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NADIS Parasite Forecast - April

Use of meteorological data to predict the prevalence of parasitic diseases


At the start of the month, the UK was under the influence of a cold northerly weather type, but from 5th to 12th high pressure became established across the UK bringing largely quiet, dry weather. A rapid breakdown around 13th brought a return to more unsettled conditions, but at the same time it turned somewhat milder. The second half of February saw a generally westerly type, with temperatures fluctuating and some heavy rain and strong winds, especially in Scotland where there were significant snowfalls across the mountains, but with some quieter days in between(

April Parasite Forecast/Update

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



Control of PGE in adult sheep after lambing

  • The aim of anthelmintic treatment of ewes around lambing time is to reduce pasture contamination and subsequent larval challenge to lambs.
  • Ewes grazing contaminated pastures would benefit from a persistent anthelmintic treatment to prevent immediate re-infection from the larvae on pasture. Consider leaving ewes nursing single lambs untreated.
  • Ewes turned out onto safe pasture only require a short acting anthelmintic treatment to remove parasites in the gut. Consider leaving ewes nursing single lambs untreated.
  • Rams are more susceptible than ewes to PGE and must be included in the anthelmintic treatment plan.

PF 15-04 Fig 1

Plan grazing and anthelmintic control policy now before lambing time.

PF 15-04 Fig 2

Ewes grazing these contaminated pastures will need an anthelmintic treatment with persistent action at around lambing to prevent immediate re-infection from the pasture. Consider leaving ewes nursing single lambs untreated

PF 15-04 Fig 3

Ewes grazing safe pasture only require a short acting anthelmintic treatment. Consider leaving ewes nursing single lambs untreated.

Nematodirus watch

  • Severe outbreaks of nematodirosis in 6-8 week-old lambs were encountered in many March/April-lambing flocks in May and June 2013.
  • Mean UK monthly temperatures from December 2014 to February 2015 have been slightly above the 1961-1990 monthly averages.
  • Early observations suggest that late January and February-born lambs grazing contaminated pastures in the south west of England may need prophylactic anthelmintic drenching before the end of March.
  • March and April temperatures and rainfall can significantly alter any early season prediction of nematodirosis for traditional lambing flocks (March/April) and a more accurate forecast for overall incidence and peak hatch will be made in early April and will be included in the May parasite forecast.

PF 15-04 Fig 4

Severe outbreaks of nematodirosis were encountered in late March/April lambing flocks during May and June 2013.

Nematodirus control

  • The best control method is to graze lambs on pasture not grazed by lambs the previous year (safe pasture).
  • Where this is not possible, and March is mild allowing early hatching, then late January- and February-born lambs may need prophylactic anthelmintic drenching before the end of March.
  • Late March/April-born lambs may require prophylactic anthelmintic drenching in May if prolonged cold weather during April delays hatching.
  • While isolated cases of resistance have been reported, benzimidazole (BZ; Group 1) wormers are still recommended for Nematodirus control.
  • As disease is primarily caused by developing larvae, faecal egg count monitoring to time Nematodirus treatments is too risky.
  • When a BZ anthelmintic is used in outbreaks of nematodirosis, the faecal egg count of several lambs should be checked 10 days later for significant Teladorsagia infection which, where present, would likely necessitate an anthelmintic from another class.

PF 15-04 Fig 5

Excellent forward planning - nematodirosis is not a problem on safe pasture.

PF 15-04 Fig 6

No forward planning - nematodirosis affecting lambs during May 2013.


  • Coccidosis is the major differential diagnosis for nematodirosis with many lambs showing profuse diarrhoea and rapid weight loss (see below).
  • Coccidiosis is associated with intensive husbandry.
  • The choice of medication as prescribed by the veterinary practitioner will depend upon individual farm circumstances
  • Sheep must be moved from infected pastures/premises as soon as disease becomes apparent.

PF 15-04 Fig 7

Coccidiosis is a significant risk in lambs managed intensively indoors (this image) but also at pasture.


  • Sheep on premises with a known fluke population will already have been dosed in the autumn and early winter and should be dosed again in March/April.
  • Newly diagnosed cases of chronic fluke disease (fluke eggs in faecal samples) can be treated with any flukicide.
  • Triclabendazole should not be used in the spring because a high level of activity against immature flukes is not required.


  • Sustainable control of PGE in growing cattle is best achieved by annual rotational grazing (cattle/sheep/crops) but this is not possible on most farms
  • To control ostertagiosis in the autumn, dairy calves and suckled calves born during the previous autumn require preventive treatment in their first full grazing season unless they are on safe grazing.
  • If pasture egg contamination is suppressed by pulse/continuous release bolus, repeated or long-acting anthelmintic injection until at least mid-summer, most pasture larvae will have died off by that time and the pasture should remain safe for the rest of the season.
  • Alternatively, calves can be dosed and moved to aftermath at mid-summer.
  • Where lungworm is a particular problem, there is still time to discuss control, including vaccination, with your veterinary surgeon.

PF 15-04 Fig 8

Dairy calves and suckled calves born during the previous autumn require preventative treatment in their first full grazing season to control otertagiosis in the autumn, unless they are on safe grazing.

PF 15-04 Fig 9

There is still time to discuss this grazing season's parasite control with your veterinary surgeon.

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