Health officials say four E. coli cases in the U.S. may be linked to the food poisoning outbreak in Europe.

All four people were in northern Germany in May, and officials are confident they were infected with E. coli in that country.

Three of them — two women and a man — are in hospital in the U.S. with a kidney complication, which has become a hallmark of the outbreak. The fourth person hasn't developed the complication but has bloody diarrhea, a common symptom.

The four travelled separately to Germany, said officials, who did not identify the U.S. cities where they're being treated.

Officials also said they were checking two possible E. coli cases in U.S. military service members in Germany.

Elsewhere on Friday, the World Health Organization cautioned people against taking antibiotics if they fall ill from the E. coli outbreak, which began in Germany last month.

The UN health agency said it supports existing recommendations to avoid antibiotics because they could make the condition worse.

Anti-diarrhea medication also should be avoided because it stops the bacteria from quickly leaving the body, WHO epidemiologist Andrea Ellis told reporters in Geneva.

In a number of cases, the outbreak is causing complications affecting blood and kidneys.

Studies testing the effectiveness of antibiotics for E. coli suggest at best the medication has no effect and at worst the drugs may worsen the illness. By killing bacteria, antibiotics cause the microbes to release more toxins that cause illness.

Doctors are treating the kidney complication called hemolytic uremic syndrome, or HUS, with dialysis and blood transfusions when needed.

WHO has no active role in combating the outbreak that has so far sickened more than 1,700 people, mostly in Germany, and killed at least 18.

The Robert Koch Institute said 1,733 people have now fallen ill in Germany, the epicentre of the outbreak. They include 520 with a life-threatening complication that can cause kidney failure.

Since May 2, reported cases of the complication — called hemolytic uremic syndrome, or HUS — have totalled 551 in European Union member states, including 520 cases in Germany, the European Centres for Disease Prevention and Control said.

Nearly 200 new cases of E. coli infection were reported in Germany the first two days of June, the national disease control centre reported Friday, but officials said there are signs the European bacterial outbreak could be slowing.

The World Health Organization said that as of Tuesday, nine other European nations had reported a total of 80 people sick from the bacteria, most of whom had recently visited northern Germany.

While suspicion has fallen on raw tomatoes, cucumbers and lettuce as the source of the germ, researchers have been unable to pinpoint the food responsible.

The outbreak is considered the third-largest involving E. coli in recent world history, and it is already the deadliest. Twelve people died in a 1996 Japanese outbreak that reportedly made more than 9,000 people sick, and seven died in the Walkerton, Ont., tainted water outbreak in 2000.

'There is no reason for hysteria, because it's not spreading and it's not increasing — it's decreasing.'— Dr. Reinhard Brunkhorst, German Nephrology Society

Kidney specialist Dr. Reinhard Brunkhorst, president of the German Nephrology Society, told reporters in Hamburg that hospitals are now seeing fewer new infections reported each day, but cautioned "it may be less, but it's not over yet."

"There is no reason for hysteria, because it's not spreading and it's not increasing — it's decreasing," he said.

There are several aspects of this outbreak that are unique, said Robert Tauxe of the U.S. Centers for Disease Control and Prevention, which has been working with German health officials since last week.

"First of all, it’s mostly adults that are being affected rather than children, and a very large [number] are developing a kidney failure complication called HUS," Tauxe said in an interview with CBC News on Friday. "Whether that is because this organism is particularly virulent or whether it’s because a lot of people have been exposed to something like a food contaminated with the organism is not completely clear."

Researcher Dag Harmsen at German's Muenster University Hospital, which has been closely involved in the investigation of the outbreak, said scientists hope to know enough about the E. coli strain by next week to be able to prevent new infections and better treat patients.

To avoid food-borne illnesses, WHO recommends that people:

• Wash their hands
• Keep raw meat separate from other foods
• Thoroughly cook their food
• Wash fruits and vegetables, especially if eaten raw

Experts also recommend peeling raw fruits and vegetables if possible.

"Cook it or don't eat it," Hamburg kidney specialist Rolf Stahl told reporters Friday. "That's my personal recommendation."

In Germany, schools have pulled raw vegetables from menus and piles of cucumbers sit untouched on shop shelves.

As the number of consumers avoiding vegetables grows, European farmers say they are losing millions of euros every day.

Russia bans EU vegetables

On Thursday, Russia extended a ban on vegetable imports from Spain and Germany to the entire European Union to try to stop the outbreak from spreading east, a move the EU quickly called disproportionate and Italy's farmers denounced as "absurd." No deaths or infections have been reported in Russia.

Meanwhile, Chancellor Angela Merkel agreed with Spanish Prime Minister Jose Luis Rodriguez Zapatero in a telephone conversation late Thursday to push for EU help for affected farmers, Merkel's spokesman said.

Merkel, however, also defended the decision of state officials in Hamburg to announce their suspicions that Spanish cucumbers were the possible source of the outbreak. The warning was given after three cucumbers from Spain tested positive for E. coli, but further tests revealed it was a different strain from the one that has made so many people sick in the northern port city and elsewhere.

With files from CBC's Kelly Crowe