LETHBRIDGE, Alta. -- The chief medical health officer for an area of southern Alberta says an outbreak of whooping cough is spreading through communities with poor vaccination rates.

Alberta Health Services says there have been 17 confirmed cases of pertussis in the south zone so far this year, with 12 cases just in the last week.

"What is unique about the last 12 ... is that they are linked, and are linked also to geographic communities with extremely low immunization rates," Dr. Vivien Suttorp said Wednesday.

"This is why an outbreak was declared. We were seeing that transmission within these communities, and the risk of transmission is obviously high."

The area affected stretches from Fort Macleod to Coaldale and takes in Lethbridge County and the city of Lethbridge.

Alberta Health's website says that last year, 68 per cent of children in the southern zone had received all four rounds of the whooping cough vaccine by the age of two. The provincial average was 77 per cent.

But Suttorp said rates were below 50 per cent in the County of Lethbridge and in Fort Macleod.

Whooping cough is a bacterial infection that causes severe coughing that lasts for weeks. Any age group can be affected, although children under a year old are the most at risk for serious complications, which include convulsions, brain damage or death.

Suttorp said a four-week-old baby died of whooping cough in the south zone in 2012.

The latest cases are spreading in schools, school buses, preschools and daycares, she said. The ages of those affected range from one to the mid-70s.

The region also had a whooping cough outbreak in 2009 which Suttorp said lasted 10 months.

Alberta Health Services said there have also been 13 confirmed cases in the Calgary zone since the beginning of this year. Five weren't linked, but eight cases involved two schools and two families.

The health agency said the vaccine is free and is reminding everyone who lives in the south zone about the importance of being immunized.

Suttorp said that, overall, vaccination rates in the south zone are comparable to Calgary or Edmonton. But there are rural pockets and schools where rates are much lower and it's in those areas that vaccine-preventable diseases such as whooping cough or measles are common.

She said trust in vaccines is low in those communities and sometimes leaders in churches and schools, doctors and alternative health practitioners advocate against immunization.

"Of course, there's always the Internet, and on the Internet you can find whichever information you wish to support your cause."

Whooping cough starts with symptoms similar to a cold, but coughing becomes more severe over the course of a week. In younger children, coughing spells are usually followed by a whooping sound when inhaling. Vomiting following a coughing spell is also common.

Older children and adults may experience milder symptoms.

AHS says parents who aren't sure of their children's immunization history can contact their local community health centre or public health office.

It also reminds people not to share water bottles, lipstick, lip balm or drinks.

People who suspect they, or a family member, have whooping cough should stay at home and call a doctor or Alberta's 811 Health Link line.