Five cases of illness associated with consuming fiddleheads — the edible shoots of ostrich ferns — have recently sprouted up in Toronto, according to a health advisory from Toronto Public Health.

Fiddleheads are believed to carry an unidentified natural toxin that can sicken some eaters unless the food is thoroughly washed and cooked before consumption. After concerns about the mystery toxin first arose in 1994, the U.S. Centers for Disease Control and Prevention began recommending people boil fiddleheads for 15 minutes or steam them for 12 before eating.

Toronto Public Health did note that it is rare for people to become ill from eating fiddleheads. Those who do become ill may have diarrhea, nausea, abdominal cramps and headaches usually lasting fewer than 24 hours.

The health advisory detailed a number of fiddlehead safety tips, which include:

– Never eat raw fiddleheads.

– Remove as much of the brown, papery husk as possible

– Wash well using fresh, cold water

– Discard water used for boiling or steaming, as it may contain the toxin.

Health Canada has more information on preparing fiddleheads on their website.