Moorland grouse - Flubendazole use for parasitic worm control: preliminary environmental assessment

1. Introduction

In 2017 the Scottish Government established the Grouse Moor Management Group to examine the environmental impact of grouse moor management practices. In November 2019 the group submitted its final report^[1] (“the Werritty report”; Werritty et al 2019) to the Cabinet Secretary for Environment, Climate Change and Land Reform. The report included an investigation into the use of medicated grit for the treatment and prevention of the strongyle worm (Trichostrongylus tenuis) in the gut of Red Grouse. It concluded that “there is some evidence that prescription levels are too high, that gritting holidays are not always observed, and that grit may not always be withdrawn from grouse at least 28 days before Red Grouse enter the food chain. At present there is little evidence of a resistance problem with the use of medicated grit, but there is some evidence that flubendazole is toxic to aquatic organisms”. The Werritty report made several recommendations in relation to the use of medicated grit, as well as an additional recommendation that “SEPA should initiate a desk-based study to determine the appropriate nature and extent of a monitoring programme to ascertain whether flubendazole residues exist in water bodies on or downstream from where it is being used, including in association with grouse moors, to conduct such a monitoring programme and to report on its findings”. Following publication of the report and subsequent discussion in the Scottish Parliament, SEPA committed to undertake a desk-based study to assess the potential impact on the wider environment of the use of flubendazole in medicated grit, including an assessment of the effects on soil dwelling organisms, aquatic organisms and the indirect effect on predatory animals (from the consumption of contaminated prey). This report documents this study.

Products containing the active ingredient flubendazole are usually added to feed to control worm infestations in chickens and fowl. The strongyle worm can cause cyclical fluctuations in grouse numbers every 6-9 years in Scotland, and the use of grit coated in a product containing flubendazole has substantially suppressed these cycles since its introduction in 2007. Flubendazole medicated grit for grouse is not officially authorised by the Veterinary Medicines Directorate (VMD). However, use is allowed under the “cascade” process as part of the Veterinary Medicines Regulations 2013. Medicated grit is offered through prescription by a vet and it is recommended follows predetermination of worm levels in grouse. There is currently no overarching system in place that monitors the use of medicated grit. There is no obligation for landowners to follow best practice, as laid out in Moorland Management Best Practice Worm Control in Red Grouse Guidance (Scotland’s Moorland Forum, 2018) and Best practice use of medicated grit (GWCT, 2019). This means there are likely to be cases where treatment is offered as a precaution without prior determination of worm burdens, and it is “contested evidence” in the Werritty report that led to the concerns that prompted this report.

The Werritty report found that, when used correctly, the use of flubendazole has been highly effective in reducing endemic strongyle levels in grouse and that residues in grouse for human consumption present a very low risk. There is a trade off between frequency of treatment and breeding success, with “gritting holidays” recommended when worm burdens are low (to lower the risk of drug resistance developing). Treatment must also be ceased 28 days before birds are shot by law. Initiatives such as Scotland’s Moorland Form’s Worm Control in Red Grouse Guidance and GWCT’s Best practice use of medicated grit, plus workshops provided by the Moredun Research Institute, aim to address this at the voluntary level. However, the Werritty report found anecdotal evidence that grit trays were not being used (open piles of grit), including near to watercourses (GWCT recommends trays not be placed near to surface waters, although no minimum distance seems to be recommended) leading to the additional recommendation for SEPA quoted above.