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Toronto researchers have found a clue in the blood of people killed by peanut allergies that could help identify those most at risk of a fatal reaction.

There is no way yet to know when peanut allergies, which have doubled in industrialized countries in the past decade, will trigger a mild reaction such as hives, or the life-threatening anaphylactic shock.

"We haven't been able to stratify kids in terms of their risks," said Peter Vadas, director of allergy and clinical immunology at St. Michael's Hospital in Toronto.

"We have to take everybody and treat them the same way. . . . We have to tell them to remove all nuts from their home, all traces of it, keep EpiPens [emergency epinephrine injectors] everywhere.

"That might not be necessary for everyone, but you can't tell anyone to let down their guard, and God forbid they have a severe reaction."

But Dr. Vadas, working with an international research team, has found that nine people who died of peanut allergies had a significantly lower level of a particular enzyme in their blood. At normal levels, the enzyme breaks down the chemical that causes bronchial spasms, and blood pressure drops during an allergic reaction.

The nine who died also showed the expected high levels of the IgE antibodies that are activated in response to an allergen.

"To be at risk for fatal peanut anaphylaxis, I think that people need to have [these] two factors come together," Dr. Vadas said. "This is clearly not the only reason that a person may be at risk for fatal peanut anaphylaxis, but it is a . . . biochemical abnormality which may allow us to identify at least one subset of patients at risk."

This dual biochemical profile was not found in the blood of any other adults and children who had died from a range of different causes, from drowning to sudden infant death syndrome, and allergic reactions to substances other than peanuts.

"If it's borne out, then there is a way of stratifying individuals," said Dr. Vadas, who is preparing the finding for publication. "If there are expensive therapies coming, you would know who should get it first. . . . My ultimate goal would be to find a drug that targets this enzyme."

The work, done with researchers from the United States, Britain, the Ontario Ministry of Labour and Toronto's Hospital for Sick Children, is part of an urgent and growing effort to understand, prevent and treat peanut allergies, which afflict more than 1.5 per cent of North American children. In Canada, more than 150,000 people are allergic to peanuts, resulting in roughly 15 deaths a year.

The trend is part of a wider rise in food allergies and asthma generally. It has Canadian researchers planning to conduct the world's largest allergy study in a hunt for answers.

AllerGen, short for the Allergy, Genes and Environment Network, one of Canada's multi-institution Centres of Excellence, plans to start enrolling 10,000 to 30,000 babies, children and parents in a long-term observational study next year.

Judah Denburg, AllerGen's scientific director and a professor of medicine at McMaster University, said the project will follow children and their families and consider a host of factors associated with allergies, including indoor air quality, outdoor pollutants, pets, diet and DNA.

"This is a very big project," Dr. Denburg said. "And it's based on one premise: We just don't know enough."

Scientists have no concrete evidence to explain the rising number of peanut allergies in the West. Several factors are thought to be involved, including greater exposure to peanut proteins in everything from lotions to processed foods, coupled with a genetic predisposition. The hygiene hypothesis is the most popular theory. It blames modern society's sanitized living for an increase in immune systems that overreact to allergens in the absence of other infections.

Strategies to prevent and treat peanut allergies have been complicated by the ethical dangers of testing experimental therapies for peanut exposures, since the exposure itself could be deadly.

"Children are going to have to be fed the food in question, and you don't know how severe their reaction will be and then your aim is to test [a new] therapy," said Susan Waserman, president of the Canadian Society of Allergy and Clinical Immunology. "It makes recruitment for these studies so difficult."

Scientists in the United States announced in 2003 that they had developed the first drug to counter deadly peanut reactions. Trials showed that monthly injections of the medicine, which is sold as a treatment for severe asthma under the brand name Xolair, could protect people against small, accidental peanut exposures by boosting tolerance.

But in trying to establish a baseline of the amount of peanut that could be safely tolerated, two children suffered severe reactions, Dr. Waserman said. The trial had to be stopped and redesigned.

It was the second time the trials of the much-awaited therapy had to be halted. The first delay was the result of legal wrangling between the drug companies that owned and developed the injection.

Dr. Waserman said Dr. Vadas's work to identify those most at risk, "is an interesting and important idea. . . . The relevance of his work is that he's examined very many groups of people who died . . . even people who are alive and well living with a peanut allergy."

But, she said, identifying those who face the deadliest risk would not necessarily help in recruitment for clinical trials. It would raise questions, she said, about whether a therapy found to be effective in children with mild allergies would work the same way in those with lethal allergies.

The only current method to prevent a reaction is strict avoidance, and the only potential treatment to halt anaphylaxis is an immediate shot of epinephrine, which can improve blood pressure and open airways.