Public health officials in New Brunswick say they are still a long way from identifying the source of an E. coli outbreak that has sent two Fredericton teenagers to hospital.

There have been four confirmed cases of a potentially deadly strain of E. coli in the Fredericton area. Macaella Boer, 18, and a male friend remain in Saint John hospital undergoing treatment. 

Dr. Denis Allard, the province's acting chief medical officer of health, said there has been a delay in administering an 18-page questionnaire to patients.

"You can imagine that when some people are sick, not feeling well, sometimes it may take more than one session to administer the questionnaire fully to find out all the range of foods they may have eaten in the past week," he said.

"Then we collate all that information from all the cases together and look for a common food that may have been eaten, or common restaurants, or maybe it might have been a party or some other instance where they may have come together to eat a meal."

Allard said it could be another week before he has any new information for the public, but in many E. coli outbreaks, the source is never identified.

"There is certainly a chance that we may never know. When we look at statistics from all the investigations, it's sometimes more than 40 per cent of the time where the source is never known," he said.

In the meantime, he is urging people to take precautions when handling and preparing food and suggests checking the latest health inspection report before eating at a restaurant.

E. coli O157: H7 secretes a powerful toxin that can destroy red blood cells leading to severe illness, high blood pressure and kidney damage.

It is the same strain that killed seven people in Walkerton, Ont. in 2000.