The U.K. Food Standards Agency said eight out of 206 horses it checked had tested positive for phenylbutazone, commonly known as bute. The drug is banned for human use in countries including Britain and the U.S.
Almost no horsemeat is consumed in Britain, where hippophagy — eating horses — is widely considered taboo. But thousands of horses killed in the country each year are exported for meat to countries including France and Belgium that have a culture of eating horsemeat.
The scandal has also raised the uncomfortable idea that Europeans may unwittingly have been consuming racehorses, which are often treated with bute.
Britain's Aintree race track said a slaughterhouse in northern England shut down this week by government investigators had a contract to dispose of fatally injured racehorses. The racecourse said it was "as confident as we possibly can be" that none of the meat had entered the human food chain.
Despite the ban on bute for humans, Britain's chief medical officer, Sally Davies, said that horsemeat containing the drug "presents a very low risk to human health."
Davies said the drug was once prescribed to patients with severe arthritis, and while it sometimes produced serious side effects including the blood disorder aplastic anemia, it was "extremely unlikely" that anyone eating horsemeat would experience them.
"If you ate 100-percent horse burgers of 250 grams (8.8 ounces), you would have to eat, in one day, more than 500 or 600 to get to a human dose," she said. "It would really be difficult to get up to a human dose."
In other, separate developments in the multi-tentacled horsemeat scandal Thursday:
— Police in Wales said two men — ages 64 and 42 — were arrested at Farmbox Meats near Aberystwyth, in Wales, while a 63-year-old man was arrested at the Peter Boddy Slaughterhouse in Todmorden, West Yorkshire.
—Germany said food containing horsemeat has also surfaced there, as two German national supermarkets have pulled frozen lasagna from their shelves.
—In Ireland, where fears about undeclared horsemeat first surfaced earlier this year, processing factory Rangeland Foods said it had withdrawn some batches of burger products that contained beef supplied from Poland after it tested positive for up to 30 percent horse meat. Food Safety Authority of Ireland said the products had been sold to the catering and wholesale sectors and distributed to Ireland, Britain, Spain, France, Germany and the Netherlands.
—In Britain, supermarket chain Asda said it was withdrawing a beef Bolognese sauce from its shelves after tests indicated the presence of horse DNA and would pull three other products from the same supplier as a precaution. The company previously has withdrawn four other products which tested positive for horse DNA.
—In the Netherlands, the Economic Affairs Ministry said Thursday the country's Food Safety Authority is carrying out a large-scale investigation in coming weeks and plans to inspect some 100 companies from slaughterhouses to supermarkets and take 200 samples including meat cuttings, mincemeat and other prepared food. The authority also is investigating labeling at 40 companies, identifying meat and checking paperwork.
Lawless reported from London. Cassandra Vinograd in London, Angela Charlton in Paris, Alison Mutler in Bucharest, Romania, Mike Corder in the Netherlands and David Rising in Berlin also contributed.