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The Otago Regional Council said the algae had been spotted recently and the signs would likely be in place for the rest of the summer. Exposure to the algae in humans may cause symptoms such as skin rashes, nausea and tingling and numbling around the mouth or tips of fingers.
The council is encouraging dog owners to keep their pets on a lead and out of the water and make sure they do not consume vegetation from the water's edge.
Signs a dog has been poisoned include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions. It is important to treat the first signs of illness as an emergency and contact a vet immediately.