Nematodirus risk

The beautifully warm spring has meant an early Nematodirus hatch this year. Last week Aberdeen had three submissions of 6-8 week old lambs which have been diagnosed as acute nematodirosis. 

As of 13 May 25, the SCOPS Nematodirus risk for Aviemore, Altnaharra Saws, Kirkwall, and Skye is high and very high for Wick and Stornoway, see Nematodirus Forecast.

Fields where young lambs were grazed last year are likely to have a Nematodirus larval challenge over the next month or so.  The timing and impact of the Nematodirus hatch is individual to the field and the age of lambs grazing it. March born lambs are likely to be ingesting sufficient grass to be at risk of acute nematodirosis on contaminated pastures. In later lambing flocks, where lambs are not ingesting significant amounts of grass yet, the ewes may ‘clean’ the pasture somewhat as they graze, leading to a lesser risk of acute nematodirosis in these lambs.  

Lambs with nematodirosis often have scour and appear empty, although disease can occur suddenly with several deaths in a 24hr period sometimes with little scour noted in the group. There is limited resistance of Nematodirus to benzimidazoles, and where it does occur it appears to be at low (subclinical) levels, therefore white drenches remain the treatment of choice, although concurrent coccidiosis or infection with BZ resistant Teladorsagia or Trichostrongylus can result in an apparently poor response to treatment.

Guidance on how to perform worm washes to help diagnose nematodirosis can be found on pages 66/67 of the SRUC sampling guide.

Posted by SRUC Veterinary Services on 20/05/2025