If you suspect your pet (yorkie, other breed of dog, or cat) has allergies, this is the absolute BEST site I have found on the web to help you with this problem. It explains IN DETAIL all the steps you need/should take to eliminate the problem of alleriges in your pet. Surprisingly, the steps are rather simple! It's all about reading the labels.
http://dogtorj.tripod.com/id5.html