in my experience ( former Abattoir owner/ still a slaughterman/ butcher) I would say that worm damage was in 20% of all the pigs that came into the abattoir, normally finding that if one pig in a batch of say 4 pigs had worms/ milk spots then the other 3 pigs would also have picked up the worm too.
The main thing my meat inspector found was what was commonly known as milk spot, which looks like white blotchy 10p piece size marks on either part, or all over the infected pig liver and this whole live would be condemned.
Hope this is of help.