The most common parasitic worm infecting sheep in the UK is the brown stomach worm, Teladorsagia circumcincta. The adult worms live in the abomasum (sheep gastric stomach) and produce eggs, which are passed out in the sheep faeces. The eggs hatch and release larvae, which develop to the infective stage (L3) on the pasture and are ingested when the sheep graze, and the cycle continues. Teladorsagia causes diarrhoea and weight loss in infected sheep.
The most important parasitic worm infecting sheep worldwide, is the barber’s pole worm, Haemonchus contortus. This worm is a closely related species to Teladorsagia, with a similar lifecycle, but causes more severe disease because the worms ingest large volumes of the host’s blood. Haemonchus causes anaemia and weight loss; acute infections cause sudden death.
In a recent survey, Teladorsagia was found on 100% of UK sheep farms and Haemonchus was found on 50% (Burgess et al, 2011). In addition to reducing animal welfare, parasitic worms cost the sheep industry an estimated £84 million per year, through production losses and the cost of treatment and control (Nieuwhof and Bishop, 2005).