Nematodirus warning: The signs, effects and preventative measures

The Department of Agriculture, Food and the Marine has issued a statement warning and advising farmers on the predicted risk of Nematodirus infection in lambs, based on the information received from the Nematodirus Advisory Group.

What is Nematodirosis?

Nematodirosis is a disease in lambs aged between six and 12 weeks which become infected through ingesting large numbers of infective larvae present on contaminated pasture.

The life cycle of Nematodirus battus is unlike that of other roundworms in that typically it takes almost a year before the egg hatches, releasing the infective larva.

There is a mass hatching of larvae in spring when the soil temperature increases after a period of cold weather and disease typically occurs in April, May and June.

After ingestion Nematodirus larvae invade the intestinal mucosa and 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 that develops, while the ewes which are unaffected continue to graze.

This disease is best prevented by keeping the current year’s lambs off pasture that was grazed by lambs or young calves in the previous year. Enterprises with high stocking rates are particularly vulnerable.

Please note that twin lambs, or single lambs born to ewes of poor milking ability may be at a greater risk of developing disease as they begin grazing greater amounts of grass earlier in life. If ‘clean’ pasture is available, preference should be given to moving these lambs first.

When is disease predicted to occur this year?

The maximum Nematodirus larval hatching is expected to peak in the last week of April for the majority of the country, with the milder south-west likely to see peak hatching a week earlier.

Nationally soil temperatures are colder than normal for this time of year leading to maximum larval hatching occurring circa two weeks later than normal. Lambs may begin to show clinical signs of infection two to three weeks from these dates of peak hatching.

When to treat lambs?

In the south-west of the country, lambs should be dosed with a suitable wormer (anthelmintic) by the first week of May (two weeks post peak larval hatching), while lambs in the rest of the country should be dosed by the second week of May – depending on farm location and individual flock factors.

This will help to decrease the risk of clinical disease and reduce pasture contamination for the next year.

What to treat lambs with?

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 the other anthelmintic classes (e.g. macrocyclic lactones) to worms such as Trichostrongylus and

Teladorsagia at a point in the grazing season when treatment for these may not be necessary. This will help to sustain their effectiveness.

This is of even greater importance on farms with pre-existing issues of benzimidazole resistance by the common stomach/intestinal roundworms.

Currently there are no drenches with effective residual activity against Nematodirus – which means that as the lamb continues to graze it can become re-infected with larvae again and as a result may require repeated treatments at two to three week intervals.

Nematodirus and coccidiosis in lambs

The department warned farmers that other parasites can 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.

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 this parasite.

Raising feeding troughs will also help to reduce the contamination of feed with faeces.

Veterinary advice and laboratory diagnostic aids

The department advises farmers to consult a private vet for an accurate diagnosis and advice on appropriate medication if lambs with severe diarrhoea and straining are observed.

This is especially the case where there has been little or no response to an initial anthelmintic treatment.

Both nematodirosis and coccidiosis can occur at the same time in the same lambs, so treatment may need to be directed at both pathogens.

It is recommended that any lambs that die unexpectedly are referred by your vet to a regional veterinary laboratory for post-mortem examination as N. battus can cause death even 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. As a result of this, and coupled with the fact that this worm is a poor egg producer, you should not rely on the use of faecal egg count monitoring as a sole guide for treatment, the department has warned.