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Managing Nematodirus risk this spring

Nematodirus battus is a species of gastrointestinal worm that can cause scour, reduced growth rates and even death in naïve young lambs.ÌýIt is the immature (non-egg producing) stages of the worm which cause disease; therefore,Ìýworm egg counts are not a useful tool in predicting and preventing losses due toÌýNematodirus.Ìý

The patterns of disease and infection withÌýNematodirusÌýare changing due to evolution of the parasite in the UK climate, and with the warming global climate.ÌýWe are increasingly seeing peaks inÌýNematodirusÌýegg output from lambs in both spring and autumn.ÌýPastures which carried lambs last spring are at greatest risk, and holding lambs off these fields until later in the summer can reduce infections.ÌýÌý

However, even pastures which carried lambs in the previous autumn (18 months ago) cannot be considered reliably ‘clean’.ÌýMost eggs will lie dormant low down in the grass, or in the soil until the following spring, and so grazing adult animals or cows over these pastures will not reduce the risk ofÌýNematodirusÌýas the eggs are not consumed (but grazing will still help reduce the pasture burdens of other worms).ÌýAlso, ensiling a paddock does not reduce the risk ofÌýNematodirusÌýthe following year for the same reason. Ìý

Fields where lambs were grazed last year are likely to have aÌýnematodirusÌýlarval challenge over the next month or so. The timing and impact of theÌýNematodirusÌýhatch is individual to the field and the age of lambs grazing it.ÌýMarch born lambs are likely to be ingesting sufficient grass to be at risk of acuteÌýNematodirusÌýon contaminated pastures.ÌýIn later lambing flocks, where lambs are not ingesting significant amounts of grass yet, the ewe’s may ‘clean’ the pastureÌýsomewhat asÌýthey graze, leading to a lesser risk of acuteÌýNematodirosisÌýin these lambs.ÌýÌý

The free SCOPS Nematodirus forecast is available on the and provides an indication of the risk of disease due to Nematodirus in areas across the UK. Local conditions on your own farm should be considered as the hatch will be later on more elevated fields and those with a northerly aspect.ÌýÌý

At the time of writing, several areas of Scotland are categorised as hatching being imminent. The risk of nematodirus will increase when the air temperature exceeds 11.5 °CÌýfor seven days and hatching of eggs on the pasture takes place.ÌýAfter hatching, larvae usually only survive forÌýone to twoÌýmonths on the pasture before dying.ÌýThe majority ofÌýNematodirusÌýdiagnoses made by ÒÁÈËÖ±²¥ in Scotland fall in the months of May and June, usually in 6-12-week-old lambs.Ìý

Lambs withÌýNematodirosisÌýoften have scour and appear empty, although disease can occur suddenly with several deaths in aÌý24-hourÌýperiod sometimes with little scour noted in the group.ÌýÌý

Timing drenching forÌýNematodirusÌýto coincide with the risk period is critical. Fixed time drenching (still common on many farms) may miss the risk period, occurring too early in a cold spring or too late in a warm one.Ìý Ìý

5 top tipsÌý

1. Assess whether the pasture is likely to have a NematodirusÌýburden. Ìý
2. Use SCOPS forecast to predict timing of the hatch on your farm. Ìý
3. Assess if the hatch on your field/farm will likely coincide with when lambs are eating a significant amount of grass (6-12 weeks, younger if ewes are not milking well). Ìý
4. Time treatment strategically to coincide with the period of risk. Ìý
5. Investigate apparent treatment failure with worm/cocci egg counts or post-mortem examinations.Ìý

ÒÁÈËÖ±²¥ Veterinary Services, vsenquiries@sruc.ac.uk

Posted by SAC Consulting on 20/05/2026