There were 1,025 press releases posted in the last 24 hours and 425,064 in the last 365 days.

The Department of Agriculture, Food and the Marine’s 2020 Nematodirus Forecast

The disease

Nematodirosis is a severe parasitic disease of lambs six to twelve weeks of age which become infected through ingesting large numbers of infective larvae from grazing on contaminated pasture. The life cycle of the causative worm, Nematodirus battus, is unlike that of other roundworms in that typically it takes almost a year on pasture before the egg hatches releasing the infective larva. There is a mass hatching of eggs in spring when the soil temperature increases after a period of cold weather and disease typically occurs in April, May and June.


After ingestion by lambs, Nematodirus larvae invade the wall of the intestine. 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 (which can be carriers of infection) 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?


Across the country Nematodirus egg hatching is expected to peak this year between the 14th and 21st of April which is a few days later than would normally be expected. This is due to the recent cooler than normal soil temperatures.


However, the south-western coastal fringes are expected to see peak egg hatching slightly earlier, i.e. before the 10th of April which is closer to the normal time of maximum egg hatching in these parts of the country.


Please see the map below (Figure 1) for expected peaks in Nematodirus egg hatching on pasture.



When should lambs be treated?


At-risk lambs (i.e. typically 6 – 12 weeks of age and grazing on contaminated pasture) should be treated approximately two weeks after the peak of egg hatching.


Along the south west coast, lambs should be dosed with a suitable wormer (anthelmintic) by the third week of April while lambs in the rest of the country should be dosed between the last week of April and the first week of May.


However, consideration should be given to dosing lambs earlier on individual farms where clinical signs consistent with Nematodirus are observed as the above treatment guidelines are based on the anticipated peak hatch of eggs.



Figure 1. Estimated date in April 2020 when maximum Nematodirus egg hatching is expected to occur on pasture

 Met Eireann map


Source: Met Éireann





What wormer should be used?

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 other 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 and is particularly important on farms with pre-existing issues of benzimidazole resistance in populations of the common stomach/intestinal roundworms.

Please note that currently there are no drenches with effective residual activity against Nematodirus. This means that as lambs continues to graze they can become re-infected with larvae, and as a result may require repeated treatments with the same or similar wormers 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 coccidia. Raising feeding troughs will also help to reduce the contamination of feed with faeces and hence transmission of coccidiosis. In some cases, nematodirosis and coccidiosis can occur together, giving rise to severe disease.


Veterinary advice and laboratory diagnostic aids

It is advisable to consult a private veterinary practitioner for an accurate diagnosis and advice on appropriate medication when lambs with severe diarrhoea and straining are observed. This is especially the case where there has been little or no improvement from an initial worming treatment. As both nematodirosis and coccidiosis can occur at the same time, treatment may need to be directed against both pathogens.


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.




Date Released: 20 April 2020