The presence of the parasitic strongyle threadworm affecting red grouse has been recognised for more than 100 years. Research started in the 1980s by GWCT established that infection with strongyle worms can cause 4-to-6 year fluctuations in grouse numbers. Subsequent research focused on ways to reduce the severity and frequency of these cyclical population crashes to help stabilise grouse numbers. Reducing parasites helps both to improve body condition and breeding success in adult grouse and reduce their scent emission, making the birds less vulnerable to mammalian predators when breeding.

This document sets out the background to parasitic threadworm infections in red grouse.

• Describes how infection can be monitored and managed through the prescribed use of medicated grit.

• Identifies the information required by a vet to assess the need for any prescription.

• Provides advice on the management of the medicated product