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NADIS Parasite Forecast - May

PF Capture

At the start of March, the UK's weather was dominated by low pressure, bringing wet and fairly cold weather with some short-lived snowfalls, mainly on high ground. There were strong winds in the south-west on the 2nd, associated with Storm Jake, and it was very wet across England and Wales on the 8th. However, high pressure became established from the 13th to the 23rd, bringing dry, settled weather, often with plenty of sunshine for western areas. More unsettled weather then returned during the last week, and Storm Katie brought damaging winds on the 28th, particularly in the south-east. (

May Parasite Forecast/Update

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


Control of PGE in adult sheep and lambs after lambing.

  • Ewes and lambs remaining on safe grazing should not require anthelmintic treatment until lambs are around four month-old but timing of treatment should be guided by pooled faecal egg counts of lambs.
  • Ewes grazing contaminated pastures should have been treated with a persistent anthelmintic before turnout which will prevent re-infection from the pasture for several months and help to reduce subsequent larval challenge of lambs.
  • The cold weather throughout April, particularly in northern areas of the UK will have delayed hatching of Nematodirus battus larvae. When the weather improves March/April lambs will face a significant challenge because they will start grazing just as the larvae hatch so drenching in early May is recommended; similar climatic conditions with a cold late spring in 2013 caused serious disease problems
  • Local conditions will affect hatching so consult the daily updates on the SCOPS website and ask your veterinary practice for local advice on Nematodirus battus risk in your area.
  • Monitoring faecal egg counts is of no value in determining when to drench for Nematodirus battus.
  • Monitor faecal eggs counts from rams regularly because they are more susceptible than ewes to PGE and often graze the same contaminated pasture every year.
Monitor faecal egg counts from rams regularly because they are more susceptible than ewes to PGE

Nematodirus control on contaminated pasture

  • Nematodirus battus will present a serious problem this spring. Similar climatic conditions caused serious disease outbreaks in 2013.
  • The cold weather throughout April will have prevented hatching of N. battus eggs present on contaminated pastures and action will be necessary soon for flocks with February/early March lambs in the southern half of England. Hatching may be up to two weeks later in the northern half of England, and Scotland where there is a very high risk of disease therefore dosing will be necessary in early May and perhaps two weeks later to be certain of correct timing.
  • Information updated daily can be accessed from the SCOPS website.
  • Adult sheep are unaffected by Nematodirus battus larvae.
  • Benzimidazole (Group 1) wormers are usually recommended for Nematodirus battus control but will be largely ineffective against other gut parasite species.
  • Faeces samples should be collected from lambs after prophylactic anthelmintic treatment to ensure that any adult Nematodirus parasites have been killed and that there are no eggs present from other gut parasites such as Teladorsagia spp.


Excellent farm planning  - these lambs on safe grazing will not require anthelmintic treatment for Nematodirus parasites.


Lambs grazing contaminated pasture.  It can prove difficult to differentiate diarrhoea caused by nematodirosis from coccidiosis and your veterinary surgeon should always be consulted.

Liver fluke

Autumn/winter 2015 was a particularly high risk year for liver fluke in western regions, sheep on farms with a known fluke problem should be dosed in April/May with a flukicide active against mature flukes. Do not use triclabendazole at this time as it may hasten the selection for resistant strains of liver fluke.



Autumn/winter 2015 was a particularly high risk year for liver fluke in western regions - comparison of normal liver (left) with diseased liver (right)


Autumn/winter 2015 was a particularly high risk year for liver fluke in western regions - comparison of cut sections of normal liver (top) with diseased liver (bottom); note the mature flukes exiting the thickened bile ducts at necropsy

Cattle nematodes

  • To control ostertagiosis later this year, dairy calves and suckled calves require preventive treatment for their first full grazing season unless they are on safe grazing.
  • If pasture egg contamination is suppressed by pulse release or continuous release bolus, repeated or long-acting anthelmintic injection until at least mid-summer, most pasture larvae should have died off by that time and the pasture should remain safe for the rest of the season.
  • Preventive strategies only work when cattle are set-stocked; movement of these cattle during the autumn onto contaminated pasture risks disease.


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