Where did you get that rat? Sounds like it has a respiratory infection. If you kill it I wouldn't feed it, I'd toss it, or complain and get another one. Or you could get some oxytetracycline powder and mix it with his drinking water and medicate him for a week or two and see if it clears up, then feed him off. You can get that powder at a feed store. I breed rats and I had a random rat that was breathing hard one evening, I woke up the next day and he just fell over dead. Strangest thing I've ever seen, I never have any kind of sickness in my rodents. I'm thinking it was a bad heart LOL.