The Otago Regional Council is warning the public, particularly dog owners, to beware of toxic algal blooms in rivers and lakes over the summer.
Director of environmental information and science John Threlfall said yesterday toxic algae could occur in relatively clean water and he identified phormidium as one of the most common.
Phormidium forms a thick, brown-black, slimy algae mat on large rocks, stones, and cobbles. It can detach in mats and get caught up in river and lake debris.
"When the phormidium mats die and dry out, they become light brown or white. Either fresh or dried, it can be highly toxic and harmful to humans and animals."
Dogs were particularly susceptible.
"Many dogs enjoy being in the water and can eat these algae intentionally or by accident, which can have tragic results," Dr Threlfall said.
Symptoms include lethargy, muscle tremors, fast breathing, twitching, paralysis, and convulsions.
"In extreme cases, death can occur within 30 minutes after signs first appear."
Toxic algal blooms were commonly blue-green in lakes or still water, but could also be red or yellow. They did not occur regularly in Otago lakes.
They typically formed thick scums on the surface, and might also develop foam at the water's edge. They could cause, in people, allergic reactions such as asthma, eye irritation, rashes, blistering around the mouth and nose, and gastrointestinal disorders, including abdominal pain, cramps and diarrhoea.
Dr Threlfall said reaction depended on the type of toxic algae, the type of toxins present, and the concentration of the toxin in the water.
Where humans became ill after swallowing water containing algal bloom, medical advice should be sought immediately, Dr Threlfall said, and a veterinarian should be contacted in cases where animals may have been affected.
