The Public Health Agency of Canada is warning U.S.-bound travellers to be on guard against whooping cough, which is also known as pertussis.

Public health officials warn that pertussis, or whooping cough, can be particularly serious in infants and children. (iStock)Outbreaks of the illness have been reported in New York State and California.

The California Department of Public Health (CDPH) says the outbreak in that state has reached epidemic levels, with 7,297 cases, including 10 deaths, nine of them in infants under two months old.

"We are facing what could be the worst year for pertussis that this state has seen in more than 50 years," said Dr. Gilberto Chávez, the chief of the CDPH's centre for Iifectious disease and the state's epidemiologist.

Another outbreak in Jefferson County, N.Y., has resulted in 463 confirmed cases, six of which required hospitalization.

Jefferson County is on the eastern shore of Lake Ontario.

Pertussis is highly contagious. It spreads through droplets in the air from coughing or sneezing. It can cause coughing spells that last from four to six weeks.

While it can infect people of any age, it's particularly serious in young children. Major complications include hypoxia or lack of oxygen, inability to breath, pneumonia and neurological complications such as seizures.

Canadian infants are routinely vaccinated against pertussis. However, some parents decline to get their children vaccinated because of a general fear of potential side-effects of vaccines.

The Public Health Agency of Canada urges anyone travelling to affected regions to ensure their vaccinations are up to date. Adults who have not been vaccinated are encouraged to visit a doctor or travel clinic and ask for a Tdap vaccination.