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A toxic algae is being blamed for the death of two dogs in Mosgiel and has prompted warnings to parents and caregivers.
Otago Regional Council director of environmental information and science Dr John Threlfall said last night he was still awaiting the result of tests.
However, it was probable the dogs had swallowed the algae Phormidium while drinking from the low-flowing Silver Stream.
The black, shiny algae gives off a toxin and Dr Threlfall said it could be harmful to humans.
"Caregivers should supervise young children playing close to the Silver Stream to ensure they don't touch or eat any algal material.
"Dog owners should also keep their pets clear of the river."
Otago medical officer of health Dr Derek Bell said the algae could cause allergic reactions including asthma, eye irritations, rashes, blistering around the mouth and nose and gastro intestinal disorders including abdominal pain, cramps and diarrhoea.
He advised anyone affected to seek medical advice.
Dr Threlfall said it was the first time the algae had been noted in the Silver Stream and council staff would be puting up warning signs today and trying to determine the extent of the problem.
The algae was common in many waterways, including the Kakanui and Waianakarua Rivers which frequently had small patches.
"As long as it's not in places where people swim or dogs frequently get walked it's probably there and nobody knows anything about it."
However, the area at Mosgiel where it had been found was popular with dog-walkers and people with children.