Warning signs going up after three dogs die on Central Otago river walk

Dog owners are on the alert after three dogs died this weekend following a walk along a stretch of river near Alexandra in Central Otago.

This morning the Otago Regional Council has staff at the spot beside the Manuherekia River taking samples and erecting warning signs after being informed of the dead dogs Sunday afternoon.

The regional council's water quality head Helen Trotter says her team is investigating if signs of toxic algae, common during hot weather and potentially deadly to both humans and animals, are evident.

"Environmental monitoring staff are on site this morning to assess the areas affected, collect samples and to install warning signs."

Ms Trotter says although warning signs are already in place a little upstream at Ophir, routine monitoring of the river closer to Alexandra, near Shaky Bridge, and closer to where this weekend's tragic incident took place, had not shown signs of toxic algae so far this season.

She thinks the algae is a possible cause of death for the three pets, although "confirmation it is present at the site is still pending".

Hundreds of people responded to an anonymous social media post made by the devastated dog walker on Sunday morning warning others of the danger.

"After a long night at the vet with three dogs, they have now all passed," the post to an Alexandra community Facebook page says.

"I urge you not to take your dogs to this spot on Galloway Road.

"All were poisoned from something on the bank."

Although the poster had already informed the council of the incident they took to social media to reach other dog owners considering taking their four-legged mates for a river walk that day.

They say they had five dogs with them, but only three started to display signs of being unwell, all within an hour of being at the river.

The dogs appeared lethargic, were swaying, shaken and cold, had rotten smelling diarrhoea and blood-stained urine, and became paralysed from the head down.

Central Otago District Mayor Tim Cadogan was one of the many people to reply to the original post, confirming a local on-call vet had been spoken to.

He says the vet's suspicion was "an algae that is toxic to dogs, which can be present in rivers when it gets hot like it has been lately" was the likely cause of the deaths.

"I will let the ORC know about this in the Manuherekia but word of mouth amongst dog owners to be extremely vigilant after this has happened will be really useful."

Toxic algae discovered in a Queenstown stream last summer, prompting warnings for dog walkers (Image/ORC).

The toxic algae is more likely to be present during summer because of warm weather and low water flows.

In rivers, it generally forms brown or black mats that grow on rocks in the river bed, but can come loose and wash up on the banks or form floating 'rafts'.

While lakes with the algae will have a murky, cloudy appearance, with a green to red scum likely on its surface.

Ms Trotter advises dog owners to keep their pets on a leash "to steer them away from the water" at problem spots.

The regional council monitors select waterways over the summer, beginning this year on Monday, December 4.

It is testing 33 popular swimming sites weekly for bacteria and 11 sites for toxic algae.

Its latest water quality results are posted to lawa.org.nz/swim (Land Air Water Aotearoa), while up-to-date information on toxic algae sightings can be found at www.orc.govt.nz/toxicalgaesightings.

Toxic algae is a common name for toxic cyanobacteria.

Known hotspots for it in Central Otago include Butchers Dam, Cardrona River, and Hāwea River, in addition to the Manuherekia River.

Members of the public are advised to report any potential sighting of cyanobacteria to the ORC via its pollution hotline on 0800 800 033.