Keep yourself, children and dogs out of the lower reaches of the Waihopai River and the river’s popular swimming hole, Environment Southland warns.
ES senior scientist Katie Blakemore said the Waihopai River, between Queens Dr and Mill Rd, was a popular spot for people to walk dogs off the lead.
"Pet owners should take extra care in this area, and all the way along the Waihopai River, as ingesting a small amount can be toxic to dogs."
While toxic algae was commonly identified in waterways in the province during warm periods and low flow times, it was infrequently found in the lower part of the Waihopai River, she said.
It was not safe for people or animals to swim in rivers with toxic algae — wetsuits offered no protection.
Vetco veterinarian Dr Michael Brannigan said two common toxins were in our rivers at this time of the year.
One toxin affected an animal’s neurological system. Symptoms included difficulty breathing, wobbly walking, disorientation, seizures, diarrhoea and vomiting.
"It can progress to collapse and coma very quickly — even within 15 minutes of ingestion. It’s pretty horrendous."
While Vetco had not treated any animals poisoned at the river, the race to save an animal was frequently lost because the toxin’s effect was so swift.
Other potentially lethal infection symptoms, such as depression, lack of appetite and vomiting, could show days later.
Dogs were particularly attracted to the algae’s musty smell and infection and contamination could happen when a dog retrieved a ball from the water, Dr Brannigan said.
People might mistake the dog’s staggering for fatigue, but they needed to quickly get the dog out of the water, he said.
"Even then getting them to the vets within an hour or half an hour might be too late," Dr Brannigan said.
Rivers were not the only risk areas people needed to be aware of at this time of the year — toxic bacteria could also be present in other slow or stagnant water such as bird baths, fountains and ponds.
Prevention was the key to avoiding a tragedy.
The algae had been identified in the river between Queens Dr and Mill Rd, and two months ago near Kennington.
"It’s the natural part of a river... It tends to grow more in warmer temperatures and with stable flow.
"The biggest thing that promotes excessive growth is a lack of flushing flows."
Other rivers infected at present included the Waiau River near Excelsior Creek.
Ms Blakemore said she hoped higher water flows over the weekend would flush the river out.
The dangerous algae was a dark green/brown slime on rocks, or dark brown/black, or dark green mats at the water’s edge and could be found accumulating at the water’s edge.
By Toni McDonald
