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.
Nematodirosis is a severe disease of six to 12-week-old lambs, which become infected through ingesting large numbers of infective larvae present on contaminated pasture. The life cycle of the Nematodirus battus worm is unusual in that, typically, there is a mass hatching of larvae in spring when the soil temperature increases after a period of cold weather.
Infection is characterised by dehydration, profuse diarrhoea and weight loss. Mortality can be high in untreated lambs.
Nematodirus larvae invade the intestinal mucosa after ingestion 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 last year.
Mean soil temperatures for March this year were slightly colder almost everywhere than their long-term average. The maximum Nematodirus larval count was expected by the start of April in the south-west and spreading to the rest of the country by the middle of the month.
Nationally, the weather conditions leading to maximum larval occurrence were similar to last year and in line with normal conditions (normal conditions are defined as the average of weather conditions between the years 1981 and 2010).
Lambs may show clinical signs of infection two to three weeks from these dates of peak hatching. In the south and west, lambs should be dosed with a suitable anthelmintic from mid to late April, while those lambs in the rest of the country should be dosed from late April to early to mid-May depending on farm location and individual flock factors. This will decrease the likelihood of clinical disease later and reduce pasture contamination for the next year.
Early lambing flocks where lambs are five to six weeks old and already grazing are particularly vulnerable; as are enterprises with higher stocking rates where lambs are grazing pastures grazed by last year’s lambs.
Due to a greater variability in the dates of peak risk this year, in addition to the fact that 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 of unknown causes are submitted to a regional veterinary laboratory for post-mortem examination as Nematodirus battus can cause death before clinical signs are apparent.
Benzimidazoles (white drench) and levamisole (yellow drenches) are effective against larval and adult stages of Nematodirus battus. 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.
Farmers are urged to comply with advice regarding the correct use of any medicines on the manufacturer’s data sheet.
There is further information about labelling of anthelminthics to distinguish to which group each labelled product belongs on the Health Protection Regulatory Authority’s website.