Adults do not die of whooping cough, though they can be sick for months. But they can infect young babies, for whom it can be fatal. The only solution may be to vaccinate prospective mothers who can pass on immunity.

"In Canada, a recent study showed there are six times as many cases of whooping cough today as 10 years ago," said Marc Struelens of Erasmus Hospital in Brussels, at an infectious diseases conference in Milan, Italy. Adult cases in the US may have risen 13-fold since 1981.

And in Finland, despite a 97 per cent vaccination rate, cases jumped five-fold between 1995 and 1999. Nearly all were in babies under three months old - too young for vaccination.

Vaccination paradox
 

No one disputes the benefits of vaccination. Whooping cough kills 400,000 children worldwide each year, mainly in poor countries. Before vaccination started in the 1950s, it killed 150 children a year in England and Wales; now Natasha Crowcroft of the UK's Public Health Laboratory Service estimates the average annual toll at nine.

And yet, vaccination may have helped cause the recent resurgence of whooping cough. Neither vaccination nor the disease confer life-long immunity. But when whooping cough was still common among young children, older children and adults encountered the bacteria frequently enough that this acted like a booster vaccine, maintaining immunity.

But as vaccination makes the bacteria scarce, this stops, and immunity among adults wanes. Now, when they do encounter the bacteria, they get sick, and a sick adult is a contagious adult.

Worse still, few adults realise they have whooping cough as their symptoms do not resemble the classic disease of babies. Diagnosis is difficult too as the bacteria are extremely hard to culture. Molecular diagnosis is possible only by using PCR to identify DNA, not a commonly-used procedure.

Inherited immunity
 

Marion Riffelmann of the Institute of Hygiene and Clinical Medicine in Krefeld, Germany used it to test German adults with chronic cough, and reported in Milan that one in 10 had whooping cough. Other studies have found rates as high as one in four.

Frits Mooi of the Dutch National Institute for Public Health and the Environment in Bilthoven, says that whatever is causing the increased cases of whooping cough in adults, those who suffer are babies too young to be safely vaccinated.

"The only way we can protect them now is to vaccinate mothers, so they will pass on their immunity," says Mooi. European public health authorities are considering offering whooping cough vaccination to prospective mothers.

The symposium on "Pertussis: the hidden epidemic" took place at the European Congress of Clinical Microbiology and Infectious Diseases in Milan, Italy.

Debora MacKenzie

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