Otago Regional Council (ORC) staff have identified the extent of the toxic algae contamination in the Silver Stream, Mosgiel. The contamination is limited to the stretch of water between the confluence with the Taieri River and Three Mile Hill Road.

ORC director environmental information and science Dr John Threlfall said the Cawthron Institute has positively identified the algal samples collected yesterday from the Silver Stream. The algae causing the death of two dogs is likely to be a Cyanobacteria called Phormidium. In watercourses such as the Silver Stream most cyanobacterial species occur in low concentrations, however the recent environmental and hydrological conditions (high temperatures and low flows) have been favourable for the Phormidium cells to multiply and form an algal bloom.

Some cyanobacterial species including Phormidium produce natural toxins (cyanotoxins). Dogs are particularly susceptible to poisoning from these toxins as they enjoy being in the water and can consume the algae by accident, particularly as most of the algae in this instance is attached to sticks.

The onset of toxin poisoning in dogs is extremely abrupt and death often occurs within 30 minutes of symptoms appearing. For this reason the public is urged to avoid using the Silver Stream until the next fresh (period of sustained rainfall), when the higher flows will dislodge the algae, Dr Threlfall said.