Phormidium found in Cardrona River

Phormidium, a common toxic algae, is toxic to both humans and dogs. Photo / Supplied

The presence of the potentially toxic alga Phormidium has been confirmed in the Cardrona River.

Its presence has prompted the Otago Regional Council (ORC) to warn people (and dogs) to avoid contact with waters in the river.

It was found at ORC's Mt Barker sampling site. Signs have been put up along the river today to warn the public of the risks associated with the algae.

ORC environmental resource scientist Rachel Ozanne said high flows during winter had kept the region's rivers relatively free of Phormidium. However, warmer temperatures since then and the resulting stable flows meant Phormidium had had the opportunity to flourish.

Where Phormidium was obvious in a river, people should assume it was unsafe for their animals to swim in, Ms Ozanne said.

In flowing rivers, Phormidium forms thick dark brown or black mats typically found on large rocks, stones, and cobbles.

"It's important that people using waterways are aware of the risks associated with its possible presence, and are alert to this," Ms Ozanne said.

Some cyanobacteria produce toxins that are a possible health risk to humans and animals if eaten, and they can also cause irritation to the skin and eyes.

There have been cases in New Zealand of deaths associated with toxic algae where dogs have eaten the mats formed by Phormidium.

"If the bed of a river is covered in thick dark brown or black mats that have a velvety texture and a musty smell, it is wise to be cautious and avoid that river site," Ms Ozanne said.

Pets should be taken to a vet immediately if they show signs of illness after coming into contact with algal mats. The greatest risk is to dogs, which are more likely to consume the mats in the water.

"The most common signs that a dog might have consumed toxic algal material are lethargy, muscle tremors, fast breathing, salivation, twitching, paralysis and uncontrolled shaking, convulsions, or frothing at the mouth soon after being in the water. Anyone concerned their pet may have consumed toxic algae should contact their vet immediately."