Nematodirosis is a severe parasitic disease of lambs six to twelve weeks of age which become infected through ingesting large numbers of infective larvae from grazing on contaminated pasture.
The life cycle of the causative worm, Nematodirus battus, is unlike that of other roundworms in that typically it takes almost a year on pasture before the egg hatches releasing the infective larva. There is a mass hatching of eggs in spring when the soil temperature increases after a period of cold weather and disease typically occurs in April, May and June.
After ingestion by lambs, Nematodirus larvae invade the wall of the intestine. 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 (which can be carriers of infection) 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 consuming 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?
Across the country Nematodirus egg hatching is expected to peak this year between the 14th and 21st of April which is a few days later than would normally be expected. This is due to the recent cooler than normal soil temperatures.
However, the south-western coastal fringes are expected to see peak egg hatching slightly earlier, i.e. before the 10th of April which is closer to the normal time of maximum egg hatching in these parts of the country.
Please see the map below (Figure 1) for expected peaks in Nematodirus egg hatching on pasture.
When should lambs be treated?
At-risk lambs (i.e. typically 6 – 12 weeks of age and grazing on contaminated pasture) should be treated approximately two weeks after the peak of egg hatching.
Along the south west coast, lambs should be dosed with a suitable wormer (anthelmintic) by the third week of April while lambs in the rest of the country should be dosed between the last week of April and the first week of May.
However, consideration should be given to dosing lambs earlier on individual farms where clinical signs consistent with Nematodirus are observed as the above treatment guidelines are based on the anticipated peak hatch of eggs