Use medicated grit wisely to offset resistance build-up
Most estates have been providing medicated grit as a safety precaution irrespective of whether measured worm burdens show that it is necessary. Historically, this type of overuse of benzimidazole worming drugs in domestic livestock has led to costly drug resistance amongst parasitic worms within only three to five years of use.
Here, resistance to one benzimidazole-based drug has resulted in resistance to others within that general drug group. Over-use of medicated grit may act in a similar fashion, accelerating the onset of resistance amongst strongyle worms infesting red grouse. Whilst the livestock industry has developed further drugs, the grouse world has only one drug it can use.
Grouse may not need medication every year. If worm burdens are low in the autumn, continued parasite monitoring through faecal egg counts in late winter is recommended. This will help in deciding whether medication is required that year. Whether to use medication should not be based solely on autumn worm burdens. Instead, grouse densities, age structure, recent weather and moor location, which determines precipitation and humidity that affects survival of free-living stages of parasite larvae, should all be taken into consideration.
To help prevent resistance, exposure of strongyle worms to flubendazole needs to be minimized rather than maximized. This is best done by only using medicated grit when totally necessary to prevent a population crash. Accepting that some worms at low to moderate levels may dampen grouse breeding success without impacting adult grouse survival will help. Ensure that when medication is used, a full treatment is provided that kills adult worms.
By careful monitoring of either adult worms of shot grouse, or worm eggs in caecal material, it may be possible to have multiple years without resorting to medication. If medication is needed, then the same monitoring may allow its provision to be delayed until March. Both these procedures will reduce worm exposure to the anthelmintic, but still achieve the required parasite control. By using medicated grit wisely, and only targeting years when it is required, we should be able to keep medicated grit for longer. If resistance by the worms to the drug in medicated grit becomes common and widespread, medicated grit will no longer kill worms.
As no alternative drugs are currently available, the boom and bust of grouse cycles, recently consigned to history, will be back.
< Return to our guidelines
Get your FREE guide on best practice use of medicated grit
Simply enter your email address below to download your free guide.
What's inside your FREE guide
✓ History of disease
✓ History of control
✓ Medicated grit
✓ Assessing grouse for worms
✓ Deployment of medicated grit and grit box hygiene
✓ Best practice checklist
* By submitting your email address you are giving your consent to receive emails from the GWCT about our work with the opportunity to unsubscribe at any time.