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Stomach worm control in lambs

Stomach worm control in lambs

Stomach worms are one of the major production-limiting diseases of growing lambs, writes Orla Keane, Teagasc Research Officer. They reduce feed intake and heavy worm burdens result in failure to thrive, scour and in severe cases even death.

There are a number of different worm species that can infect lambs; however, during the summer/autumn the major species include: the medium brown stomach worm (Teladorsagia); the black scour worm (Trichostrongylus); and the small intestinal worm (Cooperia). These worms are collectively referred to as ‘stomach worms’ or ‘strongyles’.

Lambs are most at risk as immunity takes one to two years to develop. Hence, lambs are the source of most of the worm contamination on pasture and good worm control in lambs is essential to meet performance targets and to limit the number of worm larvae shed onto the pasture, which will be available to infect stock later in the season. Therefore, finishing lambs as soon as possible and getting them off the farm will reduce pasture contamination.


As with other diseases, prevention is better than cure, which involves limiting exposure of lambs to stomach worms through grazing management and judicious use of wormers. The disease caused by stomach worms results in protein loss from the gut. Therefore, ensure nutrition is optimised through sufficient quantity of good quality feed.

Grazing lambs ahead of ewes post-weaning provides lambs with the best pick of grass while allowing the more immune ewes following to consume some of the infective larvae shed by the lambs. Using other stock, such as cattle, to clean up pastures for sheep is also beneficial, as the majority of stomach worms are specific to either cattle or sheep and so sheep stomach worms rarely cause disease in cattle and vice versa. Therefore, mixed grazing will reduce the number of stomach worms the lambs ingest.

Grazing forage crops, hay or silage aftermaths or ground not recently grazed by lambs will also reduce the number of worm larvae the lambs are exposed to.

The exposure of sheep to stomach worms can be monitored by determining the number of worm eggs in the dung, commonly referred to as a faecal egg count (FEC) test. Poor thrive can also be an indicator of heavy stomach worms burdens. However, many other factors also affect thrive.


Judicious use of wormers is another tool to limit the impact of stomach worms in lambs. However, resistance to the three commonly used wormers on Irish sheep farms (white wormers, yellow wormers and clear wormers) is now widespread. It is of vital importance that farmers know what wormers work on their farm and use a wormer that is effective on their farm.

As previously mentioned, lambs are the main source of infective larvae shed onto pasture. Use of an ineffective wormers means that treated lambs continue to shed worms onto pasture and the worms they shed are drug-resistant. These resistant worms will build up throughout the grazing season and result in poor animal performance as the season progresses. Therefore, knowing which wormers work on the farm is essential for good worm control in lambs.

Worms move from farm to farm within the stomach of infected sheep. Therefore, having a good biosecurity protocol involving the use of a new active (Zolvix or Startect) is the only way to prevent buying in resistant worms.

Further information on stomach worm control in lambs can be found here.