Nematodirusbattus
The Nematodirusdisease risk in lambs this spring isforecast to be average.
Incidence is generally high when spring is late, as thehatch of larvae is delayed until main-crop lambs are consuming significant amounts of grass. Theforecast peak of pasture larval infectivity in northeast
England is around the 12-15th April.The high-risk period is often assumed to be the6-8 weeks followingthis, although cool and damp Mayand June weather may extend this risk period byextending the period of larval survival on the pastureand/or allowing hatching to continue for longer.
Lambs are usually considered most at risk between 6 to12 weeks old, although younger lambs can be at risk assoon as they start grazing when pasture infectivity ishigh.
The best control method is to avoid grazing lambs onpasture used for pre-weaning lambs the previous year,or ideally the previous two years.If this is not possible, a risk assessment should becarried out based on farm and pasture history,forecasts, and the age of the lambs during the riskperiod. Three-weekly prophylactic treatments can thenbe given as required, aiming to cover the period whenthe high-risk age range and the high-risk time periodoverlap. In spring born lambs this may result in a singletreatment in low-risk years, and up to three in high-riskyears.
For example, this year lambs born at the beginning ofApril over much of the country may be treated in mid-May and early June. A change to cooler weather or aprolonged dry spell may mean a third dose is requiredin late June, particularly in northern regions. Theprophylactic treatment approach is always at risk ofbreaking down due to the sudden appearance of larvaeon the pasture, so the safe grazingapproach is to bepreferred.