Peak nematodirus egg hatching hitting some areas earlier

Peak hatching of nematodirus eggs has now occurred in all areas of the country. The peak hatching dates detailed in Figure 1 have occurred up to a week earlier in some areas when compared to 2024. The most significant change is a week earlier hatching in the Achill region in Co Mayo and five days earlier in the Inishowen region Co Donegal.

Peak hatching also occurred two to three days earlier in the Sligo region and from one to three days earlier in the east.

This contrasts to the southern half of the country where peak hatching was forecast to take place later on average. Peak hatching in the Limerick region was forecast five days later and three days later in Wexford and Cork.

The forecast should be used as an indicator along with taking account of farm circumstances and symptoms of disease.

A forecast for Northern Ireland is expected to be released imminently and include peak hatching in early April.

The majority of sheep farms are each year faced with the challenge of nematodirus as it is generally not possible to have clean grazing available, i.e. swards that were not grazed by lambs in the previous spring.

The life cycle of Nematodirus 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.

Given the right environmental conditions, worm eggs tend to hatch over a short period of time leading to a surge in the number of infective larvae on pasture, which lambs ingest.

The degree of risk is also influenced by the stocking rate with highly stocked sheep-only farms tending to have the highest risk profile.

The forecast outlines that at-risk lambs are typically aged six to 12 weeks of age and grazing contaminated pasture.

Younger lambs facing a nutritional challenge can also suffer from the disease as there is increased pressure to graze to meet nutritional requirements.

The advice is to treat at-risk lambs approximately two weeks after the peak of nematodirus egg hatching.

The forecast highlights that peak hatching dates are regional estimates and as such “consideration should be given to dosing lambs earlier on individual farms where clinical signs consistent with nematodirus are observed”.

Following ingestion by lambs nematodirus larvae invade the wall of the intestine.

Disease is characterised by profuse diarrhoea (normally of a green colour), dehydration and weight loss.

In the case of significant outbreaks, lambs can be seen congregating around water troughs due to the “sever thirst that develops” as a result of dehydration.

Calves can also be infected and this is usually to a lesser degree. Adult sheep are unaffected by the parasite.

The forecast is released by the Department of Agriculture in collaboration with Met Éireann, UCD and Teagasc with extensive expertise contributing to the advice. The treatment of choice is a white drench or benzimidazoles. This is effective against both larval and adult stages.

Selecting this anthelmintic class will “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 other common stomach/intestinal roundworms”.

It adds that it is also important to note that there are currently no drenches on the market with effective residual activity against nematodirus meaning, as lambs continue to graze they can become re-infected.

“As a result [lambs] may require repeated treatments with the same or similar wormers at two to three-week intervals throughout the spring”.

This will again depend on the risk profile on the farm, age of lambs etc. As touched on already, twin or single suckling lambs born to ewes of poor milking ability or a poor milk yield may be at greater risk of developing the disease as they begin to consume greater amounts of grass earlier in life. Preference should be given to offering such lambs the cleanest grazing while addressing nutritional demands will also help.

It is not just nematodirus which can trigger diarrhoea in young lambs. Coccidiosis is also a significant cause of disease-related diarrhoea in young lambs.

It can also occur on a regular basis for both nematodirus and coccidiosis to be present at the same time.

The threat of coccidiosis has been lower this year owing to the favourable weather but issues are still prevalent. Symptoms are similar in both diseases but coccidiosis is more characteristic of a dark grey or black (blood stained) scour.

“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”. The forecast advises that “rotation of pasture and frequent movement of feeding troughs 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 the transmission of coccidiosis”.

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 nematodirus can cause death before clinical signs become apparent.

In brief