By Jenifer Joseph
ABCNEWS.com
March 10 — Pediatric nurse Susie Glenn and her teenage daughter were literally sick to their stomachs with a terrible cough that persisted for three months.
     They coughed so violently that they'd choke and vomit, a situation that left them both exhausted. Doctors first diagnosed their ills as bronchitis, then pneumonia. But none of the medications they prescribed for those ailments did much good.
     After three months of coughing hell, Glenn finally returned to work, at the Kaiser Permanente Vaccine Study Center. The center's director, Dr. Henry Shinefield, overheard one of her hacking fits and diagnosed it immediately: whooping cough.

    Most people no longer worry about this bacterial infection—also known as pertussis—a once-deadly childhood disease that was virtually wiped out by vaccines. (Pertussis is the "p" in DPT shots.)
    In the 1940s, 200,000 Americans, mostly children under 10 years old, came down with whooping cough every year. By 1980, widespread vaccinations had pushed the numbers down to about 1,000 cases.
    But researchers at the infectious diseases conference now under way in Atlanta are reporting an alarming trend: the re-emergence of this age-old disease. As of 1996, some 7,800 U.S. cases were diagnosed, says Centers for Disease Control epidemiologist Dr. Dalya Guris, and that's just the tip of the iceberg.

Mistaken Identity
In an effort to see just how common pertussis is, Guris and others have been studying people with coughs lasting longer than two weeks. Their research shows that as many as 90 percent of all whooping cough illnesses go undetected. In other words, up to 500,000 Americans could have the infection right now, but think it's just a bad cold.
    "This is an enormous incidence rate that's unrecognized," says Kaiser Permanente's Shinefield, whose own study in 1996 had similar results. "There are more cases now than there were in 1945, before we had the vaccine."
    Part of the problem is that whooping cough in adults isn't usually accompanied by the characteristic "whoop" that you hear at the end of the cough in children. The only symptom in teens and adults may be a cough that simply won't go away.
    Whooping cough is easily treated with antibiotics. But all too often, doctors send patients home to wait out their "cold." That can pose a serious danger to infants, who are hit hardest by whooping cough because of their fragile immune systems.

Vaccine Wear and Tear
The reasons for the upswing in adult pertussis cases are unclear. It may be due in part to the waning effectiveness of the vaccine as people grow older. Ninety-five percent of all 2-year-olds in the United States have been vaccinated against whooping cough.
    But researchers think that, unlike vaccines for measles and mumps, the pertussis immunizations may be effective for only seven years. And since booster shots aren't approved for use in anyone older than 7, it's no wonder that adults may be more susceptible to the disease.
    That still doesn't fully explain why the numbers jumped so high, so quickly. If weakening protection from vaccine were the main reason, researchers say, such an upswing would have been evident decades ago.
    The Food and Drug Administration so far has remained reluctant to approve an adult booster until more research is done.
    So for now, Guris says, it falls on the doctors' shoulders to look more carefully at patients with long-lasting coughs.