I believe health issues are result of centuries of dog breeding. They are the result of at best ignorance or at worse "trade-offs", if you will, that people have made to obtain what they desire.
DCM for years was referred to as "sudden death". Only recently (the last few decades) have accurate diagnoses "called a duck, a duck". It is still only theory as to the genetic passing of DCM and best guessing is still utilized today to attempt to clear up lines that breeders have inadvertantly introduced DCM into. Modern medicine is currently working on identifying the genetic marker signifying that a dane is infected/carrier for DCM.
Cancer is the number one killer of danes as a whole. The problem is we have no real idea how it is passed to even begin to eliminate it. Of course there are gains being made for curing cancer, but we don't even have it figured out for humans yet.
I also applaud those that health test with the understanding it is a tool and does not guarantee results. Those that think it is the end all, be all are fooling themselves, in my opion. Two OFA excellent danes can and have produced dysplastic while two fairs have produced excellent. A dog may pass its CERF testing and its offspring have cataracts.
I hate to use use the term "fad" but especially the new crop of dane breeders are going after health like it is a fad. It is the thing to do, it is hip and trendy. In this case it is a good thing, do not get me wrong. Established breeders are health testing regularly now who may not have done so in the past. It is the nature of an american dog breeding community, especially danes, where peer pressure reigns supreme.
A century ago it was aggressive temperament that was the thing to fix. I will mention that, for the most part, the temperaments have been corrected to that of a loving, even doting, companion animal. It gives hope to this recent focus on correcting health issues.
Rob Dickens, Fall Hollow Great Danes