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Increasing numbers of adults are falling ill with whooping cough as an epidemic sweeps the country.
More than 5900 confirmed, probable and suspected cases of the disease were reported last year - more than double the 2014 reported in 2011.
And figures from the Institute of Environmental Science and Research's latest report show that notifications also more than doubled between 2010 and 2011.
Of the 309 people treated in hospital for the illness last year, two died, and 182 were aged less than a year old.
But the ESR report shows that the rate of hospitalisation for people in their 30s through to their late 60s increased markedly last year compared with 2011.
Dr Cameron Grant, associate professor of paediatrics at Auckland University, said the country was in the grip of an epidemic, which typically occurs every 2-5 years.
"Certainly it has been the experience the world over of increasing numbers of cases reported in adults.
"They have always been able to get the disease but for a long time our attention was on the little babies.
"But I think people have their radars out there more. I think our ability to confirm possibly whooping cough with a laboratory diagnosis has got a lot better in the last five to 10 years," Professor Grant said.
Whooping cough usually starts like a cold, with tiredness and sometimes a mild fever common.
Coughing then develops, usually in bouts, followed by a deep gasp, or "whoop"
In adults, the painful symptoms last from two weeks to a few months and are highly contagious - for every instance of whooping cough, there are at least 15 secondary cases.
Vaccination is recommended to start at 6 weeks of age, with other injections at 3 months, 5 months, 4 and 11.
But it eventually wears off and adults are being advised to get a booster vaccine costing about $30.
"Protection from vaccines is not life long and that is why we have added boosters to the schedule, and now that we have a [a cellular vaccine], we are able to give booster doses to older age groups," said Professor Grant.
"We are particularly looking at adult groups who are going to be in contact with little babies."
Despite the increasing prevalence of the illness among adults, infants remain the most vulnerable group. About 70 per cent of babies catch the disease from their parents or other close family members.
Professor Grant said the infection was unusual in that not much protection was provided in the womb or via breast milk, so newborns were particularly vulnerable.
Whooping cough kills one baby a year in intensive care, and an infant admitted there with it has a one-in-six chance of developing long-term lung or brain damage.
"It's a very disabling illness in any age group. The cough is always much worse at night time and comes on in long bursts, and so you have a lot of disrupted sleep.
"There are adults who have cracked ribs with the cough and have developed hernias ... Particularly with the elderly it can be awful."
A distressing illness
What is whooping cough (pertussis)?
A contagious bacterial infection of the respiratory tract that causes uncontrollable coughing and difficulty breathing.
What causes it?
The bacteria Bordetella pertussis or Bordetella parapertussis
Who does it affect?
Individuals of all ages, but it particularly strikes babies and infants and can cause disability or death if left untreated.
What complications can it produce?
Dehydration, pneumonia, seizures, long-term brain and lung damage, and in the most serious cases, death.
2011: 2014 cases
2012: 5938 cases
- James Ihaka of the NZ Herald