Nematodirus in Lambs

This page is intended to support the SCOPS Nematodirus Forecast.

Nematodirosis can strike very quickly so you can’t afford to have a ‘wait and see’ policy. And because the damage is done by large numbers of immature larvae that are not producing eggs, faecal egg counts (FECs) are not a reliable indicator of risk. Rapid action is often required and this has to be based on a risk assessment (more below) and the forecast for your area (see the forecast map).

Main risk factors

If your lambs are grazing pasture that carried lambs last spring and you answer yes to one or more of these questions, your lambs are at risk.

  • Are they old enough to be eating significant amounts of grass? (generally 6-12 weeks of age but may be younger if ewes are not milking well)
  • Do you have groups where there is also likely to be a challenge from coccidiosis? For example, mixed aged lambs are a higher risk
  • Has there been a sudden, cold snap recently followed by a period of warm weather?
  • Have you got lambs that are under other stresses e.g. triplets, fostered, on young or older ewes.

Recommended actions

  • If possible, avoid infection. Move at-risk lambs (as determined by the risk assessment) to low risk pastures (i.e. pasture that was not grazed by lambs the previous spring). See SCOPS Technical Manual. Chapter 2.4.1 and Chapter 2.4.2.  
  • If you cannot avoid high risk pasture grazed by lambs the previous spring and decide you need to treat for nematodirus, SCOPS advises farmers to use a white (1-BZ) drench. Use the SCOPS ‘Know Your Anthelmintics’ Guide to select a product. These are still highly effective against this parasite on most farms and suitable for young lambs. Check that treatment is effective by taking a FEC seven to 10 days after treatment. Remember, it may be necessary to treat lambs more than once depending on the spread of ages in a group and subsequent weather conditions.

Understanding nematodirus

Nematodirosis is a particularly nasty disease in lambs, causing a high number of mortalities and stunting the growth of many others. It is caused by the Nematodirus battus worm, which has a different lifecycle to other sheep worms. Under certain climatic conditions it can strike very quickly, with little or no warning. This means sheep farmers have to be on their guard.

The main difference in the lifecycle of Nematodirus battus compared with other parasitic worms is that development to an infective larvae takes place within the egg and infection passes from one lamb crop to the next years’ crop. Before they can hatch, the eggs have to undergo a period of cold weather followed by warmer temperatures of 10°C or more. If these conditions occur over a short period of time, triggering a mass hatch, and it coincides with the time when lambs are starting to take in significant amounts of grass (over about six weeks old), the result can be devastating. See SCOPS Technical Manual. Chapter 3.1.2

The timing of a potential problem will vary from region to region. In the south of England for example, it was traditionally likely to occur earlier in April/May, compared to early June in northern England and Scotland. However, with the weather being increasingly unpredictable, with particularly warm weather not unheard of in February, the SCOPS Nematodirus Forecast is a more useful tool than working on a calendar date, or remembering what has happened in previous years.


As explained above, SCOPS advises farmers to use a white (1-BZ) drench when treating. These are normally highly effective against this parasite and suitable for young lambs. However, the first confirmed case of nematodirus resistant to the 1-BZ group was reported in 2011 and so farmers are advised to follow SCOPS guidance on correct anthelmintic administration and to check afterward if the treatment has been fully effective. If in doubt consult your vet or adviser.