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 (Figure 1) hatches releasing the infective larvae. 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.

Infection is characterised by profuse diarrhoea, dehydration and weight loss. Mortality can be high in untreated lambs. After ingestion, Nematodirus larvae invade the intestinal mucosa and in some cases death may occur even before clinical signs of diarrhoea are observed. Ewes are not affected. 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.

When is disease predicted to occur this year and when to treat?

The peak hatching of Nematodirus larvae is predicted to be the last week of March along the south coast of Ireland, and into the first week of April for most of the rest of the country. Nationally soil temperatures are warmer than normal for this time of year leading to maximum larval hatching occurring 1-2 weeks earlier than in recent years. Lambs may show clinical signs of infection two to three weeks from these dates of peak hatching.

Along the south coast of the country, lambs should be dosed with a suitable anthelmintic by the second week of April (two weeks post peak larval hatching), while lambs in the rest of the country should be dosed from mid April depending on farm location and individual flock factors. This will help to decrease the risk of clinical disease and reduce pasture contamination for the next year.

Early lambing flocks and enterprises with higher stocking rates, where lambs are grazing pastures grazed by last year’s lambs, are particularly vulnerable. Twin lambs may be at risk of developing disease earlier compared to their single lamb counterparts as they begin grazing greater amounts of grass earlier in life.

As environmental conditions will vary from farm to farm, it is important that veterinary advice should be sought in the event of clinical cases or for a flock control program comprising specific measures for any intensive sheep flock.

It is recommended that any lambs that die unexpectedly are referred by your vet to a regional veterinary laboratory for post-mortem examination as Nematodirus battus can cause death even 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 of this, and coupled with the fact that this worm is a poor egg producer, you should not rely on the use of faecal egg count monitoring as a sole guide for treatment.

Treatment Choice

Benzimidazoles (white drenches) remain the treatment of choice and are effective against both larval and adult stages of this worm.

The use of benzimidazoles as a first choice treatment option will also help to limit the exposure of the other anthelmintic classes to worms such as Teladorsagia and Trichostrongylus at a point in the grazing season when treatment for these may not be particular warranted. This will help to preserve their efficacy and is especially important on farms where resistance to benzimidazoles occurs.

Please note that currently there are no drenches with effective residual activity against Nematodirus which means that as the lamb continues to graze it can become re-infected with larvae again and as a result may need repeated treatments at two to three week intervals. 

Nematodirus infections can resemble coccidiosis in lambs

It is also important that farmers are aware that other parasites can cause diarrhoea in young lambs and 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 this parasite. Raising feeding troughs will also help reduce the contamination of feed with faeces.

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

Figure 1. Faecal sample with Nematodirus eggs (N) among a couple of strongyle eggs (S) and coccidial oocysts (O)

To view this Press Release as a PDF:  DAFMPR 64/2017 (pdf 252Kb) ‌‌

Date Released: 12 April 2017