New Zealand Vietnam war veterans, some of whom were exposed to Agent Orange defoliant, have double the rate of chronic lymphatic leukaemia compared to the general population, an Otago University study has found.

The study, by the university's Department of Preventive and Social Medicine, looked at the medical records of many of the nearly 3400 New Zealanders who served in Vietnam from 1962 to 1971.

The cohort study, which looked at the records of 2752 men between 1988 to 2008, is the first to look at New Zealand Vietnam veterans to assess the long-term health effects of serving in a combat zone.

The researchers found while 407 veterans died over the study period, the overall death rate from all causes was 15 per cent lower than the general population.

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They also found mortality from cancer was not significantly lower or higher however than the general population.

However, lead author Dr David McBride said the study showed a doubling of the risk of mortality from cancers of the head and neck, as well as an increase in oral cancers of the pharynx and larynx.

"Lung cancer contributed the greatest burden of deaths in both New Zealand and Australian veterans,'' he said.

The study noted veterans deployed in the Nui Dat area of Phuoc Tuy province experienced a toxic environment because of the widespread use Agent Orange, which contained the carcinogen dioxin. However, the study did not have specific data on herbicide exposure of individual soldiers.

Dr McBride said the findings were not at odds with evidence needed for compensation from Veterans Affairs New Zealand for ill-health caused by service in the Vietnam War.

He said the pattern of lower overall mortality was known as the 'healthy soldier effect' which was related to the fact the soldiers would have been selected for health and fitness.

Further work was still needed, including the selection of a non-deployed comparison group to reduce the `healthy soldier effect'.

The study, funded by the War Pensions Medical Research Trust Fund, is due to appear shortly in the international journal BMJ Open.