Parasite of the month; Nematodirus

Farm vet David discusses our parasite of the month; Nematodirus. This disease can rapidly kill young lambs at grass, so knowledge is key.

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Nematodirosis is an important disease that affects (and rapidly kills) young lambs at grass.

Caused by the parasitic worm Nematodirus battus, it can take hold and cause widespread death and disease very quickly, especially in certain weather conditions.

Worm development occurs within the egg, which can survive on pasture for up to two years. Hatching occurs when temperatures see a cold snap and then reach 10°C again. As a result, we often see a mass emergence in spring which can coincide with turnout of lambs (usually affecting those over 6 weeks, however thin/weak lambs that don’t get enough milk can be affected earlier in life). Thankfully most lambs will have a level of immunity once they exceed 3-4 months.

SCOPS provide a Nematodirus forecast online:


The best method of prevention is to avoid grazing lambs on pasture that was grazed by lambs the previous year (i.e. pasture rotation).

Nematodirus is currently treated well by a Class-1 (White) benzimidazole product. Flock Health Club members have access to discounted Endospec throughout the spring!


Diagnosis is through clinical signs (profuse watery scour with dull, depressed lambs), history of the pasture (if it’s been grazed by lambs the year before they may have shed eggs that have overwintered), and through post mortem examination of dead lambs.

Faecal egg counts have limited use for nematodirus, if even ONE egg is seen then treatment is recommended as the clinical condition is caused by N.battus larvae and adults before they mature to lay eggs.

Key Questions

SCOPS identify the key questions to assess your risk as:

  • Are your lambs old enough to be at risk? (Usually 6 – 12 weeks to be eating enough grass, possibly younger if ewes lack milk)
  • Has there been a sudden cold snap followed by warm weather?
  • Have you got lambs under other stressors? (e.g. triplets, fostered or from old ewes)
  • Do you have groups where mixed infection with coccidiosis is highly likely? (e.g. mixed age groups) AND
  • Are you using pasture that was grazed by lambs last spring?

If you’d like to discuss this more with you vet, or have any concerns, don’t hesitate to contact us.

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