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Woodland fern in the young ``fiddlehead'' stage. (AP photo)

As fiddlehead season kicks into gear, the Canadian Food Inspection Agency is warning consumers to be careful in preparing the curly green fern.

Fiddleheads are the edible shoots of the ostrich fern. This is the time of year when fresh fiddleheads appear on store shelves across the country.

The agency believes the most likely cause of a number of fiddlehead-related illnesses in the country is an unidentified natural toxin present in the plant.

The federal department's website said people start to show symptoms 30 minutes to 12 hours after eating raw or undercooked fiddleheads. It said people report diarrhea, nausea, vomiting, abdominal cramps and headaches.

People usually recover in a day.

The agency tells consumers to wash fresh fiddleheads in several changes of cold water, then cook them in boiling water for 15 minutes, or steam them for 10 to 12 minutes.

It says the water used for boiling or steaming fiddleheads should be discarded as it may contain the toxin.

The CFIA said fiddleheads should also be boiled or steamed prior to sautéing, frying, or baking.