Skip to Navigation | Skip to Content

Lambs & The Threat of Nematodirus


Nematodirus - A gut Worm that Affects Lambs.


Nematodirus is a gut worm that affects lambs in the UK, as older sheep have acquired natural immunity. It is common for it to pass from lamb crop to lamb crop. Lambs are at greatest risk if a mass egg hatch of nematodirus occurs whilst the lambs are 6-12 weeks of age, as lambs are yet to acquire immunity and grass becomes a substantial part of their diet.

The life cycle of nematodirus is heavily influenced by weather conditions, so we can predict when lambs are at high risk. In the life cycle, eggs are excreted in sheep faeces and then develop into infective larvae (L3). Lambs ingest these larvae off pasture and the migration of these larvae causes damage and clinical disease.

Before eggs can hatch, a period of cold weather is required, followed by warmer temperatures greater than 10°C. Therefore, we recommend farmers check the regional parasite forecast, which can be found on the NADIS website (

Whilst we do encourage farmers to carry out faecal egg counts to assess gut worm burden, this is not appropriate for nematodirus, as it is the larvae, not the adult worms that cause disease. Clinical signs of nematodirus occur 11-12 days post infection, but eggs are only apparent in the faeces 15 days post infection.

Hence, by the time eggs are detectable by faecal egg counting, it may be too late for some lambs, as nematodirus can be very aggressive and often presents as sudden death.

Clinical signs include black/green diarrhoea, rapid weight loss, dehydration and a tucked up appearance. If you see these signs the parasite will already have taken hold of your flock and the production losses can be significant. It is therefore very important to be aware of the parasite forecast and to treat any lambs at high risk before their growth rate is significantly affected.

Most wormers do treat nematodirus, however the Sustainable Control of Parasites in Sheep (SCOPS) suggest treatment with Benzimidazoles (white drenches). Due to increased anthelmintic resistance, these wormers have become less useful against the other species of gut worms, but they are still very effective against nematodirus. Targeted therapy such as this, is one way to reduce anthelmintic resistance according to SCOPS principles. In the South West, April and May are the usual risk period, so check the parasite forecast regularly and if there is a nematodirus risk, do not hesitate to treat your susceptible lambs preventatively with a white drench.



Source Details

Sylvaine Lacrosse BVetMed MRCVS, Molecare Vet Services

Click here to return to the News page.

Store Locator For website enquiries and online orders phone 01769 576419 (8.30am - 5.30pm 5 days a week)