Because you mentioned it is a tri bike, likely with short chainstays, I'm going to guess that you have the panniers set up pretty far back to avoid heel strike. Likely weighted mostly, if not entirely, behind the rear hub. If that is how you are set up, you are putting even greater stress on the rear wheel. Not much you can do to change that if you want to continue using panniers with that bike. As suggested, I would get a 36 spoke count wheel.