Vet
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If you ha enjoy already tried it, I would suggest Deosect spray. This is licenced for use in horses, so technically use in a donkey if off licence but it should still be safe and effective.
See https://www.viovet.co.uk/Deosect-Spray/c9041/?pk_campaign=AdWords-548&pk_kwd=deosect%20fly%20spray%20for%20horses&oa_matchtype=e&oa_advert=3697&oa_network=g&gclid=Cj0KCQjwzLCVBhD3ARIsAPKYTcRqn5xCna3aP2ItoC_smFLUUXw9Tv82ik1wcuDSSiKFWb9zMf6eqwgaAv9-EALw_wcB
For an example of the product.
I hope that answers your question? Please let me know if you need any further help or advice.