Department releases nematodirus forecast for 2026
Nematodirosis is a severe parasitic disease which effects lambs typically between six to 12 weeks of age after becoming infected by ingesting large numbers of infective larvae from grazing on contaminated pasture. File picture
The 2026 nematodirus forecast, which helps to safeguard lambs from disease pressures, has been published.
Each spring the department in collaboration with Met Éireann, University College Dublin (UCD) and Teagasc advises farmers of the predicted risk of disease, primarily in lambs, caused by the roundworm nematodirus battus.
Read More
The analysis of March soil temperatures allows the period of disease risk to be accurately forecast. This can be used to gauge the optimal time to administer anthelmintic (wormer) treatment to prevent disease.
For 2026, the peak nematodirus egg hatching is expected to occur generally at the end of March and early April in many areas of the country, however, some inland areas will see maximum egg hatching around mid-April.
Nematodirosis is a severe parasitic disease which effects lambs typically between six to 12 weeks of age after becoming infected by ingesting large numbers of infective larvae from grazing on contaminated pasture.
The life cycle of N. battus is unlike that of other roundworms in that eggs deposited on pasture do not hatch until the following year to release the infective larvae.
This happens during a mass hatching event in spring when soil temperatures increase after a period of cold weather. Disease typically occurs in April, May and June.
After ingestion from lambs the larvae invade the wall of the intestine. Disease is characterised by profuse diarrhoea, dehydration and weight loss. Calves may also be affected, but usually to a lesser degree.
In outbreak situations, lambs can be seen congregating around troughs due to severe thirst that develops. Adult sheep are unaffected by the parasite.
This disease can be prevented by keeping the current year’s lambs off any 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 to the disease.
Important to note is that 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.
At-risk lambs should be treated approximately two weeks after the peak of nematodirus egg hatching.
Consideration should be given to dosing lambs earlier on individual farms where clinical signs consistent with nematodirus are observed.
The department stressed that other parasites can also 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.
Although some low levels of resistance to benzimidazoles (white drenches) have been detected elsewhere, this class of wormer remains the main treatment of choice for nematodirus infections and is effective for both larval and adult stages.
Using this form of drench will also help reduce exposure of other worms to other anthelmintic classes helping to sustain effectiveness of other drugs. This is particularly important on farms with pre-existing issues of benzimidazole resistance in populations of the other common stomach/intestinal roundworms.
There are no drenches with effective residual activity against nematodirus. This means that as lambs continue 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 throughout the spring.
Read More
If a farmer suspects that the treatment with the wormer used has been ineffective, a follow-up faecal sample is advised in order to test for the presence of nematodirus eggs, which may indicate wormer resistance, or the presence of other pathogens such as coccidia.
A private veterinary practitioner should be consulted for an accurate diagnosis and guidance 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.
It is recommended that any lambs that die unexpectedly are referred to a regional veterinary laboratory for post-mortem examination as nematodirus can cause death before clinical signs become apparent.
