The nematode worm Trichostrongylus tenuis, the causative agent of the disease strongylosis, is widespread in red grouse and high levels of infection can cause significant reductions in both breeding success and direct mortality. Research in the north of England has shown that this parasite is largely responsible for the cyclical fluctuations in grouse numbers on moors in this region. The parasite is most prevalent when grouse stocks have been high in the previous year, but it may also reduce breeding success on low-density moors.
The Game & Wildlife Conservation Trust's Grouse Technical Services team can provide advice about strongyle worms and treating grouse against them. They also provide a worm counting service to analyse worm presence.