Preliminary genetic sequencing suggests the strain is a mutant form of two different E. coli bacteria, with aggressive genes that could explain why the outbreak appears to be so massive and dangerous, the agency said.
"This is a unique strain that has never been isolated from patients before," Hilde Kruse, a food safety expert at WHO, told The Associated Press. The new strain has "various characteristics that make it more virulent and toxin-producing" than the many E. coli strains people naturally carry in their intestines.
Researchers have so far been unable to pinpoint the cause of the illness, which has now spread to at least 10 European countries and fanned uncertainty about eating tomatoes, cucumbers and lettuce. The strain has sickened more than 1,500 people, including 470 who have developed a rare kidney failure complication, and killed 18 - most of them in Germany, the country hardest hit.
Fearful of the outbreak spreading east to Russia, the country extended a ban on vegetables to the entire European Union from just Germany and Spain, a move the bloc quickly called disproportionate.
Scientists sequencing the bacteria strains found in the outbreak said it was caused by "an entirely new super-toxic E. coli strain" that several antibiotic resistant genes, according to a statement from the Shenzhen, China-based laboratory, BGI. The researchers were working together with scientists from the University Medical Center Hamburg-Eppendorf.
The scientists found the bacteria is similar to a strain isolated in the Central African Republic known to cause serious diarrhea, the Chinese lab said.
Kruse said it's not uncommon for bacteria to continually mutate, evolving and swapping genes. It is difficult to explain where the new strain came from, she said, but said strains of bacteria from both humans and animals easily trade genes, similar to how animal viruses like Ebola sometimes jump into humans.
"One should think of an animal source," Kruse said. "Many animals are hosts of various types of toxin-producing E. coli." Some scientists suspect the deadly E. coli might have originated in contaminated manure used to fertilize vegetables.
Previous E. coli outbreaks have mainly hit children and the elderly, but the European outbreak is disproportionately affecting adults, especially women. Kruse said there might be something particular about the bacteria strain that makes it more dangerous for adults.
But she cautioned that since people with milder cases probably aren't seeking medical help, officials don't know just how big the outbreak is. "It's hard to say how virulent (this new E. coli strain) is because we just don't know the real number of people affected."
Nearly all the sick people either live in Germany or recently traveled there. British officials announced four new cases, including three Britons who recently visited Germany and a German person on holiday in England.
The WHO recommends that to avoid food-borne illnesses people wash their hands before eating or cooking food, separating raw and cooked meat from other foods, thoroughly cooking food, and washing fruits and vegetables, especially if eaten raw. Experts also recommend peeling raw fruits and vegetables if possible.
Russia had earlier this week banned fresh imports from Spain and Germany, but it expanded the ban Thursday to include the entire EU. The United Arab Emirates issued a temporary ban on cucumbers from Spain, Germany, Denmark and the Netherlands.
Lyubov Voropayeva, spokeswoman for the Russian Agency for the Supervision of Consumer Rights, told the AP the Russian ban has been imposed immediately and indefinitely. No fatalities or infections have yet been reported in Russia.
"How many more lives of European citizens does it take for European officials to tackle this problem?" the agency's chief Gennady Onishchenko said to the state-owned RIA Novosti news agency.
Frederic Vincent, a spokesman for the EU's Health and Consumer Policy Commissioner John Dalli, said Thursday that the European Commission would write to Russia to demand further clarification. The Italian farmers association Coldiretti criticized the ban as "absurd."
One expert said the fact the strain is new may have complicated the response to the outbreak. "Officials may not have had the correct tests to detect it, which may explain the initial delay in reporting," said Paul Hunter, a professor of health protection at the University of East Anglia in England.
He said the number of new cases would likely slow to a trickle in the next few days. The incubation period for this type of E. coli is about three to eight days, and most people recover within 10 days.
"Salads have a relatively short shelf life and it's likely the contaminated food would have been consumed in one to two weeks," Hunter said.
But Hunter warned the outbreak could continue if there is secondary transmission of the disease, which often happens when children are infected. The disease can be spread when infected people don't take proper hygiene measures, like bathing or handwashing.
Meanwhile, Spain's prime minister slammed the European Commission and Germany for early on singling out the country's produce as a possible source of the outbreak, and said the government would demand explanations and reparations.
Jose Luis Rodriguez Zapatero told Spanish National Radio that the German federal government was ultimately responsible for the allegations, adding that Spain would seek "conclusive explanations and sufficient reparations."
Spanish farmers say the accusations has devastated their credibility and exports. In Valencia, protesting farmers dumped some 300 kilos (700 pounds) of fruit and vegetables - cabbage, tomatoes, peppers, cucumbers and other produce - outside the German consulate.
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 sickened more than 9,000, and seven died in a 2000 Canadian outbreak.
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Vasilyeva contributed to this story from Moscow. Associated Press writers Kirsten Grieshaber in Berlin, Gabriele Steinhauser in Brussels, Ciaran Giles in Madrid, Karel Janicek in Prague and Adam Schreck in Dubai contributed to this report.
