I'd try changing their bedding. Spruce is an evergreen, which can release toxic gases when cut up. It has to be kiln-dried, NOT air-dried, in order to be safe, but even then, since you're having issues I'd switch it anyway and see what happens.
Are the mice having any sneezing or nasal discharge before they die? Do they look thin or dehydrated before they die? You can check for dehydration by pulling their skin on their back upward then seeing how quickly it snaps back. If it doesn't go back in the blink of an eye, they're dehydrated.
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