Welcome to NADIS. This map will only appear on the initial visit to the website. Please select your region, and you will be automatically be taken to your region on future visits

UK Map Northern Ireland North Scotland South East Scotland South West Scotland North East England Yorkshire & Humber East Midlands East of England South East England North West England Wales West Midlands South West England
Download this page as a PDF

Download now

NADIS Parasite Forecast - August

PF Capture 16-08

At the start of June, Britain's weather was dry and settled, but often cloudy away from sheltered western areas, with north-easterly winds bringing low cloud in from the North Sea. Showers and thunderstorms increasingly broke out inland after the 5th, although there was further warm sunny weather at times, particularly in the west. From the 10th onwards, the weather was generally unsettled, wet and cloudy with low pressure often in charge. There were also thundery downpours at times, and heavy rain and thunderstorms caused significant disruption in the south-east on the 23rd.   (

August Parasite Forecast/Update

The most recent version of this monthly parasite forecast may be accessed at


Remember to check a pooled faecal egg count of your rams as this group of sheep are often neglected at this time of year.  Note that rams are generally much more susceptible to PGE than ewes.


Rams are generally much more susceptible to PGE than ewes and are often forgotten when drenching.


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).

Parasite control in lambs after weaning:

Sustainable parasite control in lambs after weaning is about achieving acceptable growth rates while managing the parasite burden; sheep are rarely, if ever parasite free and freedom from parasites is therefore not an achievable target.

"Safe grazing"

If summers are wet, peak pasture larval infectivity occurs in July and August derived from eggs passed mainly by lambs throughout the earlier grazing season. This larval challenge can be avoided by moving lambs to aftermaths not grazed by lambs this year, and preferably to pasture not grazed by lambs last year. The value of such "safe grazing" is critical to effective and sustainable parasite control on sheep farms. In general, these lambs will not require anthelmintic treatment for up to 8 weeks by which time the majority will have reached 36-40 kg and been slaughtered otherwise there is a serious problem or a conscious management decision to delay marketing.  This grazing strategy also avoids restrictions over meat withholding times after anthelmintic treatment and whether to dose or not - "will this lamb be fat in three to four weeks or should I drench it"?  Please see below for recommendations concerning which anthelmintic to use at weaning and before movement onto "safe grazing".


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:

  • Targeted anthelmintic treatment based upon liveweight gain
  • Anthelmintic treatment based upon lamb faecal worm eggs counts.
  • Whole group anthelmintic treatment at regular intervals (not sustainable and certainly not recommended)

Targeted anthelmintic treatment

  • Targeted anthelmintic treatment selects only those lambs for treatment that are failing to meet expected growth rates (assumed to be caused by a significant parasite burden).
  • Lambs are weighed regularly; at least every 3-4 weeks.
  • In general, around 40-60 per cent of lambs require treatment saving money on drugs.
  • Most importantly, targeted anthelmintic treatment reduces  selection for resistant strains of worms because a percentage of lambs are not treated  leading to a dilution effect with unselected worms. 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.
  • Progressive farmers are also using lamb growth rate data to select the genetics best suited to their flock/management system.

Anthelmintic treatment based upon faecal worm eggs counts.

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

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 helminth parasites especially when lambs are grazing relatively safe pastures (pasture containing low numbers of eggs and developing larvae "in refugia").
  • 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.

Selecting the most appropriate anthelmintic at weaning

Debate centres on the best use of group 4-AD and group 5-SI anthelmintics especially in those flocks where triple resistance to Groups 1-BZ, 2-LV and 3-ML, (which includes moxidectin) has been demonstrated.  As resistance to these new groups of anthelmintics has yet to be recorded in the UK, specific veterinary advice and guidance is recommended on their use.

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 anthelmintics will be especially useful in weaned fattening lambs before moving to safe pasture. Around 10 per cent of the strongest lambs could be left untreated to  ensure that some susceptible worms  are carried over onto the new pasture to reduce selection on these new groups of anthelmintics.

Flocks with triple resistance grazing contaminated pastures

There are no proven strategies on how to best use group 4-AD and group 5-SI anthelmintics in weaned lambs grazing contaminated pasture, but leaving up to 10 per cent of  stronger lambs untreated is likely to be more critical on the assumption that there will be a very high "in refugia" population on contaminated pasture in peak summer.


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.

Blowfly strike

  • Blowfly strike continues to be a major risk.
  • Dimpylate (diazinon) dips are effective against blowfly strike for up to six weeks and will eliminate lice and sheep scab.
  • High cis cypermethrin pour-on preparations provide protection against fly strike for 6 to 8 weeks at the site of application but require re-application in most situations.
  • Products containing the insect growth regulators should have been applied at the start of the season but may need to be re-applied after their periods of protection have ended, typically 10 or 16 weeks for cyromazine and dicyclanil respectively.
  • Be aware of withdrawal periods for all products and consult the data sheet.



  • Lungworm disease appears from June onwards, often peaking 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).
  • 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
  • Prompt anthelmintic treatment is essential. Use group 2-LV and 3-ML anthelmintics in preference to group 1-BZ drugs.


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


  • Incidents of type 1 ostertagiosis in cattle usually peak during August and September.
  • 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.
  • During the risk period,  clinically unaffected groups of calves should be given either targeted anthelmintic treatment based upon liveweight gain, or anthelmintic treatment based upon faecal worm eggs counts.PF16-08F6
Yearling Holstein heifer during her first summer at pasture with clinical signs of ostertagiosis.

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