Just something I found on a tanking forum. Don't know if this would suit you at all or not.

Regarding Parasites on Magmaw

If you are seeing parasites appearing that far away from the cluster, it's because they are infecting people and those people are vomiting them up. Essentially, someone is spawning a freshly healed parasite into your group.

Tip: Don't use shadowfury. Have your lock go demo for this fight. A Felguard's Felstorm is enough to cause the felguard to have aggro on all of the parasites (not to mention do more damage than any other AOE in your raid) and you can use this tactic on almost all waves of adds (it will be on cooldown for a few seconds if you get two waves before exposing the head). Have your warlock using hellfire (at a safe distance) to supplement this and you won't have anymore issues with parasites.

I was Destruction the first time we attempted (and failed miserably) at this fight. I came back as Demo and the adds were so incredibly much easier to control and kill. It will make a huge difference.

Edit: Make sure the felguard isn't positioned on the spawn point of the adds, or else he will get knocked up just like a player would... and that's a wasted felstorm.