Health experts are baffled by New Zealand's accelerating rates of legionellosis - the disease commonly known as legionnaires' disease.
A recently released report - prepared for the Health Ministry - revealed a 141 per cent rise in the number of cases of the illness reported between 2009 and 2010.
Historically the pneumonia-like infection has been caught by inhaling potting mix or exposure to contamined water in plumbing systems, air conditioning units or cooling towers.
Of 178 reported cases of the disease in 2010 there were five related deaths and 136 hospitalisations.
The disease has continued to develop with 89 cases reported during the first six months of this year. At the same time last year 63 cases had been reported.
Symptoms included muscle aches, headache, tiredness, chills, shortness of breath, loss of appetite, coughing and stomach pain.
Public health physician and University of Otago associate professor Michael Baker said he was surprised by the report's findings.
"I'm a bit perplexed," he said. "There certainly is a big jump in 2010."
"It appears to be accelerating in the first six months of this year and requires more investigation."
Smokers and the elderly have a higher chance of contracting the disease but the data did not strongly support this pattern, Dr Baker said.
The report was prepared by the Institute of Environmental Science and Research - the principal science advisor to the ministry.
Institute communicable disease scientist David Harte said he could not pinpoint any clear rationale for the rise, but a possibility could be global warming.
"We don't know the reason for the increase. We are still trying to work that one out.
"[It may be] simply due to warmer weather - or other seasonal factors we don't understand,
"Warm, humid weather will allow legionellosis to multiply and survive but we haven't got any hard evidence that this results in an increased incidence of the disease."
The disease could not spread between people.
Patients became "accidental hosts" after being exposed to a contaminated environmental source, commonly compost or cooling towers, Mr Harte said.
The Public Health Service recorded that 80 per cent of patients in 2010 had previously come in contact with compost or potting mix.
Although compost and potting mix were the most recognised source of the disease, tests on the patients' leftover compost sometimes showed no sign of the guilty bacteria, Mr Harte said.
"Occasionally when we test the [patient's] compost we don't find anything there, which suggests another environmental source."
While compost and potting mix posed a real risk and health warnings on the products were not mandatory.
Many suppliers labelled their product but were often vague about the associated dangers, Mr Harte said.
The simpliest way for gardeners to protect themselves was to wear a mask.
"Some labelling is a bit misleading. No one wants to sell a product that says it is a health hazard - that's not a good marketing ploy.
"The common transmission route for the disease is breathing in dust or water aerosols contaminated with the bacteria. A face mask is a simple way to prevent this from happening."
The ministry said revised guidelines for controlling legionellosis would be released this year.
Gardeners would be alerted of the dangers of using potting mix, via media statements, following outbreaks, the ministry said.
Canterbury was worst hit by the disease in 2010 with 58 reported cases but it was not an outbreak but a lower "cluster" classification, Mr Harte said.
Although the disease was soaring, the death count was not at record highs, and Mr Harte recommended seeing a doctor if you suspected you were hosting the disease.
"There have been years where we've had seven or eight deaths. The death rates are usually higher if there has been no medical intervention. We get people who get sick and don't go to the doctor."
- NZPA