Rates of rheumatic fever may be 18 percent higher than official figures show, according to new research by the University of Otago.
Rheumatic fever is a rare autoimmune reaction to a streptococcal infection that results in fever, joint pain and heart valve problems about three weeks after a streptococcal infection.
To be diagnosed with rheumatic fever in New Zealand, people must have a level of streptococcal infection that is much higher than overseas requirements.
The research paper, published in The Pediatric Infectious Disease Journal, found that if New Zealand used the Australian criteria for diagnosis, cases would increase by up to 18 percent, or 16 cases a year.
Lead author Professor Susan Jack said there were 20 to 40 cases of rheumatic heart disease a year in patients with no previously documented rheumatic fever.
"Once people have established rheumatic heart disease that means that their heart valves are damaged and that can then lead on to more serious heart failure and needing heart valve replacements with all the dangers of that kind of operation," Dr Jack said.
"So part of rheumatic fever management is picking up cases as early as possible and then giving them these penicillin injections to prevent them getting another episode.
"If they didn't have that (treatment), it could lead on to the more serious sequelae of rheumatic heart disease."
Dr Jack wants New Zealand to lower its threshold for making a rheumatic fever diagnosis.
"These cut offs for New Zealand were set a couple of decades ago and have never been updated.
"Now we have better evidence, it's time that New Zealand looked at reviewing its guidelines."
The Ministry of Health ran a rheumatic fever prevention programme from 2012 to June last year, investing $65 million to reduce the disease by two-thirds.
The ministry is still working closely with 11 district health boards where there is a high incidence of rheumatic fever.
But Dr Jack said the number of cases has still gone up since 2015.
"I would like a focus to remain in place to address rheumatic fever," she said.