I am so sorry to hear that Alex killed a possum.
I live in Southern Calif, and ROcky
unfortunately kills squirrels and possums too.
I was concerned about diseases also, but when
I called my Vet he said don't worry, since my dogs
are up to date on their shots and Rabies is rare in our area.
(I also have them on flea medication and I periodcally
turn in a stool sample for worms)

I would call your vet to make shure their are
no problems in your area. My vet never charges for
a phone call.