Sheep producers need to be mindful to the possibility that nematodirus infection may present a challenge to their lambs at an earlier date in 2024.

The 2024 Nematodirus Forecast released on Thursday 4 April by the Department of Agriculture, Food and the Marine shows that on average peak hatching will occur a week to 10 days earlier than in 2023.

The forecast highlights that peak nematodirus egg hatching will generally occur at the end of March in much of Munster and south Leinster, while the midlands and northern half of the country will see peak egg hatching in early April (Figure 1).

Northern Ireland

The Northern Ireland nematodirus forecast, also released on Thursday 4 April by the Agri-Food and Bio-Sciences Institute (AFBI) paints a similar picture. The forecast states that 10% of the annual hatch is likely to have occurred in mid-March with peak hatching taking place elsewhere in the first week of April. It is probable that the earlier hatching dates are in coastal regions.

Figure 1: Estimated dates in March and April 2024 when peak nematodirus egg hatching is expected to occur on pasture.

Risk period

Lambs facing the greatest risk are those typically aged six to 12 weeks old and grazing on contaminated pasture. The timeline from lambs ingesting significant numbers of larvae to requiring treatment is generally two weeks.

Both forecasts, however, caution that as the above guidelines are based on regional estimated peak hatching of eggs, consideration should be given to dosing lambs earlier on individual farms where clinical signs are observed.

The lambs facing the greatest risk are also those most reliant on grazed grass to meet nutritional requirements and where there is likely to be a large egg burden on pasture. As such, enterprises operating at high stocking rates are particularly vulnerable while lambs born to ewes of poor milking ability may also face a greater challenge.

The continued adverse weather is also putting lambs at greater nutritional stress this spring.

After ingestion, nematodirus larvae invade the wall of the intestine giving rise to profuse diarrhoea (green in colour), dehydration and weight loss. The forecast advises that in outbreak scenarios lambs can often be seen congregating around water troughs due to the severe thirst that develops.

Lambs will develop an immunity to disease over time but still need treatment. Adult sheep are unaffected by the nematodirus parasite.

Farmers are advised to treat lambs with benzimidazoles or white wormers. The reasoning for selecting benzimidazoles is to reduce exposure of other worms such as Trichostrongylus and Teladorsagia to other anthelmintic classes (eg macrocyclic lactones) at a point in the grazing season when treatment for these may not be necessary.

There is no product available with effective residual activity to nematodirus.

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

Faecal egg counts are not reliable in determining the presence of disease as parasites may be present in larval stage and causing huge issues without being identified. It should also be noted that diarrhoea can also be caused by coccidiosis and to diagnose the disease accurately where any doubt exists.