There's some weird counter-intuition at play when it's argued that thoroughbred racing has its doping problem under control because there are so many positive tests. But with so much money at stake—much more than in some small-change sport like baseball—there's a secret arms race between the good scientists and the bad scientists, between those developing new untestable drugs and those developing tests for them. It's not a surprise that the former group is faster at what they do.
The New York Times reports on the newest scourge of the racetrack, an opiate derived from a toxin found on the back of a certain South American frog. Just a quick tongue of the Waxy Monkey Leaf Frog (the horses are presumably getting synthesized dermorphin, not actually licking frogs), and you'll be feeling no pain. The horses can't sense their own wear and tear, and instead feel euphoric and hyperactive, and run faster. This is horse racing in a nutshell: dope 'em up and hope they don't break down.
For an unknown amount of time, dermorphin was untestable. Then one lab figured out a way, and within weeks 30 horses from four states tested positive. It's probably much more widespread than that, considering the majority of racing board-approved laboratories still don't test for it. And then, of course, there's all the other toxic amphibian juices we don't even know about yet.