Sally Davies, England's chief medical officer, told BBC News that the highest level detected was 1.9mg of bute per kg of horse meat: “We know from calculations that if humans have eaten contaminated meat, there is a very low risk to health,” she said.
“A person would need to eat 500–600, 100% horsemeat burgers a day to get close to consuming a therapeutic or treatable dose of phenylbutazone. So by the time the person eats it, it’s going to be a trace amount.”
The chief medical officer for Scotland Harry Burns said the positive samples showed “minute traces of phenylbutazone” compared with the normal dose which used to be given to humans.
“Even the sample with the highest trace is still at a level many hundred times lower than those previously used in humans on a daily basis,” he said.
"Furthermore, even at the much higher level previously given to humans, serious adverse reactions were rare and usually associated with prolonged exposure. It is, therefore, my view that the risk to human health is likely to be very low.”
Risk to human health
But Burns added that no level of this drug in the human food chain can be tolerated.
Their comments followed FSA confirmation that of 206 carcasses tested between January 30 2013 and February 7, eight horses tested positive for the drug.
Six of the carcasses – slaughtered by LJ Potter Partners at Stillman’s in Taunton – were exported to France and “may have entered the food chain”, said the FSA.
The other two carcasses did not leave the slaughterhouse at High Peak Meat Exports in Nantwich, Cheshire.
French authorities later confirmed that they had located the six carcasses and destroyed them.
The news followed revelations in January that five horses which tested positive for bute – claimed by shadow environment secretary Mary Creagh to be "a known carcinogen" – were exported to France for food last year.
Last week it was revealed that some Findus beef lasagne – produced by its French supplier – Comigel contained up to 100% horse meat.
Food minister David Heath has confirmed that no bute was found in Findus beef products identified to contain horse meat.
A spokesman for David Cameron said the UK authorities were co-operating with French officials to track the carcasses.
Horse meat contamination
The FSA ordered all processors and retailers to test products – including beef burgers, meat balls and lasagne – to assess the extent of horse meat contamination. The agency required firms to send the testing results to it by today (February 15) – a timescale some industry insiders described as unrealistic.
From this week, the FSA has operated a ‘positive release’ system for horses slaughtered in the UK. “This will mean that all horse carcasses have to test negative for bute before they can enter the food chain,” said the FSA.
Test results are available about 48 after testing.
Meanwhile, Scottish rural affairs secretary Richard Lochhead has welcomed news of a new EU-wide meat testing regime, to be introduced from March 1.
The Scottish government said the move followed “the action which has already been taken in Scotland which was the first part of the UK to undertake these inspections”.
Inspections are currently being carried out in plants approved to manufacture processed meat products throughout Scotland as part of measures introduced after horsemeat contamination was found in beef in Ireland. Most inspections are expected to be completed by February 22.
Lochhead said: “I welcome the plans for a new EU-wide testing regime, mirroring what is already happening in Scotland, which will give increased protection to consumers.”