A startling spike in the number of whooping cough cases, in Minnesota and across the nation, has prompted state officials to urge that all Minnesotans consider getting booster shots for the disease.

Nearly 400 cases have been reported in Minnesota since June 30, pushing the year’s total to 2,160 and making 2012 the worst year for pertussis since vaccines came into wide use in the 1950s.

Nine children have died of the respiratory disease this year nationwide, though no deaths have been reported in Minnesota, state health officials said on Thursday.

Pregnant women and people around infants should be vaccinated as soon as possible because infants are most at risk from the disease, said Claudia Miller, an epidemiologist at the Minnesota Department of Health.

“We’re not sure exactly what’s going on, but with the cases so far, we’re seeing the highest numbers since the 1940s and ’50s,” she said. “And we’re only halfway through the year.”

Nationally, nearly 18,000 cases have been reported — twice last year’s total and at a pace to be the highest since 40,000 cases in 1959, the U.S. Centers for Disease Control and Prevention said on Thursday.

Beside Minnesota, major outbreaks have shown up in Wisconsin, Washington, New York, Kansas and Arizona.

In the past, whooping cough season generally came in late summer and fall, but it has become essentially year-round in recent years, Miller said.

Whooping cough moves in cycles, with Minnesota cases typically ranging between 200 and 1,500 cases a year.

This year, state officials recorded 600 cases by early May — the same total as for all of 2011. That soared to nearly 1,760 by June 30 and 2,160 now. Since the 1950s, the record was 1,571 in 2005.
The disease is spread via droplets coughed by infected people and often begins with coldlike symptoms. It usually can be treated effectively with antibiotics.

Infants, however, may not cough and instead suffer episodes of apnea and difficulty breathing — “very alarming to parents,” Miller said.

Mystery increase

Whooping cough, or pertussis, has been on the rise nationally since the 1990s, though total cases still remain far below the hundreds of thousands of cases that were common in years before the 1940s.

Federal health officials recommend that children get vaccinated in five doses, with the first shot at 2 months and the final between ages 4 and 6. A booster shot is recommend at about age 11.

With vaccination rates still fairly high, health investigators are trying to figure out what’s causing the increase. Theories include better detection and reporting of cases and some sort of change in the bacteria causing the illness. But one theory holds that the vaccine’s effectiveness is wearing off sooner than expected.

The vaccine that was given to children for decades was replaced in the late 1990s following concerns about rashes, fevers and other side effects. While the new version is considered safer, it may not be as effective for the long term, Miller said.

“We’re seeing more cases here and nationally among vaccinated older elementary school children who ought to still be protected,” she said.

The Associated Press contributed to this report.

Warren Wolfe • 612-673-7253