Reducing anthelmintic intake by grouse

Reducing anthelmintic intake by grouse

Medicated grit did not have to be put out every year on four of eight study moors where medication was experimentally withdrawn. © Laurie Campbell

BACKGROUND

Medicated grit has been used since the mid-1980s, but a revised formulation involving a change in benzimidazole drug from fenbendazole to flubendazole and incorporation of a more temperature-resistant binding fat, together with a new mode of grit delivery (withdrawal using flip-lid trays) occurred in 2007. Medicated grit use is popular with grouse managers who use it as an annual insurance against strongyle-induced grouse crashes rather than when it is strictly needed, causing concern that it could cause a widespread build-up of anthelmintic resistance in parasites. The strongyle worm, a nematode parasite, can limit grouse survival and breeding success causing quasi-cyclical fluctuations in abundance. To control worms in grouse, gamekeepers provide medication, in the form of quartz grit covered with a fat coating containing a benzimidazole-based anthelmintic, in each grouse territory. Grouse consume this grit to help digest heather and by doing so, they obtain a split-dose worming treatment, which has been experimentally shown to kill 95% of worms in gamebirds.

Medicated grit has been used since the mid-1980s, but a revised formulation involving a change in benzimidazole drug from fenbendazole to flubendazole and incorporation of a more temperature-resistant binding fat, together with a new mode of grit delivery (withdrawal using flip-lid trays) occurred in 2007. These changes have proved popular and now almost all grouse managers use medicated grit every year. Accordingly, worm burdens in grouse have been uncharacteristically low since 2009, suggesting high levels of control from medicated grit. Despite this, most managers use medicated grit as an annual insurance against strongyle-induced grouse crashes rather than when it is strictly needed. This is despite a veterinary prescription being required for its use. It is evident that decisions on whether to prescribe anthelmintics are seldom based on measured abundances of parasite intensities. Over-reliance and inappropriate use of worming drugs amongst domestic livestock has rapidly resulted in a widespread build-up of anthelmintic resistance in parasites. Although resistance in strongyle worms in grouse to flubendazole has not been found, more frequent and widespread use of medicated grit in the last decade may increase the risk of it happening in grouse as well.

To encourage grouse managers to reduce anthelmintic use, we conducted a trial where we experimentally withdrew medication from parts of moors. Few participated, only seven of 25 invitees joining in northern England and only one of 18 in the Scottish Highlands. We invited grouse managers to refrain from using medicated grit over a three-year period (2012-14). However, large and sudden parasite increases in early spring 2012 led to one grouse manager resuming medicating grouse later that spring. Thereafter, we asked managers to decide whether to use medication based on actual parasite data collected from their moor in late winter in each year of the trial.

We monitored parasite and grouse responses in relation to whether medication was used by counting worm eggs and adult worms and measuring indices of grouse mortality and breeding success. Rapid increases in worm egg counts in early spring culminated in resuming medication at three wet blanket-peat sites: one in the first spring of the trial and two in the second. At a fourth moor, medication was inexplicably restored by the moorland manager, despite low parasite counts. On the remaining four moors, all drier heaths in the east, parasite levels remained low in the absence of medication; there was no increase in grouse mortality, but breeding success was 16% lower. High humidity levels associated with wet blanket peat may favour survival of both worm eggs and free-living pre-infective larval stages, increasing the likelihood of an annual need for medication. At dry heath sites, particularly those in the drier eastern parts of the country, parasite pick-up rates by grouse were presumably lower and anthelmintics may be required only every two to five years.

We demonstrated that annual provision of anthelmintics is unnecessary on some moors and that there needs to be greater awareness of parasite levels in grouse among grouse managers and vets alike before medication is prescribed. Better parasite monitoring may reduce anthelmintic use, thereby helping the likelihood of drug resistance among worms, but this may be offset by reduced grouse productivity.

There needs to be greater awareness of parasite levels in grouse before medication is prescribed. © GWCT

KEY FINDINGS

Medicated grit did not have to be put out every year on four of eight study moors where medication was experimentally withdrawn.

Withdrawal of medication was not associated with increased grouse mortality, but breeding success was 16% lower.

Better monitoring of strongyle worms, including faecal egg counts in spring, may result in reduced use of anthelmintics, which in turn may delay the onset of resistance to the drug by the parasite.

David Baines David Newborn Michael Richardson

Medicated grit can be withdrawn by sliding the lid on grit boxes to allow grouse access to normal quartz grit. © Henrietta Appleton/GWCT