The Department of Agriculture has warned sheep farmers that their lambs are likely to suffer from a Nematodirus outbreak this month.

According to the Department, Nematodirus is a severe disease affecting lambs between six-to-12 weeks of age, leading to a reduction in lamb growth rates and in severe cases death.

The disease is most likely to occur in early lambing flocks where lambs are five-to-six weeks of age and are already grazing.

Lambs become infected after ingesting large numbers of infective larvae present on contaminated pasture, typically during April, May and June.

Maximum Nematodirus larval counts:
  • West of Ireland – first week of April
  • Connacht and coastal counties in Munster and Leinster – second week of April
  • Inland counties: third week of April

According to the Department, lambs may show clinical signs of infection two-to-three weeks from the peak Nematodirus larval counts.

In the extreme west of the country, lambs should be dosed from mid-April on, while lambs in the rest of the country should be dosed from late April to early May depending on the farm location, the Department recommends.

It also says that farmers should seek veterinary advice in the clinical cases or for a flock control programme for intensive sheep flocks.


Teagasc research also shows that mortality can be high in untreated lambs suffering from a Nematodirus outbreak.

Infected lambs can be easily identified, as their heads will be down and their ears will be dropped along with having a tucked up abdomen, as they are in pain.

If the infection is untreated, the lambs will also develop a scour, which may lead to profuse diarrhoea and wasting. But, lambs may also stop eating and the scour may only consist of a slimy mucus.

Infected lambs can also be seen gathering around water troughs, due to thirst and dehydration.

Prevention and Control

According to Teagasc, the best way to stop a Nematodirus outbreak is to keep lambs off grassland that was grazed by lambs last year.

This could be ground that was closed from early spring until after the first cut or silage was taken or ground that had been grazed by adult cattle last year.

If clean pasture is not available, lambs will require a dose to control the parasitic infection. According to Teagasc, a white (Benzimidazole) or yellow (Levamisole) dose works best to control Nematodirus.

Main risk factors:
  • Lambs grazing pasture that was grazed by lambs or young calves the previous spring
  • Lambs old enough to eat a large amount of grass face an increased risk of ingesting large numbers of worm larvae
  • Twin lambs are more likely to suffer from a Nematodirus outbreak younger, as the begin to graze earlier
  • A cold period followed by warm weather triggers the synchronised hatching of eggs, which have been on the pasture since the previous spring