Northland doctors say Health Ministry figures for one year they have audited under-estimated by half the true burden of acute rheumatic fever in the region.
The results of their study are published in today's issue of the New Zealand Medical Journal.
Rheumatic fever is an auto-immune disease triggered by a Strep A infection, frequently but not always a sore throat. The illness can sometimes progress to the stage where the heart is damaged.
Last year 171 people were hospitalised nationwide for what's known as a first episode of rheumatic fever, a national rate of 3.6 cases per 100,000 people.
For their research, doctors at Northland District Health Board audited local rheumatic fever records from 2012 to 2017 and compared those cases with the ministry's numbers.
One of the team, Kate Wauchop, said in 2017 they found Northland had an extra five cases that weren't included in the ministry's tally of six.
She said the ministry relies on hospitalised cases and information is lacking for people who are diagnosed in primary care and hospital emergency departments.
Listen to an Insight documentary on rheumatic fever's impact on New Zealanders.
NZ's heart-breaker: rheumatic fever rates on the rise
Dr Wauchop said rheumatic fever tends to be regarded as a disease of childhood but their audit found 10 cases in people over 20.
"That was quite an unexpected finding. Seven patients were diagnosed in outpatient clinic and two of those who had that diagnosis who were older than 20 had moderate to severe carditis at the time of diagnosis which is somewhat concerning in that there may have been a delay in their diagnosis."
Dr Wauchop said 6 percent of cases were Pasifika when they made up only 2 percent of Northland's population. When this was combined with the high impact of the disease on the Māori community, it was unacceptable. "And if you look at individual cases it's just so sad to see that this is what's occurring to people's lives and it's from a preventable disease."
Professor of Public Health at Otago University in Wellington Michael Baker agrees.
"We need to identify people who've had the disease so they can have long-term regular penicillin treatment. We are looking at monthly injections of penicillin for maybe 10 years to stop or reduce the chance of those children developing very serious heart damage and that's rheumatic heart disease."
He said a national register of all cases is needed urgently and has been needed for 10 years. He said it was ridiculous that $65 million is being spent on a national prevention programme with no real way of counting the disease cases accurately.
The health ministry wouldn't be interviewed but acknowledged the current method of reporting fails to capture the complete burden of the disease and case numbers that fell between 2013 and 2015 were now rising again.