Toxic algae has killed a dog in Masterton and is overwhelming stretches of the Ruamahanga River amid heightened warnings of its potentially deadly spread.

Toxic algae - or cyanobacteria - forms brown or black clumps on river beds and can kill livestock and dogs. The clumps can peel off and often wash up on the riverbank.

Toxic algae can in humans cause vomiting, diarrhoea, skin irritations and other allergy-type symptoms.

Masterton veterinarian John McLaren said yesterday he had unsuccessfully treated a pig dog that had ingested the cyanobacteria while drinking from the Kopuaranga River about three weeks ago.


The owner had brought the animal to his surgery the day after the toxins were ingested, Mr McLaren said, but by that stage its liver and kidneys had been irreparably damaged. "It appeared okay at first but it went down over about three days. They go downhill very quickly.

"It may have had a better chance if it was brought in earlier but the toxins had already done their damage. All we could do was put the dog on a drip and give nursing drugs."

Mr McLaren said it was vital dog owners brought in their pets immediately if they suspected the animal had ingested toxic algae.

Jennifer Taylor, South Wairarapa Veterinary Services practice manager, agreed that treatment must be as quick as possible to give the animal a fighting chance.

Greater Wellington Regional Council spokesman Jim Flack said the spread of toxic algae and its associated risks had been upgraded from moderate to high at Kokotau Bridge on the Ruamahanga River this week.

"Toxic algae covers more than half of the river bed at Greater Wellington Regional Council's monitoring site at Kokotau and is washing up at the water's edge, where it is a health risk to people, dogs and stock," he said.

The Waipoua River in Masterton remains a high risk to users.

The regional council, Carterton District Council and Wairarapa Public Health, which is part of Regional Public Health, are urging river users to protect themselves and their dogs from toxic algae and warning signs have been put up along key access points to the river.


GWRC senior environmental scientist Summer Greenfield said the warm weather is a key factor behind the spread of toxic algae in the region.

She said the algae can be abundant in New Zealand waterways during summer and the risk to river users and dogs is likely to remain until significant rainfall flushes the rivers clear.