You tell me the how many good racers make it into the stock loft pout it anther way after thay make 4 years old out of 100 how many are left that's not ploders that are good enough to go into the stock loft not many I would say to worry about there immune system or constitution I think if thay have won well and breeding the goods at 4 years old I don't think you have much to worry about lol