What is Nematodirus ?

Nematodirus spp. is a thread like roundworm that infects the small intestine and causes disease in lambs, goats and occasionally cattle causing a high number of deaths. Nematodirus spp. infection largely affects lambs at spring time 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 Nematodirus battus the most pathogenic. Other species are Nematodirus filicollis and Nematodirus spathiger, which tend to be less pathogenic.  This species is typically associated with disease observed in British sheep, but has also been reported in other European countries. Nematodirosis can cause stunted growth in lambs and mortalities in many others; In worst case scenarios, Nematodirosis can cause fatalities in up to 30% of lambs.

How do lambs become infected with Nematodirus?

Nematodirus spp. worms have a completely different lifecycle to gut ruminant roundworms. The main difference seen in the life cycle whencompared with other parasitic worms is the development of infective larvae (L3), which takes place within the egg. In contrast with other parasitic worm life cycles, larvae will hatch as L1 or L2 and develop into infective larvae (L3) on pasture.

Nematodirus spp. isn’t passed from sheep to lambs like many other parasites, but rather the infection passes yearly from old to new lamb crops. Before the larvae hatches, the eggs must undergo a period of cold weather, followed by warmer temperatures of 10° C or more. This can trigger a mass hatch if the conditions occur over a short period of time. The outcome of this can be a large batch of infective L3 on pastures in early spring, typically known as the ‘spring flush’, where a huge flush of the infective larvae hatch onto the pasture. If this coincides with lambing then typically lambs will not show signs of disease, as Nematodirosis is a disease of abundance i.e. a heavy burden must be present for the disease to come forward. 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, FEC (faecal egg counts) are not always useful to diagnose Nematodirus infection in lambs during early stages of the disease, meaning 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 herd which this knowledge used to prevent future outbreaks. Post-mortem examination may reveal very large numbers of developing larval stages and adults within the small intestine. Recovered lambs often acquire a strong immunity to re-infection.

Some clinical symptoms to look out for:

  • Ingested third stage larvae (L3) disrupts the mucosa in the small intestine affecting absorption
  • Protein/muscle loss
  • If left untreated during the early stages of disease, deaths occur from dehydration
  • Weight loss
  • Lambs stop suckling
  • Dull and rough fleece
  • Watery diarrhoea
  • Presence of other parasites e.g. coccidiosis

Control and prevention of Nematodirus

  • Awareness of the disease can be regulated by FECs (although eggs not always seen in early infection) will help to recognise infection, as mass- hatching is temperature dependent.
  • Monitor the parasite forecast for your region and keep in contact with your local vet and other farmers. NADIS provide a ‘Nematodirus forecast’. It can be found here: www.nadis.org.uk/parasite-forecast.aspx.
  • It is advisable that lambs are not grazed on land grazed by similar aged lambs or calves during the previous year because adult sheep are highly resistant to infection and only lambs produce significant numbers of eggs. Alternating pasture can work as a strategy to reduce worm burden.

Treatment for Nematodirosis:

  • Sheep should be moved from infested pastures whenever possible and treated with an anthelmintic immediately as directed by a veterinary surgeon.
  • Targeted treatments with a white drench can also be used and are usually given three weeks apart during May in normal risk years. As always with worming animals, it is vital that it is performed correctly in order to reduce the increasing levels of resistance in worms.
  • It is advised to 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 to 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 the lambs from grazing fields grazed by lambs or calves in the previous year
  • Base diagnosis on clinical sings and grazing management
  • Undertake a drench check to ensure any treatments have been effective


 Anon, (n.d.). Farm Health Online – Animal Health and Welfare Knowledge Hub – Nematodirus. [online] [Accessed 28 Jan. 2021].

Nematodirus in Lambs | SCOPS. [online]

Taylor, M.A. (2012). Emerging parasitic diseases of sheep. Veterinary Parasitology, 189(1), pp.2–7.


Rlckard, L., Hoberg, E., Blshop, J. and Zimmerman, G. (1989). Epizootiology of Nematodirus battus, N. filicollis, and N. spathiger (Nematoda: Trichostrongyloidea) in Western Oregon1. Proc. Helminthol. Soc. Wash, [online] 56(2), pp.104–115.