TOPEKA, Kan. (AP) -- Health officials say Kansas and other parts of the United States are experiencing a cyclical peak in the number of cases of whooping cough.

Since the beginning of the year, the Kansas Department of Health and Environment has recorded 56 confirmed cases of whooping cough. The respiratory illness is marked by severe and frequent coughing, especially among young children.

An epidemiologist from the Centers for Disease Control and Prevention in Atlanta said the United States is in the midst of another peak outbreak of the illness. The most pronounced outbreaks are in Washington state, where there have been more than 2,300 cases reported as of June 9. There were just 171 cases reported in the state through the same period in 2011.

Stacey Martin, a CDC epidemiologist, said the cycles typically last two to five years at a time, but that the intervals between the outbreaks appear to be shrinking. Part of the explanation is the change in the type of vaccine given to children in the 1990s, which has a shorter period of effectiveness.

Martin said that vaccine has fewer adverse side effects and reactions than the older one, but the downside is that it may not last as long. Some of the new cases are in children who are at or approaching adolescence, she said.

"It emphasizes the need for the booster for 11 to 12 year olds," she said.

Health officials say the best prevention is to make sure young children are immunized between the age of 2 months and 6 years, with boosters during adolescence.

Charles Hunt, state epidemiologist for the Kansas Department of Health and Environment, said there was no pattern to the whooping cough cases in the state. Eight counties are reporting cases and there have been eight widespread outbreaks since 2001.

"It's probably consistent with what's going on in most other states," Hunt said. "Probably 80 percent of the cases are in children under age 15."

Hunt said the department's main focus has been communicating the dangers of the spread of whooping cough and the importance of vaccinations.

Dr. Gianfranco Pezzino, head of the Shawnee County Health Department, said one factor for fewer cases in Kansas -- compared to Washington -- was the state's policy that limits the ability of parents to opt-out of immunizations.

The higher vaccination rates and lower number of school immunization exemptions means more of the population is protected from the illness and its spread is limited, he said. However, Pezzino said the outbreak is a good learning tool for health officials and parents.

"The silver lining is this can be a wake-up call," he said. "You have to remind parents over and over that these illnesses aren't gone" despite available vaccines.

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Online:

Centers for Disease Control and Prevention: http://www.cdc.gov

Kansas Department of Health and Environment: http://www.kdheks.gov