German health authorities Friday continued to advise against eating raw tomatoes, cucumbers and lettuce from Northern Germany, and provided more details about their efforts to find the source of the country’s massive outbreak of E. coli O104:H4 infection.

Meanwhile, the World Health Organization said 1,823 people have been sickened, 18 fatally.

Six Americans — four tourists who had been in Germany in May and two U.S. military personnel stationed in Germany— may be linked to the outbreak, a federal food safety panel revealed during a conference call with journalists Friday afternoon.

Although one European official said the outbreak appears to be stabilizing, the WHO case count is up 200 from Thursday’s report. The numbers include 1,271 with enterohemorrhagic E. coli (EHEC), plus another 552 suffering from hemolytic uremic syndrome (HUS), the acute kidney disorder associated with some E. coli infections.

Most of the case patients are in Germany (1,213 EHEC infections and 520 with HUS) and the areas hardest hit are the Hamburg and Schleswig-Holstein states, followed by Bremen and Mecklenburg-Western Pomerania.  There are people sick with the outbreak strain in 11 other countries; all of them recently had been in Germany.

Three of the four American tourists, hospitalized in the U.S., have developed HUS, Dr. Chris Braden of the Centers for Disease Control and Prevention said during the Friday news conference. The CDC is analyzing laboratory specimens from the two women and one man to confirm if they have been infected with the outbreak strain of  E. coli O104:H4. A fourth American suffered from severe E. coli symptoms, including blood diarrhea, but did not develop HUS, Braden said.

The Department of Defense has not released any information on the American service members who are ill.

Still no source

In its latest report, Germany’s Robert Koch Institute (RKI) said the source of the outbreak has not been identified, but fresh tomatoes, cucumbers and lettuce from the northern part of the country “must be considered to have the highest relative risk for infection compared to other foods investigated,” and these raw vegetables should be avoided “until the definitive source of the outbreak has been identified.”

Describing the preliminary results of two more epidemiological studies, RKI said 46 patients with HUS or EHEC infection from Bremen, Hamburg and Lubeck were questioned in detail from May 29 to June 2 about the foods they had eaten.

These cases were compared with 2,100 healthy individuals — the control group — matched for age, gender and region of residence. These were the results:

— Lettuce had been eaten by 84 percent of those ill, but only by 47 percent of the controls.

— Cucumbers had been eaten by 75 percent of those ill, but only 50 percent of the controls.

— Tomatoes had been eaten by 80 percent of those ill, but only 63 percent of the controls.

A total of 95 percent of those ill had eaten at least one of these vegetables.

In a second, separate study, investigators found that people who had eaten from the salad bar at a company canteen in Frankfurt were 7 times more likely to have developed bloody diarrhea than those who did not eat from the salad bar. There was no association with illness for other foods investigated, such as dessert, fruit and asparagus.

“These two unrelated and methodologically distinct studies support the results of the previously performed case control study,” RKI concluded, referring to an earlier epidemiological investigation conducted in Hamburg and published May 26.

Meanwhile, the U.S. public authorities sought to calm any concerns that potentially contaminated vegetables might have reached domestic stores.

David Elder, director of regional operations for the U.S. Food and Drug Administration, said less than 2/10th of 1 percent of fresh produce imported by the United States originates in Europe and not much arrives this time of year.

Nevertheless, Elder said fresh produce from Germany and Spain will be tested in U.S. labs for E. coli contamination before shipments are allowed to enter the country.

Testing for non-O157 E. coli strains

The size and severity of the German outbreak have led some to question whether it’s long past time for the United States to classify the most prevalent strains of pathogenic non-O157 E. coli as adulterants in food and test for them. Currently, only E. coli O157:H7 is officially considered a food poison.

The Seattle food safety law firm, Marler Clark, sponsor of Food Safety News, has petitioned the U.S. Department of Agriculture to mandate beef testing for other E. coli, and ban the sale of ground beef if it is contaminated with the bacteria.

Responding to questions at Friday’s news conference, David Goldman of the U.S. Department of Agriculture said the timetable for whether to declare non-O157 strains of E. coli as adulterants in food, or for imposing stricter regulations, won’t be affected by the outbreak in Germany.

The outbreak strain — O104:H4 — is not one of the strains being considered for adulterant status, he noted.

The so-called “Big Six” under consideration are E. coli O26, O45, 01O3, O111, O121 and O145.  The CDC estimates that, taken together, these toxic E. coli cause 160,000 illnesses in the U.S. each year.

Goldman acknowledged that the proposal to expand the list of adulterants is at the rule stage in the Office of Management and Budget (OMB), and said it involves complex issues with multiple stakeholders.

However, while the proposal sits at OMB, Don Kraemer, deputy director of the FDA’s Center for Food Safety and Applied Nutrition, had this to say in a statement issued by the agency Friday: “The FDA considers any disease-causing strain of E. coli in food to be illegal. The FDA has provided scientific guidance to the produce industry on ways to minimize the risk of E. coli, and these methods will reduce the risk of the strain of E. coli causing the European outbreak as well as the more common strains.”

The unusual O104:H4 strain

Helge Karch, director of the HUS Consultant Laboratory in Muenster, said on Friday that  “reports stating that the current pathogen is a completely new strain are not correct.”

The CDC’s Braden told reporters that a similar organism may have been involved in a 2009 outbreak in the Republic of Georgia and with a 29-year-old women who developed HUS in South Korea in 2006.

“We’ve seen similar strains in other parts of the world,” he said. “It’s rare and we haven’t seen much of it.”

Braden said the outbreak bacteria appear to differ from other Shiga toxin-producing E. coli (STEC) in the way they attach to the intestine. This strain may have a longer incubation time — up to 12 days as opposed to about one week — in contrast to most other STEC.

But it’s not clear that this is a new or especially virulent pathogen. Press reports characterized it as such after the Beijing Genomics Institute, BGI, in China and Life Technologies Corp. of California, announced they had completed preliminary DNA sequencing of E. coli O104:H4.

In other news about the outbreak:

— Reinhard Brunkhorst, president of the German Nephrology Society, told reporters in Hamburg that the number of new infections appears to be “stabilizing somewhat.”

— Chancellor Angela Merkel defended Germany’s earlier false alarm that Spanish cucumbers might be the source of the outbreak. Tests had found that organic cucumbers from Spain were contaminated with toxic E. coli, although it turned out not to be the outbreak strain and Germany withdrew the warning.

Merkel told Spain’s prime minister in a telephone call that Germany’s health authorities were “duty-bound to inform the public at all times.”

— U.S. military commissaries in Europe are not selling or receiving lettuce, tomatoes and cucumbers from any European sources until further notice, according to a directive from the U.S. Army’s European Regional Veterinary Command.

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Dan Flynn contributed to this report.