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NADIS Parasite Forecast - August

Use of meteorological data to predict the prevalence of parasitic diseases


At the start of June an intense low pressure system brought rain and unseasonably strong winds to the UK. After this, the month was typified by rather quiet weather generally; after a spell of fine, settled weather there was rain or showers from time to time, and more unsettled conditions over Scotland, although generally much drier further south. Much of June was rather cool in an often westerly or north-westerly flow, but it became very warm at the end of the month. (

August Parasite Forecast/Update

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

Parasite control in lambs is a balance between an anthelmintic policy that limits selection for resistant strains and acceptable lamb growth rates.  Management utilising "safe grazing" wherever possible has a crucial role to play in parasite control programmes.  A policy of regular and repeated anthelmintic treatments at short intervals has proven unsustainable manifested by the development of triple resistance.


Advantages of early lamb sales

Despite very disappointing market prices for fat lambs this summer, lambs sold before mid-July will have largely avoided the mid-summer PGE challenge and consequent checks in growth rate.  The very dry weather in many regions during June will have resulted in reduced larval challenge in general.

PF Fig 1

Lambs reared on safe grazing off to market in early July before significant parasite challenge.

Advantages of 'safe grazing' after weaning

  • After weaning and a move to "safe grazing" lambs may not need to be treated with an anthelmintic for up to 6-10 weeks, however this will depend upon the amount of infection carried over, stocking rate and weather conditions (large numbers of lambs left untreated, high stocking rates and wet weather increase challenge).
  • Monitoring of pooled faecal worm egg counts (FWEC) every 7-10 days with fresh faeces collected from around 10 lambs after gathering into a corner of the field can help to decide whether anthelmintic treatment is necessary.
  • Groups 4-AD and 5-SI anthelmintics could be especially useful at weaning for lambs which have grazed permanent pasture where triple resistance has previously been confirmed on the farm.  When such lambs are dosed and moved to "safe grazing" (e.g. silage aftermath), around 10 per cent of the strongest lambs could be left untreated to carry some anthelmintic-susceptible worms over onto the new pasture to reduce selection for anthelmintic-resistant worms.  Alternatively, all lambs could be left to graze contaminated pasture for 4-7 days before movement to "safe grazing".
  • Groups 4-AD and 5-SI anthelmintics are POM-V products and farm-specific veterinary advice given at the time of purchase is very important.

Overcoming problems of contaminated pastures for weaned lambs

  • Where weaned lambs must remain on pasture grazed by lambs this year, further pasture contamination with eggs passed by the lambs must be limited by anthelmintic treatment to achieve acceptable future growth rates.
  • Consider using targeted anthelmintic treatments whereby only those lambs failing to grow at accepted rates are treated.
  • Targeted anthelmintic treatments avoid unnecessary treatments (up to 40% on average), save money, and reduce the selection pressure for resistant strains of worms.
  • Where lamb growth rates are universally poor, check for resistance to anthelmintic groups 1-3 ("triple" resistance).

Potential complicating factors

  • Cobalt deficiency should also be considered as a potential cause of poor growth in weaned lambs and may exacerbate PGE.  Your veterinary surgeon should be consulted for professional advice.

PF Fig 2

Design a grazing plan such that lambs are not challenged by high pasture larval burdens - the rewards are obvious with improved growth and fewer treatments.

PF Fig 3

Around 10 per cent of the heaviest lambs without diarrhoea in the group should be left un-treated ('in refugia') to carry some anthelmintic-susceptible worms onto safe grazing.

PF Fig 4

Lambs should be dosed for the heaviest lambs in the group.

PF Fig 5

Sustainable parasite control requires considerable advance planning and snap decisions 'on the day' may well be the wrong course of action.

Blowfly strike

  • Flystrike on the back related to rain scald has been seen if wet weather prevails in many regions
  • Control of PGE will reduce faecal soiling of the tail and perineum.
  • Control measures using either dips or pour-on preparation for lambs will be restricted by meat withhold times.

PF Fig 6

Blowfly strike affecting the lamb on the right. Evidence of faecal staining of the perineum probably indicates lack of effective PGE control in both ewe and lamb.



  • Lungworm disease reaches a peak in August.
  • Unvaccinated dairy calves at grass for their first season without an appropriate parasite control programme are most at risk.
  • Weaned beef calves are typically most at risk during their second grazing season
  • Adult cattle may show clinical signs after grazing heavily contaminated pastures
  • Frequent coughing, even at rest, is the most common sign of lungworm.
  • Most cattle in the group will develop clinical signs within several days if left untreated
  • Clinical signs may be present before lungworm larvae are detected in faeces
  • All cattle in the group should be treated with an anthelmintic.


PF Fig 7

Frequent coughing, even at rest, is the most common sign of lungworm.


  • Incidents of type 1 ostertagiosis in cattle 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.

PF Fig 8

Incidents of type 1 ostertagiosis with profuse diarrhoea peak during August and September.

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