WEDNESDAY, Aug. 4 (HealthDay News) -- Amidst the largest outbreak of whooping cough in decades, public health officials in California are urging residents, particularly pregnant women and those who come into contact with infants, to make sure they're immunized for the highly contagious disease.

With the incidence of whooping cough also higher than last year in Michigan, South Carolina, Ohio and upstate New York, there's growing concern whooping cough will continue to spread, said Jennifer Liang, an epidemiologist with the U.S. Centers for Disease Control and Prevention. Whooping cough, or pertussis, is named for the wheezing sound, or "whoop," sufferers make when they try to breathe during a coughing fit.

"Pertussis is a cyclic disease, and we do see peaks every three to five years," Liang said. "The last peak was 2005, when we had 25,000 reported cases nationally, and we may be on the upswing of another cycle."

So, should you be worried? In adults, whooping cough can cause a barking cough that lasts for weeks, but it's treatable with antibiotics and rarely life-threatening, said Jeff Dimond, a CDC spokesman.

But in infants too young to be immunized, whooping cough can be deadly. Last week, the seventh California baby died in what public health officials are called the largest outbreak in 50 years.

About two-thirds of infants who get pertussis will be hospitalized, according to the CDC. About one in 10 children who are infected develop pneumonia, while in one in 250 get the disease that affects the brain, called encephalopathy.

In the first half of the year, California has seen nearly 1,500 reports of pertussis. That compares to a little more than 300 cases in the first part of 2009, according to the California Department of Public Health.

As for the smaller outbreaks in other states, Liang cautioned that reporting by local and state public health departments varies widely, so the CDC's national surveillance data can lag. Local health departments tend to be more attuned to localized outbreaks, Liang said.

"The magnitude of the increase in California is concerning, and that's why we are trying to emphasize the importance of vaccination," Liang said.

Vaccination guidelines call for children to receive doses of pertussis vaccine at 2 months, 4 month, 6 months, between 15 and 18 months and then again at school age, between 4 to 6 years. Children should also receive a booster between the ages of 11 and 12.

Previous recommendations called for women to get vaccinated right before or right after pregnancy to protect infants. But in the wake of the epidemic, California public health officials are now advising pregnant women to get vaccinated.

Since 2005, the CDC has also recommended adolescents and adults up to age 64 receive a one-time booster shot because immunity from childhood pertussis immunization wanes over time, said Christina Chambers, an epidemiologist and a professor of pediatrics at the University of California, San Diego.

The shot itself, called Tdap, typically also includes a tetanus and diphtheria booster.

Yet few adolescents and adults are following expert recommendations, Liang said. A 2008 CDC study found about 40 percent of adolescents aged 11 and 12 had gotten the booster, while only 6 percent of adults had.

Though there little data explicitly looking at whether the vaccine is safe for pregnant women, there is no evidence that getting the vaccine can bring on the disease or carries any other risks. The pertussis vaccine is made up of an inactivated, or "killed," virus, Chambers explained.

"There is no data to suggest [that] giving the pertussis vaccine during pregnancy will cause harm," said Chambers, director of the California Teratogen Information Service (CTIS) Pregnancy Health Information Program, which provides counseling for pregnant women on the safety of exposures to medications and vaccines.

Chambers added: "Though it's not a 100 percent guarantee, that's reassuring. And there is definitely concern about the mother getting the disease and the infant not being protected."

Adults who will be around infants and other family members should also make sure they get vaccinated or booster shots, Dimond said. "You make a cocoon around an infant so people coming into contact with the infant don't spread it," Dimond said.

Peak season for whooping cough is late summer and early fall. The disease usually begins with symptoms similar to those of a common cold, such as runny nose, congestion and a mild fever, and then progresses to a dry, hacking cough and prolonged, violent coughing spells that can cause vomiting.

More information

The CDC has more on the pertussis vaccine.