The Department of Agriculture, Food and the Marine (DAFM) has issued its forecast on the Nematodirus (nematodirosis) disease for 2018 based on the advice received from the Nematodirus Advisory Group.
The disease affects lambs between six to 12 weeks of age which become infected through ingesting large numbers of infective larvae present on contaminated pasture. The lifecycle 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. The maximum Nematodirus larval hatching is expected to peak in the last week of April for most of the country, with the southwest likely to see peak hatching a week earlier.
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.
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. The DAFM noted 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.
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 (eg. 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.
Please note that 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.
It is also important that farmers are aware that other parasites, like coccidal infection can cause diarrhoea in young lambs which require different control measures and medication. 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.
Contact your vet
It is advisable to consult your 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.