I do believe that you can breed to improve the immune system.
My rats are kept outside and even though I cover them and monitor temperatures, some still get fairly sick in the winter from the temp swings bringing their immune system down. Just like people. I have a 1.4 that is over 2 years old that has never been sick and the females are still going strong and producing. I want to breed that line into all my color rats to improve them.
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