Nematodirus in Lambs

Nematodirus is a roundworm that causes diarrhoea in young lambs during late spring and early summer. We discuss the risk factors and how it can be prevented.

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What is Nematodirus?

Nematodirus battus is a roundworm that causes diarrhoea in young lambs during late spring and early summer.

The development of the eggs on pasture into infective larvae depends on a very specific environmental temperature change (a cold snap of weather followed by a period of warmer weather). When conditions are right, huge numbers of eggs can all develop into infective larvae on the pasture at once, exposing lambs to an enormous number of larvae. Because of this, there can be absolutely no delay in treatment if Nematodirus is suspected. Your regular vet should be contacted to discuss the best course of action before any more damage occurs.

Because the parasite is so dependent on weather patterns in order to become infectious, it is possible to forecast the level of Nematodirus risk from year to year. The NADIS parasite forecast ( is an excellent service which is freely available online, and provides a monthly, weather-based estimate of risk levels for Nematodirus, as well as other parasites. This forecast is now complemented by a new interactive SCOPS map ( that allows greater precision in predicting risk by giving very localised data that is updated daily.

Nematodirus Risk Factors

The main risk factors for lambs being affected by Nematodirus include:

  • A sudden cold snap followed by a period of warm weather.
  • Young lambs that are eating a lot of grass (e.g. 6-12 weeks old. Adult sheep are very resistant to infection).
  • Lambs grazing pasture that was used for lambs the previous year.
  • Lambs under other stresses, or under challenge for coccidiosis (if a high-risk Nematodirus period coincides with a high-risk coccidiosis period, i.e. when lambs are 4-8 weeks old, very severe mixed infections can result).

Prevention and Treatment of Nematodirus

Prevention of Nematodirus problems involves avoiding contaminated pastures which might have been grazed by infected lambs the previous season. Targeted treatments with a white drench can also be used and are usually given three weeks apart during May in normal risk years. As always with worming animals, it is vital that it is performed correctly in order to reduce the increasing levels of resistance in worms.

Treatment plans are hugely dependent on the risk forecast, and higher-risk years will require very different protocols to a low-risk year. Keep up-to-date with the latest parasite forecasts to make sure your management practices are appropriate for the risk.

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