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Trust confirms reduced immune function in 2009 dolphin deaths

The cause of multiple dolphin deaths in the Swan Canning Riverpark in 2009 has become clearer, with the Swan River Trust confirming the presence of the morbillivirus in two of three dolphins tested.

This is the first time the morbillivirus has been found in Western Australian marine mammals and is only the second confirmed case in marine mammals in Australia.

The tests were carried out in Ireland on the dolphin samples in 2010 following a report on the dolphin deaths by the state’s Chief Scientist Lyn Beazley.

A collaborative agreement between the Swan River Trust and Murdoch University enabled further investigations to be carried out.

Trust Principal Scientist Kerry Trayler said while only two Swan River dolphins tested positive for morbillivirus, the presence of the virus in other dolphins that died cannot be ruled out.

“Two of the dolphins found in Swan River were so badly decomposed that they were not able to be tested, but results from another Swan River dolphin are pending,” Dr Trayler said.

“It is difficult to detect this pathogen in post-mortem analysis.

“Samples sent to Ireland were taken from dolphins that all showed some sign of reduced immune function, which typically occurs when the morbillivirus is present.

“The presence of morbillivirus could explain why there was so much variation in the symptoms that have been observed among the dead dolphins. Some had ulcerative skin disease, one had encephalitis and others had succumbed to pneumonia.

“Urban estuaries are challenging environments in which to live and we cannot rule out morbillivirus acting in concert with a range of environmental stressors.”

The state’s Chief Scientist Lyn Beazley said morbillivirus affects a dolphin’s immune system and its ability to resist other stressors and fight off infection. As a consequence animals may be more vulnerable to infections such as pneumonia and encephalitis.

“It is however reassuring that morbillivirus infection seems not to have recurred given that we have not had another spike in dolphin deaths in the Swan Canning Riverpark,” Prof Beazley said.

“However, should this occur again, mechanisms are now in place to test for the virus in an Australian laboratory and thus provide results more quickly in future.

“Community awareness of dolphins in the rivers has increased with reporting of their activities helping us to better understand the population and threats to them.

“Programs such as the Swan River Trust’s Dolphin Watch are vitally important because they involve the community in monitoring dolphin activities.”

Dr Trayler said while it was not known exactly how the Swan River dolphins contracted the virus, one possibility was through contact with offshore cetacean populations.

“Cetacean morbillivirus has been associated with a number of large-scale mortality events of marine mammals in other parts of the world,” Dr Trayler said.

“The disease is not transmissible to humans.”

Samples taken from Bunbury dolphins were also analysed and tested negative for morbillivirus.