I use the same as you Purina One ( for large breed puppies) and I also had the same problem happen to me one time so I called them and they said that the worms would not hurt him but the lil worms can come from the warehouse where the stores keep it stocked. ( Mine came from Wal Mart) I wasnt about to feed him that food so I dumped out out and Purina sent me coupons for 2 free 20lb bags and a bunch of 2.00 off coupons. I only ran into this problem 1 time and I still feed him Purina One. He loves it and is a healthy and growing boy!
The Number to call is on the bag and If you call them make sure you have the infested bag there as they will need some numbers off of it!