An outbreak of E. coli linked to romaine lettuce is being investigated in three provinces.

The Public Health Agency of Canada says 21 people in Quebec, New Brunswick and Newfoundland and Labrador became sick from E. coli in November.

Many of the individuals said they ate romaine lettuce before getting sick.

The Public Health Agency of Canada says it is working with Health Canada, the Canadian Food Inspection Agency and provincial public health agencies to look into the source of the romaine lettuce.

Symptoms of E. coli infection include nausea, vomiting, headache, fever, severe stomach cramps and watery or bloody stool.

The Public Health Agency says the health risk of E. coli is low, and that most people who are infected show symptoms for five to 10 days before making a full recovery.