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Nematodirus (sometimes called thread-necked worms) is a parasitic roundworm that can infect young lambs, typically in spring when lambs are around 6-8 weeks of age.  There are several species of Nematodirus that contribute to economic losses in lambs.  Most of the losses in the UK occur as a result of Nematodirus battus.

Signs of infection include reduced appetite, reduced production, acute yellow/green diarrhoea and subsequent dehydration. 

The ingested infective larvae can disrupt the normal integrity, structure and function of a lamb’s small intestine.  Intensity of the infection varies between lambs.  Whilst the majority will recover within a month, mortality rates can be as high as 10-30% of the lamb crop in which animals suffer intense scouring and succumb to dehydration quickly. Other clinical signs include sunken eyes and rough wool.

Acute disease occurs when synchronous hatching of worm eggs into infective larvae coincides with the grazing of young lambs.  The timing of development and hatching generally depends on climatic conditions (chilling followed by a mean day/night temperature of greater than 10°C for 10 consecutive days) and hence most larvae will overwinter within the egg and hatch in the following year.  Recent climatic changes and alteration of parasite behaviour have resulted in a second hatching event in autumn in some N. battus populations, thus providing the potential for a second generation per annum. 

Key Points

  • Nematodirus is a roundworm that infects young lambs, typically in spring when lambs are around 6-8 weeks old
  • Although thought of as a spring disease in very young lambs, in recent years outbreaks of the disease have been seen in older lambs later in the grazing season
  • Onset of disease can be extremely rapid and deaths of lambs can occur before parasite eggs appear in faeces
  • Acute disease occurs when synchronous hatching of worm eggs in spring coincides with the grazing of young lambs
  • Signs of heavy infection are profuse, watery, yellowy-green diarrhoea which leads to severe dehydration.  The fleeces of infected animals can become dull and rough and the animals may show a “tucked up” appearance
  • Lighter infections reduce appetite and productivity resulting in classical ‘ill thrift’
  • Risk of disease forecasts can be found at www.scops.org.uk/nematodirusforecast
  • Benzimidazole (white drench; 1-BZ) resistance has recently been identified in Nematodirus battus populations in the UK. However, given that white drench resistance in Nematodirus battus appears to be at low levels in the UK at present, white drenches (1-BZ) are still recommended for the control of this parasite in young stock
  • Speak to your veterinary surgeon or advisor to ensure that you use a suitable anthelmintic to control Nematodirus on your farm

Research at Moredun

Current interests and future aims:

  • Develop a molecular test to look for resistance markers in Nematodirus populations
  • Survey populations of Nematodirus from UK sheep flocks for presence of white drench resistance (1-BZ) markers
  • Questionnaire survey assessing current Nematodirus management strategies from across UK
  • Undertake risk factor analysis to examine correlations between the presence of white drench resistance, animal movement and general management practices
  • Investigating the egg hatching patterns, i.e. requirement for chilling/non chilling prior to hatching of isolates of Nematodirus from different geographical regions

Farming and Veterinary Outreach

Please see Moredun Newssheet 6.13 - Nematodirus: A changing parasite: