Berlin/London, June 3 (Reuters): More than 1,700 people have been infected by a toxic strain of E. coli bacteria that has killed at least 19 and may be the deadliest yet in human history, health officials said today.

Germany is at the centre of the outbreak. But with the exact source of the illness still a mystery, consumers appear to be increasingly nervous around the world.

Some 1,733 people in Europe and the US have so far become ill, probably from eating contaminated vegetables and salads. The WHO said the strain was a rare one, seen in humans before, but never in this kind of outbreak.

In Germany, health officials said they recorded 199 new cases of the highly toxic strain of the infection in the past few days. People have also become ill in Austria, Denmark, France, Netherlands, Norway, Spain, Sweden and Switzerland, Britain and US.

“All these cases except two are in people who reside in or had recently visited northern Germany during the incubation period for the infection — typically 3 to 4 days post-exposure — or in one case, had contact with a visitor from northern Germany,” the WHO said in a statement.

Scientists struggled to pinpoint the contamination, assumed to be poor hygiene at a farm, in transit, a shop or food outlet. Health institutes across Europe have tried to reassure the public by stressing that E. coli, a frequent cause of food poisoning, can generally be tackled by washing vegetables and by washing hands before eating to reduce the risk of bacteria being passed on from the faeces of an infected person.

E. coli bacteria themselves are harmless. But the strain that is making people sick in Europe has the ability to stick to intestinal walls where it pumps out toxins, sometimes causing severe bloody diarrhoea and other complications. People have become ill probably from eating lettuce, tomatoes, cucumbers or other raw salad vegetables in Germany. The region around Hamburg is at the epicentre of the sickness.

Robert Tauxe of the US Centers for Disease Control and Prevention, which has been working with German health officials since last week, said the strain was likely the most deadly yet in terms of the number of deaths recorded. “I believe it is,” he told Reuters when asked about that possibility. He said it was unclear how the bacteria became so resistant.

The outbreak has put strains on trade relations, with Russia drawing EU criticism after banning raw vegetable imports from Europe.

German officials originally blamed the infections on contaminated cucumbers imported from Spain but later backtracked and apologised to Madrid. E. coli infections can spread from person to person but only by what is known as the faecal-oral route. Health experts in Germany have recommended consumers avoid eating raw vegetables.