the group. Affected cattle rapidly lose weight and body condition.
  • Immediate treatment of all cattle in the group is important when first signs of acute profuse diarrhoea appear.

17-07 PF F4

Yearling Holstein heifer during her first summer at pasture with clinical signs of ostertagiosis.


  • Lungworm disease has been increasingly reported in first-year grazing animals in summer or early autumn, and over the past few years in older animals including adult cattle.
  • Infection often peaks in July and August in unvaccinated calves, those cattle without an effective anthelmintic programme, and non-immune adults (either unvaccinated or lacking previous lungworm exposure).
  • In areas of the country where lungworm is endemic, first year grazers should have been vaccinated prior to turnout.
  • Unvaccinated calves should be monitored closely for signs of lungworm.
  • Early signs of lungworm include coughing, initially after exercise then at rest, and an increased respiratory rate.
  • Affected cattle rapidly lose weight and body condition.
  • Parasite-naïve milking cattle may experience a sudden and dramatic drop in milk yield.
  • The presence of lungworm larvae in faeces can be readily undertaken by your veterinary practice with results available within 24 hours but note that clinical signs of lungworm may be present before the infection becomes patent.
  • Prompt anthelmintic treatment is essential. All available wormers are highly effective against adult lungworms and parasitic larval stages.

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 logoImproving Sheep & Cattle Health