[Split from http://chowhound.chow.com/topics/547904 ]
Farmed salmon (mainly the Atlantic salmon species) has to go through strict health and safety regulations, and that is probably why it is used in sushi or other raw recipes. However, salmon are mainly saltwater fish (except in Lake Ontario), and only spawn in fresh water, which farmed salmon don't do. Trout are mainly fresh water with a few small exceptions, and are farmed in fresh water. Fish in any type of water can have parasites and that is why medications are often used in farmed fish.
An important part of eating raw fish is that it has been visually examined for parasites. I have caught wild trout that had worms in their guts, not their flesh. I have also caught wild trout without any signs of parasites from pristine waters and had the best tasting sushi ever. Some people looked at me as if I were crazy, but I'm still here and healthy!