Farmers need to be mindful to the risk of nematodirus occurring earlier in 2021. The Department of Agriculture released its nematodirus forecast last Thursday, three weeks earlier than in 2020.

The forecast highlights that nematodirus larval hatching is expected to peak between 23 March and 13 April. This is a week earlier, on average, than would normally be expected, with the forecast citing higher than normal soil temperatures as being responsible.

Farmers should also be mindful that there is quite a bit of variation across the country. The peak larval hatching period for most of the country is expected in the week of 5 April to 10 April. The exception to this is the southwest and northwest coastal region, where peak hatching is expected to have occurred in the period between late March and the first few days of April, as reflected in Figure 1.

At-risk lambs

Lambs which are at risk are those consuming significant quantities of grass on contaminated pastures and as such are also likely to be ingesting high numbers of nematodirus larvae. The highest risk is with lambs aged six to 12 weeks of age but younger lambs which are under nutritional stress and as a result forced to consume higher levels of grass earlier can also face a significant challenge.

The advice is to treat these lambs approximately two weeks after the peak egg hatching. On farms along the west coast with lambs of this age, it is recommended to treat lambs by the second week of April while for the rest of the country treatment should take place for lambs at risk in the last two weeks of April. These dates will be pushed out further for later-lambing flocks. Likewise, treatment may be required earlier where clinical signs of a nematodirus infection are present.

Product selection

Treatment for nematodirus is the second component of a four-part initiative set up by Teagasc in tandem with the Department of Agriculture and UCD to tackle a worrying increase in anthelmintic resistance. The programme, which is being supported by Chanelle, Elanco, Norbrook and Zoetis, focuses on promoting best practice at key time frames during the season.

Teagasc sheep specialist Damian Costello says: “Anthelmintic resistance is developing at an alarming rate to group 1-BZ (white), group 2-LV (yellow) and group 3-ML (clear) anthelmintic classes with an increasing number of instances where treatment during the main grazing season to other roundworms such as Teladorsagia and Trichostrongylus is not 100% effective. We need to use wormers strategically to reduce the rate of resistance developing and treatment for nematodirus has a major role to play”.

Damian says group 1-BZ or benzimidazoles/white drenches should be the only product selected when treating for nematodirus. White drenches are effective against both larval and adult stages and will work adequately on the high number of farms with anthelmintic resistance issues in common midseason roundworms. Adopting this practice will safeguard the other anthelmintic classes for use later in the season.

Strategic treatment

There is no product available that possesses effective residual protection against nematodirus. As such, lambs can become re-infected post-treatment and may require repeat treatments depending on the age of lambs and risk profile on the farm.

Damian explains there is also a greater risk on highly stocked farms as there is no opportunity to alternate between paddocks grazed in the previous year while swards are likely to possess a high larval burden. In such high-risk scenarios, repeat treatment may be required at two-to-three-week intervals and Damian stresses white drenches should remain the product of choice for these treatments.

Likewise, where there is a wide range in the age of lambs within a grazing group, the first dose may treat lambs that may not yet require treatment while delaying dosing will leave older lambs highly exposed. This is an added benefit of getting grazing groups established and keeping the age range between lambs to a minimum. Where this is not possible, strategic treatment will be required.

Characteristic symptoms

Following ingestion, the Nematodirus battus larvae attack the wall of the intestine. Characteristic signs of infection include lambs scouring, with a normal green-coloured scour typical. A high burden will quickly lead to profuse diarrhoea, dehydration and significant weight loss. In such cases lambs will regularly congregate around water troughs due to the intense thirst caused by diarrhoea and dehydration.

Damian says that while the peak hatch poses the greatest risk, disease may continue to occur during May and June.

While rare, there have also been instances of significant issues occurring as late as August in lambs that have yet to develop immunity.

Top tips

A number of other considerations should be borne in mind to optimally protect your flock from nematodirus.

  • The information in the map above is based on data collated by Met Éireann and while it gives a good estimate of peak larval hatching on pasture, close supervision should be adopted for clinical signs of the disease.
  • Coccidiosis can occur at the same time as nematodirus, while coccidiosis can also be mistakenly identified as nematodirus which presents a high risk as treatment is different. Characteristic symptoms of coccidiosis include a dark, often blood-stained scour and severe straining by lambs.
  • Any unexplained deaths should be submitted via your vet to the veterinary laboratory for post-mortem analysis.
  • As the nematodirus larvae cause the damage and infection can be rapid, faecal egg counts are not a reliable tool for identifying if treatment is warranted.