Canned Cat Food Going Bad?

[Syd'sMom]

Hi - I feed my cat Wellness canned food and he loves it. Recently he spent 24 hours in the emergency room because he was lethargic, not eating, just not the same sweet boy. Turns out they think it was gastro-enteritus. They couldn't tell me how he got it. But I told them that he started acting odd after I opened a new case of Wellness cans and fed him. It looked a little different than normal, like not as moist as usual. But the expiration date said '08 so I figured it was okay.

We keep the cans in a cool pantry and then store about 6 at a time in the fridge before feeding him.

My question is... could the 'bad' canned food have made him sick? Also, how can you tell when canned food goes bad? Not a lot of preservatives in Wellness, it's a quality brand. But I don't want to keep guessing when I open a new case - is this okay for him to eat?

I just need to know how to tell when canned cat food goes bad.