The Department of Agriculture Food and the Marine (DAFM) released its Nematodirus forecast late last week – with peak hatching expected to take place earlier than usual this year.

Peak hatching is likely to take place between March 23 and April 13 – which is about one week earlier than normally expected and is due to milder than normal soil temperatures.

The majority of the country will see maximum larval hatching this week with the exception of the south-western coastal region where the peak is expected to have occurred between late March and the first days of April, and coastal fringes of the west and north-west where it was expected this week.

Nematodirus: What is it?

Nematodirosis is a severe parasitic disease of lambs aged six to 12 weeks-old, 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 the eggs deposited on pasture do not hatch until the following year to release the infective larvae.

There is a mass hatching of eggs in spring when the soil temperature increases after a period of cold weather. The 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. Adult sheep are unaffected by the parasite.

This disease is best prevented by keeping the current year’s lambs off any pasture that was grazed by lambs or young calves (who can be carriers of infection) in the previous year.

Farms with high stocking rates are particularly vulnerable. Twin lambs or single lambs born to ewes of poor milking ability may be at a greater risk of developing the 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.

The map (below) gives an indication for the expected timing of peaks in Nematodirus egg hatching on pasture.

Estimated dates in March and April 2021 when maximum Nematodirus egg
hatching is expected to occur on pasture. Data source: Met Éireann

When should lambs be treated?

At-risk lambs (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, as well as coastal fringes of the west and north-west, lambs should be dosed with a suitable wormer (anthelmintic) by the second week of April, while lambs in the rest of the country should be dosed in the last two weeks of April.

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 estimated peak hatch of eggs.

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 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.

Currently, there are no drenches with effective residual activity against Nematodirus.

This means that as lambs continue to graze they can become reinfected with larvae, and as a result may require repeated treatments with the same, or similar wormers, at two to three-week intervals throughout the spring.

Nematodirus and coccidiosis in lambs

It is also important that farmers are aware that other parasites can also 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 coccidia.

Raising feeding troughs will also help to reduce the contamination of feed with faeces and hence transmission of coccidiosis. In some cases, nematodirus 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 Nematodirus 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.

Therefore, faecal egg counts alone do not provide a reliable basis for deciding when to treat lambs.