I don't know of any catfish in Ontario to have any toxins in their whiskers or spines (I may be corrected should there be any). The protective slime that covers catfish is more likely to have may bacteria that can cause easy infections if one of the spiny fins cuts or punctures the skin.
As Andrew said it very well,
For smaller fish, hold the catfish such that the hand is behind their spines. (thumb behind one of the side fin's spine, dorsal spine resting between the thumb and index finger, then in the index finger or the third finger behind the other side's fin/spine.)
For bigger fish, I'd recommend getting a lip-gripper as there would be no way to do the above.