Apparently you would have to eat 100% horse meat burgers exclusively for breakfast, lunch and dinner and one for supper every day for years before the Bute caused a health hazard in a human adult, so of little risk. Though my main concern is the quality of the meat from these dubious sources (Horse meat is delicious and we should lift our silly ban on the stuff), and what else are they putting in there? We've already heard about Donkey meat, but DNA testing will only find something that you're specifically looking for. What are they going to find if they start looking for Rat meat?
Having said, it appears that this has been going on for years, if not decades, so it seems we might have been consuming it for years unbeknown to us and I'm not aware of any human becoming ill or dying as a result. I've always been very suspicious of the burgers you buy from burger vans, i'm sure they don't contain alot of beef - but I have them from time to time anyway in full knowledge that i'm probably eating the more nasty and fleshy parts of the animals anatomy - ear lobes, eyelids and other unmentionable things. But I am a big advocate of utilising all of the animal anyway!