Iowa has recorded

92 cases of pertussis, which also is called whooping cough, so far in 2012, an

83 percent increase over the state’s five-year average, public health officials say.

Most of the increase has come from eastern Iowa, including Scott and Linn counties. In Scott County, an outbreak of the disease continues, with 36 cases reported in March through April 19, Edward Rivers, director of the Scott County Health Department, said during a meeting this week with the county health board.

The cases have mostly been in the adolescent population, Rivers reported, but other age groups have reported the illness in recent days.

Public health officials urge vaccination as a way to prevent the incidence of contagious diseases such as pertussis. Adolescents in Iowa are not required to have a pertussis booster shot, also called TDaP. However, that is a requirement in most other states, including Illinois.

“Parents need to see that adolescents get the age-appropriate boosters,” said Dr. Louis Katz, medical director of the Scott County Health Department. He added that the revised national recommendations are for pre-teens, 11-12 years old, to have TDaP boosters.

Older children in Illinois will need TDaP boosters beginning this summer.

“In Illinois, for next school year, all sixth- and ninth-grade students will have their physicals, which are required, and also need to show proof of a TDaP booster,” said Melaney Arnold, a spokeswoman for the Illinois Department of Public Health.

Illinois’ higher rate of immunization against pertussis might explain fewer incidences of the disease in Rock Island County. There were only four reported cases of pertussis from Dec. 1 through March 31, Joe Williams of the Rock Island County Health Department said.

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The Iowa Department of Public Health has begun to discuss a requirement that pre-teens have the booster. The regulation, which is expected to be enacted no sooner than August 2013, would put Iowa in line with 40 states that already have the requirement. That includes all neighboring states, except South Dakota.

Pertussis is a cyclical infectious disease that appears to be in an “up” cycle.

“Numbers will go up for a couple of years and then down,” Arnold explained.

A disease such as pertussis arrives in annual cycles because of how immunity works in a population, said Dr. Patricia Quinlisk, the medical director of the Iowa Department of Public Health. “Herd” immunity builds up for a few years, but then it begins to wane over time. More cases are recorded when the disease occurs again.

That happens now with pertussis as well as shigella (a bacterial infection that causes diarrhea in humans), Quinlisk said. It used to happen with mumps and measles before vaccines for those diseases were perfected and distributed widely enough to build up effective herd immunity for years at a time.

In 2010, Illinois reported 1,057 pertussis cases, up from 588 in 2006. Also in 2010, California had one of the worst outbreaks of the disease seen in the United States in a long time, with 10 babies dying from pertussis. Nationally, more than 21,000 cases of the disease were reported in 2010.