A girl in Canada experienced an unusual allergic reaction to blueberry pie — she was not allergic to any of the pie's ingredients, but instead reacted to antibiotic residue in the food, a new study suggests.

Shortly after eating a slice of blueberry pie, the girl experienced facial flushing, hives and abnormal breathing. She was taken to an emergency room, and treated with drugs used for allergic reactions, including epinephrine, and recovered.

A team of doctors then investigated what might have caused the girl's reaction. Although the patient was allergic to milk, an analysis showed the pie did not contain milk. Doctors also performed tests to see if the girl was allergic to other ingredients in the pie, such as blueberries, eggs or nuts, but the tests all came back negative. [8 Strange Signs You're Having an Allergic Reaction]

Further analysis showed that the pie contained residue from an antibiotic. The doctors tested the girl for an allergy to streptomycin, an antibiotic used as a pesticide on fruit. And, indeed, she reacted to streptomycin in much the same way as she had responded to the blueberry pie.

Although the researchers did not have access to enough of the pie to confirm that it contained streptomycin specifically, the study results suggests that the girl's allergic reaction was caused by streptomycin-contaminated blueberries, the researchers said.

Allergic reactions to antibiotics in food — such as beef and milk — are rare, but have been reported. The new study is the first to link an allergic reaction to antibiotics in fruit, the researchers said.

The findings  serve as a reminder to doctors in cases of unexplained allergic reaction. "Don't forget to think about antibiotics," said study researcher Dr. Anne Des Roches, an allergist at CHU Sainte-Justine, a health center affiliated with the University of Montreal in Quebec.

Allergic reaction to antibiotics in food are underdiagnosed because doctors cannot simply check a product label for antibiotics; they have to send the sample to special laboratories to perform an analysis, Des Roches said.

"This is a very rare allergic reaction" Dr. James Sublett, president-elect of the American College of Allergy, Asthma and Immunology, said in a statement. "Nevertheless, it's something allergists need to be aware of and that emergency room personnel may need to know about."

Use of antibiotics in agriculture has received criticism because it may contribute to the rise of antibiotic resistance. Some countries have banned the use of antibiotics for growing food, but the practice is allowed in the United States and Canada.

Recently, the U.S. Food and Drug Administration took steps to help phase out the use of certain antibiotics in livestock; the drugs had been used to help animals gain weight faster.

Stricter policies to reduce antibiotic contaminants in foods will not only help to fight antibiotic resistance, but may also reduce the type of rare allergic reaction that the girl in the study experienced, the researchers said.

The study is published in the September issue of the journal Annals of Allergy, Asthma and Immunology.

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