(AP) -- An expert at the World Health Organization says time is running out for German investigators to find the source of the world's deadliest E. coli outbreak, which has spread fear across Europe and cost farmers millions in exports.
German officials are still seeking the cause of the outbreak weeks after it began May 2. In the last week, they have wrongly accused Spanish cucumbers and then German sprouts of sparking the crisis that has killed 22 people and infected over 2,400.
"If we don't know the likely culprit in a week's time, we may never know the cause," Dr. Guenael Rodier, director of communicable diseases expert at WHO, told The Associated Press on Tuesday.
He said the contaminated vegetables have likely disappeared from the market and it would be difficult for German investigators to link patients to contaminated produce weeks after they first became infected.
"Right now, (Germans) are interviewing people about foods they ate about a month ago," he said. "It's very hard to know how accurate that information is."
Without more details about what exact foods link sick patients, Rodier said it would be very difficult to narrow down the cause.
"The final proof will come from the lab," he said. "But first you need the epidemiological link to the suspected food."
Other experts issued harsher criticism of the German investigation and wondered why it was taking so long to identify the source.
"If you gave us 200 cases and 5 days, we should be able to solve this outbreak," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, whose team has contained numerous food-borne outbreaks in the United States.
Osterholm described the German effort as "erratic" and "a disaster" and said officials should have done more detailed patient interviews as soon as the epidemic began.
The medical director of Berlin's Charite Hospitals, Ulrich Frei, said it took the national disease control center weeks to send his hospital questionnaires for E.coli patients to fill out about their eating habits.
Osterholm said the Germans should have been able to trace cases of illness to infected produce by now and that tests on current produce won't be helpful.
"It's like looking at camera footage of a traffic intersection today to see what caused an accident three weeks ago," he said.
"This is an outbreak response that is not being led by the data," he added. "Solving an outbreak like this is difficult, but it's not an impossible task."
On Tuesday, the EU health chief warned Germany against premature - and inaccurate - conclusions on the source of contaminated food. The comments by EU health chief John Dalli came only a day after he had defended the German investigators, saying they were under extreme pressure.
Dalli told the EU parliament in Strasbourg that information must be scientifically sound and foolproof before it becomes public.
In outbreaks, it is not unusual for certain foods to be suspected at first, then ruled out. In 2008 in the U.S., raw tomatoes were initially implicated in a nationwide salmonella outbreak. Consumers shunned tomatoes, costing the tomato industry millions. Weeks later, jalapeno peppers grown in Mexico were determined to be the cause.
In the current E. coli outbreak, tests are continuing on sprouts from an organic farm in northern Germany, but have so far come back negative,
But Rodier said that doesn't necessarily exonerate the vegetables.
"Just because tests are negative doesn't mean you can rule them out," he said. "The bacteria could have been in just one batch of contaminated food and by the time you collect specimens from the samples that are left, it could be gone."
He said food-borne outbreaks are difficult to contain because they involve multiple industries, government departments and in Germany's case, several layers of bureaucracy to report numbers. That results in a slight reporting delay, which makes it harder for experts to know whether an outbreak is peaking or not.
The outbreak has killed 22 people - 21 in Germany and one in Sweden.
Germany's national disease control center, the Robert Koch Institute, on Tuesday raised the number of infections in Germany to 2,325, with another 100 cases in 10 other European countries and the United States. The number of victims hospitalized in intensive care with a rare, serious complication that may lead to kidney failure rose by 12 to 642.
The institute said the number of new cases is declining - a sign the epidemic might have reached its peak - but added it was not certain whether that decrease will continue.
In a major difference from other E. coli outbreaks, women - who tend to eat more fresh produce - are by far the most affected this time. The majority of the victims in Germany are between 20 and 50 years old and tend to be highly educated, very fit, and lead healthy lifestyles, investigators said.
"What do they have in common? They are thin, clean pictures of health," said Friedrich Hagenmueller of the Asklepios Hospital in Hamburg, Germany.
Explore further: NIH launches tool to advance Down syndrome research