Do you have a fish tank? Only half kidding; a doc once told me his pediatric patients got Yersinia from drinking fish tank water.
I have not heard of blood test serology for Yersinia diagnosis. It is a bacteria found in a stool culture and only takes 3 days at most to grow. So I wouldn't be anxious to take antibiotics that might not be necessary. Even if you did have a past infection, if the bug is now gone and hasn't been recovered in culture of stool, or anything else, then there is nothing for an antibiotic to kill.