Nematodirosis in Lambs

What is Nematodirus?

Nematodirus spp. are thread-like roundworms that infect the small intestine and cause nematodirosis in lambs, goats and occasionally cattle. Nematodirus spp. infection largely affects lambs in springtime as adult sheep develop a natural resistance to the infection. Due to its unique development and life cycle, a phenomenon known as the spring flush can occur.

Three species of Nematodirus spp. can affect sheep, with N. battus as the most pathogenic. Other species are N. filicollis and N. spathiger, which tend to be less pathogenic. This species is typically associated with disease in British sheep, but has also been reported in other European countries. Nematodirosis can cause stunted growth and mortality in lambs. In worst case scenarios, it can cause fatalities for up to 30% of lambs.

How do lambs become infected with Nematodirus?

Nematodirus spp. worms have a completely different lifecycle to other ruminant roundworms. The main difference seen in the life cycle when compared with other parasitic worms is the development of infective larvae (L3). Unlike other parasites, this does not take place within the egg. Larvae will hatch as L1 or L2 and then develop into infective larvae (L3) on pasture.

Nematodirus spp. isn’t passed from sheep to lambs like many other parasites. Rather, the infection passes yearly from old to new lambs. Before the larvae hatch, the eggs must undergo a period of cold weather, followed by warmer temperatures of 10°C or more. This can trigger mass hatching if the conditions occur over a short period of time. This is called the spring flush. If this happens, a huge number of the infective larvae hatch onto the pasture at the same time.

If this coincides with lambing then lambs don’t often show signs of disease, as nematodirosis is a disease of abundance. This means a heavy burden must be present for the disease to show clinical signs. If however, the spring flush happens when lambs are 6-12 weeks of age, then they can take in significant amounts whilst grazing the pasture, and the disease onset can be sudden and deaths high.

What are the clinical symptoms?

As the clinical symptoms are caused by worm larvae, FECs (faecal egg counts) are not always useful to diagnose Nematodirus infection during the early stages of the disease. By the time the eggs are detected in faeces, infection has spread amongst a flock and it is often too late to treat. However, FECs can be used to confirm an outbreak in a flock and this knowledge can then be used to prevent future outbreaks. Post-mortem examination may reveal very large numbers of developing larval stages and adults within the small intestine. Lambs which recover often acquire a strong immunity to re-infection.

Nematodirus battus, the parasite which causes nematodirosis.
Nematodirus battus, the parasite which causes nematodirosis.

Some clinical signs to look out for:

  • Sudden onset watery diarrhoea
  • Depressed, inactive lambs
  • Lambs stop suckling
  • Rapid loss of condition
  • Dull and rough fleece
  • Dehydration – lambs may gather around water sources
  • Presence of other parasites e.g. coccidiosis

Control and prevention of Nematodirus

The best way to control and prevent nematodirosis is to be vigilant. Regular checks can help you spot signs of disease and FECs can keep you informed on the parasites status of your flock. Being aware of the conditions necessary for the parasite to thrive will help you protect your flock during peak times.

Monitor the parasite forecast for your region and keep in contact with your local vet and other farmers. NADIS provide a Nematodirus forecast, here.

Adult sheep are highly resistant to infection, and only lambs tend to produce significant numbers of eggs. As such, it is a good idea to rotate pastures for young lambs and make sure they are not grazing on land which was grazed by similar aged lambs or calves the previous year.

Treatment for Nematodirosis

  • You should move sheep from infested pastures whenever possible, then treat with an anthelmintic immediately as directed by a vet.
  • Treat with a white drench if necessary. These are usually given three weeks apart during May in normal risk years. As always with worming animals, make sure you do so correctly to reduce the increasing levels of resistance in worms.
  • Do a ‘drench check’ 10-14 days after treatment to ensure the anthelmintic has been effective.
  • SCOPS advises farmers to follow the SCOPS guidelines on correct drenching technique and dose carefully for the correct weight of the lambs.

Top tips for Nematodirosis:

  • Be mindful of the ‘spring flush’
  • Check for Nematodirus forecasts on the NADIS website
  • Where Nematodirus is a known problem, prevent lambs from grazing fields grazed by lambs or calves in the previous year
  • Base diagnosis on clinical sings and grazing management


Nematodirus in Lambs | SCOPS.

Taylor, M.A. (2012). Veterinary Parasitology, 189(1), pp.2–7.

Rickard, et al. (1989). Proc. Helminthol. Soc. Wash, 56(2), pp.104–115.