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What is Nematodirus?

Nematodirus battus is a real cause for concern in the UK. Changing climatic conditions have meant that disease caused by this parasite has been a real challenge for sheep over the last few years. The Nematodirus lifecycle usually takes much longer than other roundworms . Eggs laid in spring develop slowly to the third larval stage inside the egg, after which they can survive on pasture for up to two years. Hatching takes place after a prolonged cool period (over winter), followed by an average daily temperature of 10˚C or more. If weather conditions are right, the majority of larvae will hatch at the same time (‘mass hatch’), which means that lambs are faced with a short, sharp burst of damage to their intestines, causing acute disease. If hatching is over a longer period of time, disease will not be so evident, but growth rates can suffer greatly. White drenches e.g. Rycoben™ have high activity against Nematodirus and there have been very few confirmed cases of resistance to white drenches in this parasite in the UK.

Nem Lifecycle

The ‘mass hatch’

If weather conditions are conducive, the majority of larvae will hatch at the same time. This is called a ‘mass hatch’. If it occurs when naive lambs of any age are grazing, the immature worms will be ingested and cause sudden and devastating damage to the lambs’ intestines, resulting in severe disease, even death. The first indication many farmers may see of Nematodirus is a dead lamb. If, due to weather conditions, hatching occurs over a longer period (’trickle hatching’), only small numbers of larvae will hit the intestines at a time, lambs will have the chance to gain some immunity and disease will not be so evident. However, depending on the level of gut damage, this lower level of infection can still cause a significant growth check at a time when lambs should be hitting their maximum growth rates. Similarly, if the hatch occurs early, before lambs are consuming significant amounts of grass, there is a reduced risk of disease. Once hatched the larvae don’t survive for long on pasture so damaging infections in lambs will have been avoided. Immunity to Nematodirus battus develops quite quickly following exposure, so adult animals that have had some exposure during the previous grazing season are not affected.

Paying attention to the SCOPS and NADIS forecasts, and treating as soon as appropriate, is a vital tool in managing the Nematodirus risk.

Symptoms and diagnosis - Disease is caused by larval stages damaging the small intestine. This damage prevents the gut wall from exchanging fluids and nutrients properly, leading to diarrhoea, lack of appetite, dehydration and thirst. In the case of the mass hatch, there will be a sudden onset of profuse diarrhoea with associated dehydration, and often death. As ewes are unaffected they are often seen to be still grazing whilst the thirsty lambs are crowded around drinking places.

Veterinary diagnosis is based on examination of faecal samples and post-mortem examinations and often rest on determining between Nematodirus and acute coccidiosis which have similar clinical signs, occur at the same time of year, and can be present at the same time. Faecal Egg Counts (FECs) will identify coccidiosis but not Nematodirus as the damage is done by immature larvae that are not producing eggs. It is vital that a diagnosis is made so that farmers can treat the right disease with the right product.

Managing the risk

  1. Grazing management
    Grazing management and the use of ‘low risk’ pasture can go a long way towards controlling a Nematodirus problem. Avoid grazing this years’ lambs where young lambs (3-12 weeks) have grazed the previous year, and so prevent exposure to large numbers of larvae that are ready to hatch.
  2. Monitor the risk
    Pay attention to SCOPS and NADIS forecasts. Using live weather station data, these forecasts predict the level of challenge and time of hatching at each location and are updated daily. They will show when the risk is high in the local area. For more information visit or
  3. Treat promptly
    Treatment with anthelmintics should be administered following SCOPS Nematodirus risk warnings and repeated as necessary over the risk period. It is unwise to wait for clinical signs of diarrhoea to occur before treating as severe gut damage can occur, causing long term effects on growth rates.

Beware of autumn Nematodirus

Like many parasites, this nematode has shown an ability to adapt its behaviour and in some areas some outbreaks of disease are now being seen in autumn. This occurs when eggs on pasture develop to the third larval stage – without overwintering – and hatch in the autumn, with a ‘mass hatch’ potentially occurring in the warm, wet period following a dry summer. Lambs that avoided exposure in the spring are particularly at risk.

Treatment options

In the case of an outbreak, anthelmintic treatment of all lambs, plus supportive therapy to manage dehydration, will be advised. Affected animals may need to be housed for a short period. Group 1 benzimidazole (BZ) white drenches are the active of choice against Nematodirus battus as this group has high activity against Nematodirus and there have been very few cases of resistance to white drenches in this parasite in the UK.

Act now, find out more about how you can incorporate Zolvix for your mid-late season break dose

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