New Zealand's rate of giardiasis, an infectious disease, is more than five times higher than for Britain and nearly four times the rate in the United States.

Researchers, who published their findings in the New Zealand Medical Journal, pointed to soiled babies' nappies and New Zealanders' love of the outdoors as possible reasons for the differences.

Giardiasis is the country's third most commonly notified communicable disease.

At 55.68 cases for every 100,000 people in 1996-97, New Zealand's rate of infection is thought to be one of the highest among developed countries.

Britain's rate per 100,000 in that period was 10.22 and in the US 15.39.

"We're an outdoor country. We go into the outdoors and drink the water and go swimming," said researcher Dr Virginia Hope, a public health physician with the Auckland District Health Board.

Viable giardia cysts had even been found in seawater, she said, although she was unsure how widespread they were at beaches.

"We're in contact with our environment a lot and we know giardia is pretty widespread in the environment.

"It's conceivable that that's a national behaviour and way of living that might be systematically different from other countries."

It was also conceivable that New Zealanders had different ways of dealing with soiled nappies, she said.

Dr Hope and colleagues last year reported in the British medical journal Lancet on their New Zealand study finding that people who changed babies' nappies were at a four-fold increased risk of giardia infection.

The giardia parasite, which can live in the gut of humans or animals, can be spread from faecal contamination of food or water or by person- to-person contact. Symptoms of infection, which can be treated with drugs, include violent diarrhoea and intestinal cramps.

Infections usually peak in late summer and early autumn.

In the latest study, researchers looked at Auckland cases from 1996 to 2000 and found that infection rates were mostly higher in rural areas.

"We know that we get a reasonable number of cases outside the metropolitan mains-water areas," Dr Hope said.

"So we suspect it's something about that environment that does expose people to greater risk.

"Whether that's just that it's spread within families and it's person to person, or whether it's in water supplies and we just can't find it there, we're not sure.

"The most obvious factor is that they are not always using reticulated water supplies so that would be worthy of further investigation."

Groundwater bores and stream-fed domestic supplies could be contaminated from septic tank runoff or by animals, and roof-water users might risk infection from birds and possums.

"None of those methods of supplying water are ones we would recommend without treating, boiling or filtering the water."