Nematodirosis can strike very quickly so you can’t afford to have a ‘wait and see’ policy. And because the damage is done by large numbers of immature larvae that are not producing eggs, faecal egg counts (FECs) are not a reliable indicator of risk. Rapid action is often required and this has to be based on a risk assessment (more below) and the forecast for your area (see the forecast map).
Main risk factors
If your lambs are grazing pasture that carried lambs last spring and you answer yes to one or more of these questions, your lambs are at risk.
Nematodirosis is a particularly nasty disease in lambs, causing a high number of mortalities and stunting the growth of many others. It is caused by the Nematodirus battus worm, which has a different lifecycle to other sheep worms. Under certain climatic conditions it can strike very quickly, with little or no warning. This means sheep farmers have to be on their guard.
The main difference in the lifecycle of Nematodirus battus compared with other parasitic worms is that development to an infective larvae takes place within the egg and infection passes from one lamb crop to the next years’ crop. Before they can hatch, the eggs have to undergo a period of cold weather followed by warmer temperatures of 10°C or more. If these conditions occur over a short period of time, triggering a mass hatch, and it coincides with the time when lambs are starting to take in significant amounts of grass (over about six weeks old), the result can be devastating. See SCOPS TECHNICAL MANUAL, Chapter 3.1.2.
The timing of a potential problem will vary from region to region. In the south of England for example, it was traditionally likely to occur earlier in April/May, compared to early June in northern England and Scotland. However, with the weather being increasingly unpredictable, with particularly warm weather not unheard of in February, the SCOPS Nematodirus Forecast is a more useful tool than working on a calendar date, or remembering what has happened in previous years.
As explained above, SCOPS advises farmers to use a white (1-BZ) drench when treating. These are normally highly effective against this parasite and suitable for young lambs. However, the first confirmed case of nematodirus resistant to the 1-BZ group was reported in 2011 and so farmers are advised to follow the SCOPS guidelines on correct drenching technique and dose carefully to the correct weight of the lambs and to check afterward if the treatment has been fully effective. If in doubt consult your vet or adviser.