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

3. Exposure assessment

Flubendazole is present at 5% w/w in the prescribed product, incorporated into a stearate coating onto grit. Dose quantities vary but are around 1kg of the product in 1 tonne of grit (equating to approx. 50g flubendazole per tonne). Grit trays are the preferred method of deployment since these control quantities available and minimise environmental release of uneaten grit. The grit is now stearate coated and so is less likely to allow significant leaching (72% remained on weathered grit after 1 year). The Game and Wildlife Conservation Trust recommend one grit tray per breeding pair, with no more than 500g of grit per tray (GWCT, 2015). As grouse typically consume 35g of grit/month, a tray will last a pair approx. seven months (note this approach differs greatly from other livestock, where treatment in feed lasts only for seven consecutive days). Treatment must be withdrawn at least 28 days before animals are shot for subsequent human consumption. Generally, grit trays are placed at the end of the shooting season in late autumn and are removed one month before the start of the shooting season in early august (GWCT, 2020). The period of potential release is therefore winter, spring and early summer.

The GWCT has a number of best practice guidelines which include a number of measures to reduce potential environmental exposure. These include:

• Conducting early and late autumn counts of strongyle worms from at least 20 shot birds. Where counts are low, consider delaying treatment until counts of worm eggs in grouse faeces are conducted in the winter.
• The use of grit trays as opposed to placing grit in piles on the soil surface.
• Avoid siting trays near standing or running water.
• Supply a maximum of 500g of medicated grit per breeding pair per season.

Toxicokinetic studies in other bird species (chickens and turkeys) indicate that flubendazole is absorbed rapidly but that a high proportion is eliminated unchanged in faeces. Organ specific analysis showed that, of the systemically available flubendazole, this undergoes a high level of metabolism in the liver. For this reason maximum residue limits (MRLs, relevant for human consumption) are based on the parent compound and a “marker” metabolite (EMEA, 2006). It must be noted that these data are based on in-feed studies; there may be differences in toxicokinetics from ingested grit treatments as well as species-dependent differences.

The primary pathways of exposure in the environment from use in medicated grit will include the soil compartment (mainly from unchanged flubendazole in faeces, and potentially from direct losses to moorland if best practice not followed).

For aquatic exposure, flubendazole’s affinity to sorb to soil and low water solubility means that emissions to groundwater are less likely, and releases to surface water are more likely to result from flubendazole bound to particulate runoff rather than in solution/sub-surface pore water.

3.1. Measured Environmental data

SEPA do not have any aquatic monitoring data for flubendazole. Data from one partner organisation in England indicate very low frequencies of detection at low ng/l levels in surface water, however the location of sites monitored are unlikely to relate to use of the substance in medicated grit on moorlands.

3.2 Predicted Environmental Concentrations for Soil

The Predicted Environmental Concentration for Soil (PEC_soil) has been calculated on the basis that the main release pathway is from unmetabolised flubendazole in grouse faeces and takes into account the accumulation of the substance in soil over subsequent years of treatment. The derivation follows the principles of the guidance published as part of the Veterinary International Conference on Harmonisation (VICH) for veterinary medicines registration (European Medicines Agency, 2016), although there is no exposure scenario that directly relates to releases from grouse on moorland. The closest scenario is excretion by livestock on pastureland; this scenario has been adapted to derive a PEC_soil in this assessment. The calculations and parameters used to derive PEC_soil are presented in Appendix C. Note that while this scenario does not take into account direct leaching of the substance from spilt or open grit mounds, as it assumes all of the grit is used over a season and that all of the dose is excreted by the bird, in terms of mass balance all of the flubendazole is accounted for. The final PEC_soil calculated is 1.14 µg/kg.

3.3. Predicted Environmental Concentration for Surface Waters (including sediment)

It is assumed that flubendazole will mainly enter surface waters in dissolved form (from soil porewater) and sorbed to soil particles in rain run off. Following VICH guidelines a PEC_water can be estimated from the concentration of the substance in soil pore water (derived from the soil PEC above) with an assumed dilution factor of 3 for the receiving water body. This gives a PEC_water of 0.475 ng/l (see appendix C). This approach does not account for concentrations of the substance sorbed to particulates in runoff, which is likely to be a much higher route of exposure based on the substance’s properties (and highly relevant for the sediment compartment).

The substance may also enter watercourses directly from grit trays or piles that are placed too near water courses. This route of exposure would reflect intermittent releases related to periods of heavy rainfall that could potentially result in periodically significant concentrations in water. No model is available to model such inputs, and uncertainties around specifics such as rainfall, surface absorbance, run off rates etc make this scenario difficult to model from first principles. This is further discussed in appendix C, but as a worst case (assuming 10% of offered grit is spilt) up to 1.4mg flubendazole per tray could leach over the course of a treatment season or up to 14mg in the case of a pile of grit (if for some reason the pile remains unused).

A PEC_sediment can be calculated from the PEC_water above based on the substance’s partitioning behaviour (see appendix c). As above, this only considers dissolved losses from soil that then partition to sediment once in the receiving water and not losses of flubendazole attached to particulate runoff, or intermittent direct releases from grit placed near to watercourses. The PEC_sediment is 0.238 µg/kg.