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Department issues advice to sheep farmers over Nematodirus disease in lambs


Nematodirosis is a severe disease of lambs six to twelve weeks of age.

Nematodirosis is a severe disease of lambs six to twelve weeks of age.

Nematodirosis is a severe disease of lambs six to twelve weeks of age.

The Department of Agriculture, Food and the Marine advises farmers in relation to the predicted risk of infection in lambs based on the advice received from the Nematodirus Advisory Group.

The disease

Nematodirosis is a severe disease of lambs six to twelve weeks of age which become infected through ingesting large numbers of infective larvae present on contaminated pasture.

The life cycle of Nematodirus battus is unlike that of other roundworms in that typically it takes almost a year before the egg hatches releasing the infective larva. There is a mass hatching of larvae in spring when the soil temperature increases after a period of cold weather and disease typically occurs in April, May and June.

After ingestion Nematodirus larvae invade the intestinal mucosa and infection is characterised by profuse diarrhoea, dehydration and weight loss.

In outbreak scenarios, lambs can be seen congregating around water troughs due to the severe thirst that develops, while the ewes which are unaffected continue to graze.


This disease is best prevented by keeping the current year’s lambs off pasture that was grazed by lambs or young calves in the previous year.

Enterprises with high stocking rates are particularly vulnerable. Please note that twin lambs, or single lambs born to ewes of poor milking ability may be at a greater risk of developing disease as they begin consuming greater amounts of grass earlier in life. If ‘clean’ pasture is available, preference should be given to moving these lambs first.

When is disease predicted to occur this year?

The maximum Nematodirus larval hatching is expected to peak in the last week of March along the south west coast and west of Ireland, and into the first two weeks of April for most of the rest of the country.

Lambs may begin to show clinical signs of infection two to three weeks from these dates of peak hatching.

Nationally soil temperatures are warmer than normal for this time of year leading to maximum larval hatching occurring two to three weeks earlier than last year.

When should lambs be treated?

Along the south west coast and west of Ireland, lambs should be dosed with a suitable wormer (anthelmintic) by the second week of April (two weeks post peak larval hatching), while lambs in the rest of the country should be dosed during the third week of April.

What to treat lambs with?

Benzimidazoles (white drenches) are the treatment of choice for Nematodirus infections and are effective against both larval and adult stages. The use of this anthelmintic class as the first choice treatment option will also help to reduce the exposure of worms such as Trichostrongylus and Teladorsagia to the other anthelmintic classes (e.g. macrocyclic lactones) at a point in the grazing season when treatment for these may not be necessary. This will help to sustain the effectiveness of these drugs. This is of even greater importance on farms with pre-existing issues of benzimidazole resistance in populations of the common stomach/intestinal roundworms.

The Department warned that currently there are no drenches with effective residual activity against Nematodirus which means that as lambs continues to graze they can become re-infected with larvae and as a result may require repeated treatments at two to three week intervals.

Nematodirus and coccidiosis in lambs

It is also important that farmers are aware that other parasites can cause diarrhoea in young lambs which require different control measures and medication.

Nematodirus can be wrongly assumed to be the cause of severe diarrhoea in lambs when in fact the cause is a coccidial infection. Rotation of pasture and frequent movement of feeding troughs and watering points to drier areas will help prevent coccidiosis in young lambs as localised poaching creates moist conditions suitable for the spread of these parasites.

Raising feeding troughs will also help to reduce the contamination of feed with faeces and hence transmission of coccidiosis.

Veterinary advice and laboratory diagnostic aids

It is advisable to consult a private veterinary practitioner for an accurate diagnosis and advice on appropriate medication if lambs with severe diarrhoea and straining are observed. This is especially the case where there has been little or no response to an initial anthelmintic treatment.

Both nematodirosis and coccidiosis can occur at the same time in the same lambs, so treatment may need to be directed against both pathogens.

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It is recommended that any lambs that die unexpectedly are referred by your private veterinary practitioner to a regional veterinary laboratory for post mortem examination as N. battus can cause death before clinical signs become apparent. It is important to note that most of the pathogenic effects of this parasite are caused by the larval stages. As a result, faecal egg count monitoring is not reliable as a sole guide for the requirement of treatment.

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