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NADIS Parasite Forecast - July


May began unsettled and windy with low pressure in charge, but by the 5th a warmer and more settled pattern became established with high pressure close to the UK. For the rest of the month, anticyclonic and easterly weather types prevailed and brought plenty of fine, warm and sunny weather, although it was often cooler near North Sea coasts, and scattered thundery showers sometimes affected western and southern areas. It was particularly warm in the second week. This pattern was interrupted by a brief colder interlude mid-month and an unsettled spell from the 17th to 22nd, but warm and sunny weather re-established itself for most of the last week.  (

July Parasite Forecast/Update

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


The cold weather throughout much of April led to the prediction in the NADIS May Parasite Forecast, and reflected on the SCOPS Nematodirus risk webpage, that late March/April lambs could face a significant challenge from Nematodirus battus infestation because of delayed hatching coinciding with lambs consuming significant amounts of grass and infective larvae.   This forecast appears to have been accurate with an above average number of reported nematodirosis outbreaks in many regions.

Publication of the WAARD Project (Wales Against Anthelmintic Resistance Development) has highlighted the very high prevalence of anthelmintic resistance on 47 Welsh farms tested in 2014 and 2015, where many farmers were unaware of the lack of drug efficacy.  The results indicated that 94% of farms had evidence of resistance to Benzimidazole (1-BZ), while 68% of farms had resistance to Levamisole (2-LV) drenches. Resistance was also detected on 51% of farms to ivermectin (3-ML) drench, and 19% of farms showed resistance to moxidectin (3-ML); 43% of farms demonstrated triple wormer resistance, some including moxidectin. (The pdf can be downloaded free by entering the title provided at the end of this report into any search engine).  Planning ahead, it is essential that farm-specific advice is necessary and that can be meaningful only after assessing anthelmintic efficacy. SCOPS advise that products containing group 4-AD and group 5-SI anthelmintics should be used strategically and where necessary, but they should not be left in reserve for when all other groups have failed on a farm.

Worm control in lambs up to weaning:

Sustainable worm control in lambs is about achieving acceptable growth rates while managing the worm burdens; few sheep are ever worm-free and total freedom is not an achievable target.

The control options for PGE in lambs before weaning 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 larger  the in refugia population, the slower anthelmintic resistance develops.
  • Progressive farmers are also using lamb growth rate data to select the genetics best suited to their flock/management system.
  • The major disadvantage to such parasite control is time but such limitation can be reduced by investment in efficient handling equipment.


Targeted anthelmintic treatment selects only those lambs failing to meet expected growth rates.

Anthelmintic treatments based on faecal worm egg 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 e.p.g.
  • Wherever possible, aim to leave about 10% of the flock untreated by not dosing large single lambs, or lambs not showing any clinical signs of scour.

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

Weaned lambs

Peak pasture larval contamination occurs in July and August from eggs passed by lambs but this larval challenge can be avoided by moving lambs to silage or hay aftermaths not grazing 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.


Lambs grazing 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.

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.  There are no accepted guidelines regarding the best use of these products, indeed experts gathered at the recent Sheep Veterinary Society conference in Skipton in May were divided in their opinions and farm-specific veterinary advice was strongly recommended.

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 could 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 pressure on these new groups of anthelmintics although not all experts agree on this practice.

Flocks with triple resistance grazing contaminated pastures

There are similarly no proven strategies on how to best use group 4-AD and group 5-SI anthelmintics in weaned lambs grazing contaminated pasture but leaving some stronger lambs untreated is likely to be less critical on the assumption that there will be a very high "in refugia" population 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.



Tapeworm segments are commonly seen in faeces passed by lambs at this time of year but are usually considered to be of no clinical significance.

Blowfly Strike

  • Blowfly strike is a major risk from June onwards. PGE control is a much more effective measure for reducing blowfly strike than tail docking - see video recording.
  • 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 cyromazine or dicyclanil should be applied at the start of the season prior to the anticipated period of risk
    • Cyromazine, provides up to 10 weeks protection after topical application
    • Dicyclanil affords up to 16 weeks' full body protection.
  • Dimpylate (diazinon) dips are effective against blowfly strike for up to six weeks.
  • As with tail docking of puppies, many members of the general public regard tail docking of lambs as an unacceptable mutilation.


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 receiving prophylactic anthelmintic treatments must remain on the same pasture during the entire grazing season or be moved to safe grazing such as silage aftermath from July onwards.
  • Spring born calves may benefit from turnout directly onto silage or hay aftermaths thus avoiding the build-up of pasture populations of worms 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 faecal worm eggs counts.


  • 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).
  • 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.
  • In areas of the country where lungworm is endemic, COWS recommend vaccination of first year grazers prior to turnout.


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.


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