Its presence has prompted the Otago Regional Council (ORC) to warn dog owners to be vigilant when exercising their pets alongside rivers and streams across Otago and stop them from eating any scavenged material.
The warning comes as warm weather post-Christmas causes river levels in Otago to drop. Phormidium occurs naturally in waterways, but can become a problem in summer when it forms thick mats on river beds, especially during low flows.
Otago Regional Council science manager Dr Dean Olsen said high river flows before Christmas kept the region’s rivers relatively free of Phormidium. However, warmer temperatures since Christmas and the resulting stable flows enabled it to flourish, certainly in the Cardrona and possibly in other historic trouble spots such as the Lindis, Manuherikia, Taieri (Silverstream), Kakanui and Waianakarua rivers.
"It is important that people using waterways are aware of the risks associated with its possible presence, and are alert to this wherever they are in Otago," Dr Olsen said.
Some cyanobacteria produced toxins that were a possible health risk to humans and animals if eaten, and they could also cause irritation to the skin and eyes. Phormidium mats could indicate other cyanobacteria posing a risk to human health might also be present. People were advised to avoid contact with waterways where the mats were present.
"If anyone becomes unwell after contact with affected waterways, they are advised to see their doctor."
There have been cases in New Zealand of dogs dying after eating the mats formed by Phormidium, having been attracted by their deep earthy odour.
Dr Olsen said mats could detach from the river bed and accumulate along the water’s edge, where they were readily accessible to dogs.
"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."
Pets should be taken to a vet immediately if they showed signs of illness after coming into contact with algal mats. The greatest risk was to dogs, which were 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."
