An eight-year-old boy developed an anaphylactic allergy to fish and peanuts after receiving a blood transfusion, a rare case that illustrates why parents and doctors should be aware of the possibility following a transfusion, Canadian researchers say.

The boy had no history of allergies. He received blood products including plasma and chemotherapy as part of his treatment for medulloblastoma, a type of brain tumour.

The boy was given a prescription for an epinehphrine autoinjector after his temporary salmon allergy. (Mandi Wright/Detroit Free Press/Associated Press)

In Monday's issue of the Canadian Medical Association Journal, Dr. Julia Upton of the Hospital for Sick Children in Toronto and her team describe how the boy experienced anaphylactic symptoms including swelling of the lips, facial redness, throat discomfort and fatigue within 10 minutes of eating salmon, a food he'd eaten often before without having an allergic reaction.

The symptoms resolved after a few hours of treatment with an antihistamine. The boy was told to avoid salmon and given a prescription for an epinehphrine autoinjector as a precaution.

Doctors suspected analyphylaxis from transfer of an allergy antibody in blood plasma that can react against allergens in rare cases, rather than the typical scenario where a person makes the allergy antibodies themselves.

"It's rare to have an allergic reaction to a previously tolerated food," said Upton, a staff physician in Sick Kids' clinical immunology and allergy department. "It's extremely rare to have a new allergy come from a blood product. Thankfully, the prognosis is excellent and it typically resolves within a few months."

Researchers said unusual causes of food allergy should be suspected in children who react to previously tolerated foods.

When this type of passive transfer of allergies after transfusion is suspected, the hospital's transfusion service can investigate donors to determine if they can continue to be suitable blood donors.

The researchers advised doctors to followup with the family after a few months to decide when to reintroduce the temporary allergen into the child's diet.

In the boy's case, four days after the fish reaction, he experienced an allergic reaction to peanuts minutes after eating a chocolate peanut butter cup. Again, he'd often eaten peanut products before.

Allergies among blood donors

At that point, the hospital did skin prick tests, which were positive for fish, peanut and tree nut.

When the boy's family was contacted about six months after his reactions, the boy was again eating salmon, one of his favourite foods, and peanuts. At Upton's last contact with him a few months ago, he was better.

Canadian Blood Services said the first known instance was reported in 1919. The Toronto boy's case is the second reported in the country in the past decade and the first in a child.

Upton hopes the publication will raise awareness and lead to better understanding of the rare event. Several circumstances have to come together for it to occur, she noted, including having a blood donor with a history of allergies, a child who received a lot of plasma from transfusion and ate the allergenic foods within a short window.

It is the clear plasma component of blood that contained the allergy antibody from a donor with a severe allergy to peanuts, tree nuts, shellfish and fish, the researchers said.

The overall prevalence of analyphylaxis in the U.S. is about 1.5 per cent, Upton said.

A survey Canadian Blood Services conducted around 2010 disclosed that allergies are common among blood donors and nearly eight per cent reported having severe allergies.

Asking so many people to refrain from giving blood would jeopardize Canada's blood supply to prevent a less than one in a million occurrence, said Dr. Mindy Goldman, the agency's medical director.