The World Health Organisation has said that the E coli bacterium responsible for an outbreak that has left 17 dead and infected hundreds in Europe is a new strain that has never been seen before.
Preliminary genetic sequencing suggests the strain is a mutant form of two different E coli bacteria, with lethal genes that could explain why the Europe-wide outbreak appeared to be so big and dangerous, the agency said.
Hilde Kruse, a food safety expert at the WHO, told the Associated Press that "this is a unique strain that has never been isolated from patients before".
She added that the new strain has "various characteristics that make it more virulent and toxin-producing".
So far the mutant E coli strain has infected more than 1,500, including 470 who have developed a rare kidney failure complication. Researchers have been unable to pinpoint the cause of the outbreak, which has hit at least nine European countries.
Nearly all the sick people either live in Germany or recently travelled there. Two people who were infected are now in the US and both had recently travelled to Hamburg, Germany, where many of the cases occurred.
Fearful of the outbreak spreading into Russia, the country on Thursday extended its ban on vegetable imports to all of the EU. Russia had banned fresh imports from Spain and Germany on Monday.
Lyubov Voropayeva, spokeswoman for the Russian Agency for the Supervision of Consumer Rights, told AP that the ban had been imposed immediately for an indefinite period.
The agency's chief, Gennady Onishchenko, told Russian news agencies that this "unpopular measure" would be in place until European officials informed Moscow of the cause of the disease and how it was being spread.
"How many more lives of European citizens does it take for European officials to tackle this problem?" he told the state-owned RIA Novosti news agency.
No infections have yet been reported in Russia.
Medical authorities appear no closer to discovering the source of the infection. The outbreak is already considered the third-largest involving E coli in recent world history and it may be the deadliest. Twelve people died in a 1996 Japanese outbreak that reportedly infected more than 12,000, and seven died in a Canadian outbreak in 2000.