The public has been warned to be vigilant after Otago Regional Council staff spotted a dog-attracting algae, which can also be dangerous to humans, in a North Otago river.

ORC said today the algae Phormidium was spotted in the Kauru River at Ewings, west of Oamaru.

Environmental Resource Scientist Hugo Borges said dogs were especially vulnerable to the algae, as they were inclined to eat it after being attracted by its smell.

"While most of the warm and dry conditions that cause these algae blooms are behind us, there is still a risk of encountering Phormidium at certain spots, and it pays to be especially cautious with pets.


"The algal mats can detach from the river bed and accumulate along the water's edge, making them more accessible to dogs. Dogs seem to like the smell of Phormidium, and it can cause severe poisoning and even death if they consume it," Dr Borges said.

People were advised to avoid contact with the water, as exposure to Phormidium could cause symptoms such as skin rashes, nausea, and tingling and numbness around the mouth or tips of fingers.

ORC also advised people learn how to recognise potentially-toxic algae in waterways, and look out for signage indicating water risks.

People who suspected their dogs had ingested the algae were advised to contact a vet immediately.

The most common signs a dog might have consumed toxic algal material were lethargy, muscle tremors, fast breathing, salivation, twitching, paralysis and uncontrolled shaking, convulsions, or frothing at the mouth soon after being near the water.

Phormidium blooms under warm conditions and when river levels are lower, as low water levels are slower to flush it out.

The algae appear in thick, dark brown or black mats on riverbeds, and may have a velvety texture and a musty smell.