Finding the dog killer

Paul McNabb of the Cawthron Institute in Nelson explains how they got involved with the dog deaths on Auckland beaches. He takes us on the journey the scientists went through to identify what killed the dogs.

Transcript

PAUL MCNABB
Cawthron became involved with the dog deaths in 2009 through the media, so we heard about the story just like everyone else. I decided that I would send one of my colleagues up to look at it, so Andy Selwood went, and he collected anything on the beach that he thought a dog might be interested in.

We collected samples from Narrow Neck and Cheltenham Beach, and they brought those samples into the lab on Monday and then we extracted those samples ready for mouse bioassay. That was our first test was to see whether any of the samples were toxic to our laboratory mice.

We got around about 50 samples of everything ranging from scraping the top of sand which had a green algal growing on it, other algal material from drains and things that were running onto the beach, seaweed, starfish and a dead fish. There was all sorts of things, and the criteria was anything a dog might be interested in, and obviously that included a sea slug also.

The samples we picked up on the beach we made extracts from very simply with methanol, and we sent those away for mouse bioassay. We were trying to reduce the numbers of mice that we use in the lab by saying which samples we thought were most likely to be of interest to dogs and potentially toxic. So it was the next day that we received results back from the mouse laboratory, which was that sample number 1 had caused virtually no symptoms and sample number 2 had caused some mild agitation, sample number 3 had caused almost no symptoms and so on until sample number 9, and sample number 9 they had written ‘death in seconds’. So we immediately phoned them up and asked them if they could tell us a bit more about sample number 9, to which they replied that it was a very interesting sample and they had diluted it 10 000 fold and it was still killing mice. So we immediately knew that this was the sample that was most likely to be the cause of the dog deaths, and we then took that sample to a biologist at Cawthron to have it identified, and he identified it as the grey side-gilled sea slug.

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