I actually was recently talking to my veterinarian about this. Surprisingly, not many rats carry rabies, and if your dog get the full suite of vaccines (the leptospirosis shot is critical), your dog will likely be fine after killing a rat. Provided it’s not eating it or eviscerating it and getting blood everywhere. Dogs that are natural rodent killers like terriers, schnauzers, and dachshunds, they kill by shaking the rat, breaking its back, and then leaving it. So very low risk for disease, and I’d wager that those rodent hunting dog breeds have been bred to be more resilient against any disease that rodents might be carrying too.