Whooping cough makes a comeback

THE CHILD had been coughing for weeks. Mum described a recent worsening, and at times her three-year-old had difficulty catching his breath. No, it hadn’t ever had a true seal-like whooping quality, but the cough continued through the day. And yes, there had been episodes of vomiting after a prolonged coughing fit.

Might it be whooping cough? It might, even though the child was up-to-date with his vaccinations. It’s a growing phenomenon: whooping cough is on the move again, even in people who are fully vaccinated against it.

It is caused by infection with the bacteria Bordatella pertussis. It is spread by coughing and sneezing, and is highly infectious. The bug is sensitive to the antibiotic erythromycin, but treatment within seven days of the onset of symptoms is required if a prolonged cough is to be avoided.

In the US, pertussis had been all but eliminated in the mid-1970s following the introduction of the three-in-one vaccine against diphtheria, pertussis and tetanus (DTP) some decades earlier. Whooping cough had gone from being a feared child killer in the 1920s and 1930s to a rare clinical phenomenon. But the state of California has declared a pertussis epidemic, with more than 2,700 cases so far this year, a seven-fold increase from a year earlier. Other states have had less dramatic increases in the infectious disease.

Last week's news of an outbreak of whooping cough in the Republic is a wake-up call for the authorities here. A paper published in the scientific journal Eurosurveillancegives details of a serious outbreak of whooping cough in Co Leitrim earlier this year. A preliminary investigation identified some 67 possible cases in the Ballinamore area between January and June. On average, about 80 cases of whooping cough are notified annually in the entire State, underlying the unprecedented scale of the outbreak in the northwest. The highest attack rates in the outbreak were in children under the age of 12 months and those aged three and under. But cases among adolescents and young adults were also noted, suggesting that waning immunity against the infectious disease could be a factor.

It is something the authors of the report commented on: “In the 1990s and 2000s, findings from various pertussis surveillance systems demonstrated a change in the age profile of pertussis cases in countries with high vaccine coverage rates in young children. A shift in the age group has been observed, with increasing pertussis incidence among adolescents and adults.”

They also make the interesting point that having an effective immunisation programme reduces the amount of Bordatella pertussiscirculating in the population. In turn, this will result in less natural boosting of immunity among those whose immunity is waning.

Best estimates for the duration of immunity to whooping cough are between seven and 20 years after infection, and between four and 12 years after vaccination. But the duration of protection after the first three doses remains unclear. Indeed, a recent meeting of a WHO group of immunisation experts called for the introduction of a pertussis booster vaccine in the second year of life.

The case definition for pertussis is a cough lasting for at least two weeks, in addition to one of the following: a “whoop” when breathing in; spasms of coughing; or vomiting after prolonged coughing. But symptoms vary according to age, something noted by the doctors investigating the Leitrim outbreak, with adults often lacking the whoop and the vomiting seen in children. This means adults may spread the infection to younger children before they are themselves diagnosed. The result is significant community transmission leading to a wider outbreak of the disease.

I didn’t think it would help mum to tell her that pertussis is sometimes known as the “100-day cough”. The bacterium releases toxins that inflame the lungs, and it takes some time for the inflammation to heal. But by the time her son reaches his early teens, we should have implemented a policy of giving booster shots to those aged 11 to 14.

mhouston@irishtimes.com