Nematodirosis can strike very quickly, meaning rapid action is often required. Clinical disease can occur in young lambs when they ingest large numbers of immature larvae off pasture, and because these larvae are not producing eggs Faecal Egg Counts (FECs) are an unreliable indicator of risk. In conjunction with a risk assessment, of the lambs and grazing, the SCOPS forecast helps sheep farmers and their advisers to take appropriate action when required.
The decision on IF and WHEN a treatment may be required has to be based on two factors. Firstly, how ‘at risk’ a group of lambs are should there be a peak in hatching larvae, and secondly the temperature profile for the area where the lambs are grazing to gauge the level of challenge they are facing from hatching larvae.
Lamb and pasture Factors – are they at risk? (Watch a short video here)
The first step is to decide whether a group of lambs is at risk of Nematodirosis, based on the following factors:
Using the forecast you can see if ‘at risk’ lamb groups are likely to be facing a challenge from large numbers of hatching larvae as indicated by a red or black dot on the most appropriate weather station. Check regularly (the map is updated daily) so you pick up when the peak is occurring and check the historical data so you can see the context. A triangular icon on the station means that peak hatch has past.
The timing of these colour (temperature) changes can vary by a number of weeks between years for each weather station. The graph below illustrates this for one weather station over a 6 year period. The variation in the date when the temperatures reached levels needed for a peak hatch was 7 weeks, ranging from late March to late May. The hatching forecast takes away the guesswork and helps pinpoint the time when lambs that are at risk, will face the biggest challenge form larvae on pasture.
Also consider the height above sea level (a.s.l) of your fields. For every 100m a.s.l between a field and the weather station there will be a +/- 7 day difference in peak hatch. The aspect of the field will also have a significant effect, with south facing fields reaching temperatures more quickly than those that are north facing.
If possible, avoid the high challenge. Move at-risk lambs (as determined by the risk assessment) to low risk pastures (i.e. pasture that was not grazed by lambs the previous spring). See SCOPS Technical Manual. Chapter 2.4.1 and Chapter 2.4.2.
Treatment – if this is required then SCOPS advises farmers to use a white (1-BZ) drench*. Use the SCOPS ‘Know Your Anthelmintics’ Guide to select a product. The white drenches are still highly effective against Nematodirus on most farms and are suitable for young lambs. However, because there have been a small number of cases of resistance confirmed you should check that a treatment was fully effective by taking a dung sample for a FEC seven to ten days after treatment. Remember, it may be necessary to treat lambs more than once depending on the spread of ages in a group and subsequent weather conditions.
*You may need to use another wormer group if there is a concurrent challenge by other strongyle species and it is known there is 1-BZ resistance.
Mitchell et al., (2011) Vet. Rec., 168 pp. 623-62. Benzimidazole resistance in Nematodirus battus.https://doi.org/10.1136/vr.d3584
Melville et al (2020) Int. J. Parasitology: Drugs and Drug Resistance; 2, pp68-76.
Large scale screening for benzimidazole resistance mutations in Nematodirus battus using both pyrosequence genotyoping and deep amplicon sequencing, indicates the early emergence of resistance on UK sheep farms.
https://doi.org/10.1016/j.ijpddr.2020.03.001
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 eggs which were put on pasture by last year’s lamb crop. Before these larvae can hatch, the eggs have to undergo a period of cold weather followed by warmer temperatures of 10°C or more. In many cases this is a gradual process and lambs acquire immunity over 8-10 weeks, but if these conditions occur over a short period of time, it can trigger a ‘mass hatch’ resulting in a very high larval challenge. If this 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 year to year, region to region and even field to field. In the south of England for example, it is traditionally likely to occur earlier in late March through to May, compared to early June in northern England and Scotland. However, with the temperatures being increasingly unpredictable, the SCOPS Nematodirus Forecast is an essential tool and when combines with a risk assessment for groups of young lambs, it can ensure that tretments are given when necessary and at the most effective time.
