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Editorial Information

Professor Mike Taylor BVMS PhD MRCVS DipEVPC DipECSRHM CBiol MRSB

Published 2017

Parasite Forecast - July 2017

17-07 PF Capture

May started with rain and showers in the south, but it was mostly dry and sunny in the north, turning unsettled around mid-month with bands of rain interspersed with brighter showery weather.  The third week was hot for much of the UK followed by widespread thunderstorms at the end of the month. The UK mean temperature was 1.7 °C above the 1981-2010 long-term average, making it the second warmest May since 1910.  Mean maximum temperatures were generally about 2.5°C above normal in Scotland and Northern Ireland, but nearer 1.5°C above in the south, while mean minimum temperatures were between 1-2°C above average in all regions.  Rainfall was above normal in East Anglia, SE and central southern England, but mostly below normal elsewhere with 83% of average overall.

July Parasite Forecast/Update



Fluke Forecasts

Summer temperature and rainfall between May and October are the major climatic drivers directly affecting the life cycle and subsequent fluke risks, based on wetness (Mt) models. This year NADIS will be producing more detailed fluke risk maps for GB based on Met Office MORECS data that divides GB into 190 * 40 km squares. A preliminary regional fluke forecast (based on May and June data) will be issued with the August parasite forecast with the full fluke forecast appearing later in the year.

Worm Control

  • Sustainable worm control in lambs is about achieving acceptable growth rates whilst managing the worm burdens.
  • Effective worm control relies heavily on the use of safe and effective anthelmintics.
  • However, the prevalence of anthelmintic resistance (AR) has risen sharply in recent years with an increasing number of flocks finding that one or more of the chemical groups are no longer effective against some worm species.
  • For worm control strategies to remain effective, farm-specific advice is essential and can be only be meaningful after assessing anthelmintic efficacy.

Worm control in lambs to weaning

  • Lambs grazing permanent pastures will become exposed to infective larvae as part of the 'mid-summer rise' that results from pasture contamination earlier in the grazing season.
  • Levels of infectivity will depend on factors such as grazing history, stocking levels and prevailing weather conditions.
  • Warm wet summers lead to rapid development and release of larvae from the faecal pellets, whilst prolonged local dry weather conditions can delay larval challenge with a return to infectivity when wet weather arrives.

Weaned lambs

  • Peak pasture larval infectivity occurs in July and August but this larval challenge can be avoided by moving lambs to silage or hay aftermaths not grazed by lambs this year, and preferably to pasture not grazed by lambs last year.
  • The value of such clean grazing is critical on most sheep farms.
  • It is now recommended that if lambs are wormed, the move to aftermath fields should be delayed slightly to allow some re-infection with unselected parasites.

17-07 PF F1

Fig 1 Lambs grazing aftermaths 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

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.
  • Target selective treatments for 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  caused 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.

17-07 PF F2

Fig 2 Target selective treatments for only those lambs failing to meet expected growth rates.

Anthelmintic treatment based upon 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.

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.
  • The cheapest and simplest way is to perform a drench test to check on anthelmintic efficacy.
  • Collect FECs both prior to worming, and again at either 7 days for 2-LV; or 14 days for 1-BZ and 3-ML wormers post-treatment.
  • If mean FECs have been reduced by <95% then resistance is suspected.
  • 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 triple 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 triple resistance grazing contaminated pastures

  • There are no proven strategies on how to best use group 4-AD and group 5-SI wormers in weaned lambs grazing contaminated pasture but leaving some stronger lambs untreated is likely to 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.

17-07 PF F3

Fig 3 Fattening lambs weaned onto safe grazing.  Consult your veterinary adviser about the best worm control strategy which may include group 4-AD and 5-SI wormers.


  • Haemonchosis, due to the presence of the blood-feeding, abomasal nematode Haemonchus contortus, is a sporadic disease affecting all ages of sheep in the UK.
  • Infection causes an acute haemorrhagic anaemia due to the blood-sucking habits of the worms.
  • Each worm removes about 0.05 ml of blood per day, so that a sheep with a worm burden of 5000 adults may lose about 250 ml daily.
  • Acute haemonchosis is characterised by anaemia, variable degrees of oedema, and may lead to sudden death.
  • Chronic haemonchosis is associated with progressive weight loss and weakness.
  • Unlike with other worm infections, diarrhoea is not generally a feature.
  • Sudden outbreaks may occur as the female worms are prolific egg-layers, resulting in high FEC and rapid pasture contamination under optimal conditions.
  • Most wormers are effective, as are the flukicides  nitroxynil and  closantel, although resistance has been reported historically to 1-BZ products.


  • Tapeworm segments (Moniezia) are commonly seen in faeces passed by lambs during the summer months but are usually of no clinical significance.
  • Lambs become infected by the ingestion of the intermediate hosts which are oribatid, forage mites on pasture.
  • White drench products (1-BZ) are generally effective against tapeworm segments although re-infection is common.

17-07 PF F4

Fig 4 Tapeworm segments (Moniezia) are commonly seen in faeces passed by lambs during the summer months.

Blowfly strike

  • Blowfly strike is a major risk during the summer months.
  • Preventing diarrhoea caused by worm infections will greatly reduce the risk of blowfly strike on the breech.
  • 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

  • Careful planning when treating lambs is required due to some of the long meat withdrawal periods.

17-07 PF F5

Fig 5  Tail docking has failed to prevent blowfly strike in this lamb where faecal staining of the perineum is the greater risk factor.


Parasitic Gastroenteritis (PGE)

  • Cattle that were given prophylactic worming treatments earlier in the season should remain on the same pasture during the entire grazing season, or be moved to aftermaths from July onwards.
  • Late spring born calves may benefit from turnout directly onto silage or hay aftermaths thus avoiding the build-up of pasture larvae that may have otherwise occurred from contamination earlier in the season by older calves.
  • Incidents of clinical PGE occur from mid-July onwards peaking during August/September.
  • For calves on permanent pasture, the Control of Worms Sustainably (COWS) group recommend either targeted anthelmintic treatment based upon liveweight gain or anthelmintic treatment based upon FECs.


  • Lungworm disease appears from June onwards 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
  • Prompt anthelmintic treatment is essential and supportive therapy may be required depending on clinical presentation. Affected animals should be removed from infected pasture.
  • Most 3-ML products are not indicated in milking cows; the exception being eprinomectin which has a zero-milk withdrawal period.


17-07 PF F6

Fig 6  Early clinical signs of lungworm in a parasite-naïve beef cow - the source of lungworm was purchased store cattle which had not received any quarantine treatments.

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

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