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Even grapes from the garden, grown organically?

Yes. There is a chemical called tannin in the skins. Raisins are even worse because it concentrates the 'dose'. Dog and cat livers can't process it and it can make them very sick. Some handle it better than others, but it's best to avoid them altogether. I recommend replacing them with frozen blueberries.

Thank you for the advice!

You're welcome! We want to make sure everybody stays healthy and happy. ;-D