It could either be giardia or a colitis. Either way, if this were my dog I would switch to a bland diet and take a fecal to the vet (especially due to the age). I would also be looking for a different vet who won't focus on raw as a reason for everything. I ended up switching vets not long after we started feeding raw for that reason.
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Leyna vom Grunenfeld, HT, PT, HSAs, HRDs, RLFs
Paisley vom Kirschental, 3/25/13, aka "the devil"
Levi vom Grunenfeld, RIP 9/5/04 - 3/4/13