Parasite Watch, Sheep farming

The Nematodirosis risk

Parasitic gastro-enteritis (worm scours) will affect all lambs to a greater or lesser degree. It can be caused by many different worm species, but the two most important in the UK are Teladorsagia circumcincta and Nematodirus battus. Teladorsagia tends to build up on pasture through the season after being shed by older animals the parasite has overwintered within, especially ewes around lambing time, the so-called “Spring Rise”.

Nematodirus can cause explosive outbreaks of disease and even death amongst young grazing lambs. Ewes are immune to Nematodirus worms and do not carry significant numbers of adult worms and therefore do not contaminate pasture. In lambs, the worms affect the surface of the small intestine where they damage the intestinal wall as they feed from the digesta. The parasites are relatively large and usually numerous. They cause severe inflammation of the intestine, with acute onset scour, lethargy, abdominal pain, rapid weight loss, dehydration and death (see figure 1).

Figure 1 – Nematodirus battus from the intestine of a lamb


This condition typically affects young lambs between six and twelve weeks of age.  This happens when large numbers of eggs deposited on pasture by lambs the previous year,  hatch together in spring (see figure 2), triggered by a period of chilling over winter followed by a period of time spent between 11.5°C and 17°C.

Figure 2 – The 2016 spread of Nematodirus battus across the UK

Risks are highest when we have a cold spell followed by an increase in temperature. Young lambs take in large numbers of larvae as they graze, which damage their gut leading to profuse scouring and even death. Damage is not done by adult worms, but by large numbers of infective larvae burrowing into the gut simultaneously. Signs of disease may precede the appearance of significant numbers of Nematodirus eggs in the dung, making a diagnosis of the condition through worm eggs counts tricky.  Because of this, any unexplained deaths affecting lambs should be thoroughly investigated, particularly if there is also evidence of scouring. 

Unlike other sheep nematode worms, the larvae of Nematodirus battus develop to the infective third stage larvae (L3) within the egg.  These eggs have a double wall to allow survival on pasture over winter for over two lambing seasons and are thus much larger (figure 3) than other Strongyle-type worm eggs. This means high survival rates for the parasite and large burdens occurring at once.

Figure 3– comparison of Nematodirus eggs to Strongyle eggs

Where possible farmers should avoid putting young lambs on fields that may have been contaminated with Nematodirus eggs the previous grazing season, particularly those paddocks previously grazed by ewes with young lambs. The good news is that lambs will develop a natural resistance to Nematodirus by 6 months of age.

Nematodirus forecasts and risk maps have been provided for the past four years by SCOPS and the University of Bristol, featuring updates from local weather station data, providing localised risk and this year there is a real time monitoring website available not just for Nematodirus but other sheep parasites too.

The aim of Parasite Watch is to not only inform the farmers involved in the project about parasite levels on their farm, but to alert SQPs and livestock farmers to parasite risks in their local area by an interactive map, which can be found at

Parasite data on the interactive map from each of the farms will be updated regularly. This will allow SQPs and farmers to see if there are spikes in certain parasites in their area and enable them to take appropriate action.

The results and stories from these farms will also be shared on Facebook (SheepFarmersUK) and Twitter (@Sheep_farmers), so make sure you follow us to stay up to date.

For Control of Nematodirus more than one treatment may be necessary at 3 week intervals dependent on the spread of ages in the group and if the high-risk forecast is prolonged. Anthelmintics in the 1-BZ (white) class are still the treatments of choice for Nematodirus battus.

However, due to the widespread resistance of other worm species to the white wormers, if there is a mixed burden of worms, speak to your advisor about the correct treatment choice.

For further information, please see the product SPC, or contact your veterinary surgeon, SQP or Zoetis UK Ltd, Walton Oaks, Dorking Road, Walton on the Hill, Tadworth, Surrey, KT20 7NS. Customer Support 0845 3008034. . Always seek the advice of your medicines provider. Use medicines responsibly ( AH304/17