The bacterial infection spreads through coughs and sneezes.
Initially mild symptoms give way to severe coughing fits that can last for weeks, making it hard for a child to eat, drink or even to breathe, doctors say.
"The way you can tell really is when you are three or four weeks out and you are still really coughing then it is much less common for it to be something else," said Dr. Allison McGeer, director of infection control at Toronto's Mount Sinai Hospital.
"There may be as much as a 40 per cent chance that it is going to be whooping cough, but you have already been sick for six weeks and you have already infected a whole lot of people."
Public health officials advise that children should receive a full series of immunizations for pertussis, also called whooping cough, and a booster shot in their teens.
"It is very important that adults — particularly adults who are working with or caring for very young children — to get immunized to create kind of a bubble of protection around children," said Dr. Arlene King, Ontario's chief public medical officer of health. "It is critical."
Last week, the family of one-month-old Harper Whitehead, who died of whooping cough, pleaded with parents to get their children immunized.
The pertussis vaccine prevents the spread of serious illness and death among young children who haven't yet been fully immunized, King said.
For children over six months of age, whooping cough is miserable though rarely life-threatening. In contrast, the airways of the youngest babies aren't well developed, which increases their susceptibility, doctors say.
Children under six months of age are mostly infected by someone who lives in the house with them and often that is a parent, McGeer said.
The province expanded its free immunization program last summer to provide another dose of the tetanus, diphtheria and pertussis vaccine to adults aged 19 to 64.
In southwestern Ontario, Wellington-Dufferin Guelph is currently experiencing an outbreak, local and provincial health officials report. Public health units in New Brunswick, Nova Scotia, British Columbia, Alberta and Yukon also reported outbreaks this year.Suggest a correction