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Department's Nematodirus Advisory Group Issues its Forecast for Spring 2016 to Enable Well-Timed Treatment of Lambs This Spring

The Department of Agriculture, Food and the Marine advises farmers in relation to the predicted risk of infection in livestock based on the advice received from the Nematodirus Advisory Group.

About the disease

Nematodirosis is a severe disease of lambs six to twelve weeks of age, which become infected through ingesting large numbers of infective larvae present on contaminated pasture. The life cycle of the Nematodirus battus worm is unlike that of other roundworms in that, typically it takes almost a year before the egg hatches releasing the infective larvae. 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.

Infection is characterised by profuse diarrhoea, dehydration and weight loss. Mortality can be high in untreated lambs. After ingestion, Nematodirus larvae invade the intestinal mucosa and in some cases death may occur before signs of diarrhoea are observed. Ewes will appear clinically normal. This disease is best prevented by keeping the current year’s lambs off pasture that was grazed by lambs or young calves last year.

Forecasted timing of infection and treatment this spring

The maximum Nematodirus larval count on pasture is expected by the end of the first week of April in the extreme west of the country, with peak larval counts occurring in the rest of the country by the second week (in Connaught and coastal counties in Munster and Leinster) and third week (in inland counties) of April. Lambs may show clinical signs of infection two to three weeks from these dates of peak hatching.

In the extreme west of the country, lambs should be dosed with a suitable anthelmintic from mid April onwards (two weeks post peak larval counts), while lambs in the rest of the country should be dosed from late April to early 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. Early lambing flocks where lambs are five to six weeks of age and already grazing are particularly vulnerable; as are enterprises with higher stocking rates where lambs are grazing pastures grazed by last year’s lambs. As environmental conditions will vary from farm to farm, it is important that veterinary advice should be sought in the event of clinical cases or for a flock control program comprising specific measures for any intensive sheep flock.

It is recommended that any lambs that die from unknown causes are referred by your vet to a regional veterinary laboratory for post-mortem examination as Nematodirus battus can cause death before clinical signs are 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 nematode is a poor egg producer, you should not rely on the use of faecal egg count monitoring as a sole guide for treatment.

Selecting a treatment

Although all three common classes of anthelmintics can be used to treat lambs with Nematodirus infections, benzimidazoles (white drench) remain the treatment of choice and are effective against both larval and adult stages of this nematode. The use of benzimidazoles as a first choice treatment option will also help to limit the exposure of the other anthelmintic classes to nematodes such as Teladorsagia and Trichostrongylus at a point in the grazing season when treatment for these may not be particular necessary. This will help to preserve their efficacy and is especially important on farms where resistance to benzimidazoles occurs. 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 important to ensure that drenches are given over the back of the tongue to avoid stimulating the oesophageal groove reflex.

Disease caused by Nematodirus can resemble coccidiosis in lambs

It is also important that farmers are aware that other parasites cause diarrhoea in young lambs and 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 reduce the contamination of feed with faeces.

It is advisable to consult a private veterinary practitioner 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 treatment. Both nematodirosis and coccidiosis can occur at the same time in the same lambs, so treatment may need to be targeted at both pathogens.

 View Press Release as a PDF:  DAFMPR 37/2016 (pdf 364Kb) 

Date Released: 20 April 2016