The number of cases of whooping cough associated with an outbreak in southern Alberta has almost quintupled since the first week of June, officials say.

Alberta Health Services declared on June 5 that the southwestern areas of Fort Macleod, County of Lethbridge and Coaldale are the most affected.

Three days later, AHS reported 12 confirmed cases of the illness. The number jumped to 38 just a week later.

On Monday, that number rose again to 58. Vivien Suttorp, Alberta Health Services' lead medical officer of health for the south zone, says the outbreak as now touched into Lethbridge.

"Those numbers are continuing to be on the rise as staff as receiving more and more lab reports," Suttorp said.

Suttorp says in nearly 90 per cent of all the confirmed cases, the patients have not received any immunizations in their lives.

Alberta Health Services' south zone has seen 69 confirmed cases of whooping cough this year — 58 of those cases linked to the current outbreak.

Young children most at risk

Whooping cough, or pertussis, is a bacterial infection of the respiratory tract. Young children are the most at risk and suffer a more severe form of the disease, Suttorp said.

However, the illness can present mildly in adults and older children, who can often be unaware they are infected and spread the disease.

Whooping cough is a difficult outbreak to manage because the symptoms are quite varied and milder cases will usually go unreported, Suttorp said.

Patients infected with whooping cough remain contagious for up to 21 days, Suttorp said. Once infected, it can take up to three weeks for symptoms to appear.

Outbreak could last 6 months

The low immunizations numbers in some part of southern Alberta will likely mean the outbreak could last up to six months, Suttorp said.

"Historically in southern Alberta, when we've had whooping cough outbreaks, we see ongoing transmission over six to 10 months in a community settling with very low immunization rates," she said. "If we get one case in a school for example where almost all of the children are immunized, we would only see one or two cases."

AHS is continuing to follow the current immunization schedule. But with the outbreak, officials have begun immunizing women in their third trimester of pregnancy with the pertussis vaccine to help protect the unborn child.

No deaths of been reported in relation to the outbreak.