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The endangered Maui's dolphin faces extinction from a bacterium which can kill unborn dolphins and destroy the mammals' ability to reproduce.
Brucella was identified in a baby dolphin found dead at the mouth of the Waikato River in November.
Dr Wendi Roe, a Massey University pathologist, said the
dolphin was born alive but did not make it to the surface to take its first breath.
The bacterium was identified by DNA analysis at the Biosecurity New Zealand investigation and diagnostic centre, which has also found evidence of marine strains of brucella in the Hector's dolphin.
"The test results cannot confirm if the dolphin died as a result of brucellosis," Dr Roe said.
"However, brucella does cause this type of problem in animals, and this finding in the Maui's dolphin population is of real concern for the future of the species."
The population of Maui's dolphins, which live in coastal waters from Dargaville to Taranaki, has been reduced to just over 100 by human activity such as commercial and recreational fishing.
Dr Roe said two other Maui's dolphins found dead this summer did not show signs of brucella bacteria.
Further screening would be required to determine the prevalence of the bacteria in the Maui's dolphin population, and its effects.
Department of Conservation spokeswoman Nicola Vallance said the Maui's dolphin was considered to be the rarest marine dolphin.
Ms Vallance said the effect of brucella in marine mammals was unknown but it caused abortions and reproductive failure in farm livestock.
There was some evidence from international examples that might also be the case for dolphins.
Vaccines were available for livestock, but not for marine mammals.
Ms Vallance said Brucella could be transferred from animals to humans. Brucellosis in humans caused fever, headache, arthritis and neurological symptoms, and was treated intensively with antibiotics.
People would become at risk of infection if they had had direct contact with body fluids from a clinically infected animal.
The greatest risk of transmission was to those who worked with infected animals, such as conducting autopsies.
Infection could gain entry through abraded skin, mucous membranes or through inhalation.
WWF-New Zealand yesterday renewed its call for the Government to introduce emergency measures to protect the Maui's dolphin from extinction.
Species At Risk
* The Maui's dolphin used to be called the North Island Hector's dolphin.
* Researchers now say Maui's dolphin has a distinct genetic and skeletal structure, as well as a slightly different body form.
* These differences mean that Maui's and other Hector's dolphins are unlikely to interbreed.
* There are just over 100 Maui's dolphin and it is feared it may be extinct within 25 years.
Source: WWF