NEW YORK - Dubbed a "mutant" strain, a deadly E. coli breakout in Europe has resulted in 18 deaths and has sickened more than 1,500 people, the Associated Press reports.

At first, cucumbers from Spain were thought to be the culprit, but tests have revealed that??s not the case. Hilde Kruse, a food safety expert at the WHO, told the AP "this is a unique strain that has never been isolated from patients before."

Researchers have not been able to pinpoint the cause of the outbreak, which has hit at least nine European countries, and prompted Russia on Thursday to extend a ban on vegetables to the entire European Union, the news source writes.

Some scientists, meanwhile, are saying that the mutant E. coli strain could have originated in contaminated manure used to fertilize vegetables.

Germany has been hit hard by the outbreak, prompting officials to warn people not to eat lettuce, tomatoes and cucumbers.

The AP writes that this outbreak is already considered the third-largest involving E. coli, and it may be the deadliest. In 1996, 12 people died in a Japanese outbreak and seven died in a 2000 Canadian outbreak.