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Officials advise people and pets not to swim in the area because ingesting the water can cause serious illness.

PMG FILE PHOTO - The Willamette River from Cathedral Park in Portland.A toxic algal bloom in the Willamette River near North Portland's Cathedral Park has prompted state health officials to advise people and pets not to swim in the area.

Blue-green algae, called cyanobacteria, which produce harmful cyanotoxins, were detected in the water above levels safe for human exposure, the Oregon Health Authority said in a health advisory Wednesday, Aug. 17.

Poisoning from digesting the cyanotoxins can cause acute illness, which can sometimes be fatal.

The advisory is limited to the Cathedral Park area based on the available data, officials said.

People should avoid swimming and high-speed water activities such as water skiing or power boating in the area because the risk of ingesting water where cyanobacteria are present is higher. Boating is safe if speeds are low enough not to spray water.

Portland Parks & Recreation has posted signage at the park warning people of the danger, said Erica Heartquist, spokesperson for OHA, in an email.

Scientists predict human-caused climate change will make toxic algal blooms more common, according to the U.S. Environmental Protection Agency. Blooms develop in slow-moving, warm water of rivers and lakes. More frequent hot summer days means waterways are more likely to warm and maintain favorable conditions for toxic algae.

Health advisories related to toxic algal blooms have been issued in areas of the lower Willamette River five of the last eight years, including last year, Heartquist said.

Temperatures reached 97 degrees on Wednesday amid Portland's third heatwave this summer.

Willamette River's temperature spiked to 78 degrees and favorable nutrient conditions — another factor in algal growth — triggered the bloom near Cathedral Park, Heartquist said.

The specific cyanotoxin present in the water is microcystin, water samples show. The amount detected was 105 parts per billion, which is substantially higher than levels safe for exposure — less than 8 parts per billion, Heartquist said.

Cyanotoxins are not absorbed through the skin, but people with skin sensitivities can develop a puffy rash if exposed, officials said.

COURTESY PHOTO: OREGON HEALTH AUTHORITY - A water sample taken from the Willamette River near Cathedral Park contains high levels of harmful cyanobacteria from a recent algal bloom.Children and pets are at increased risk for exposure because of their size and levels of activity.

"Dogs can get extremely ill and even die within minutes to hours of exposure to cyanotoxins by drinking the water, licking their wet fur or eating the toxins from floating mats or dried crust along the shore," officials said. They added that people should be aware of the danger regardless of whether there's a health advisory in place.

Symptoms of exposure may be similar to food poisoning, with people having stomach cramps, diarrhea, nausea and vomiting, officials said. More serious symptoms can include numbness, tingling, dizziness and shortness of breath. Symptoms may require medical attention.

Dogs can experience symptoms including drooling, weakness, difficulty walking, convulsions or seizures, lethargy and loss of appetite. People should seek veterinary treatment as quickly as possible if a dog exhibits any of those symptoms, officials said.

If pets are exposed, don't let them lick their fur and wash them with clean water immediately, Multnomah County officials said.

The toxins cannot be removed by boiling, filtering or treating water with camping filters, making drinking water directly from affected areas especially dangerous.

Officials said people should be aware that symptoms of heat stroke and water intoxication can be similar to those of exposure to cyanotoxins, officials said. Dogs can become ill and die of water intoxication, which can occur if a dog drinks excessive amounts of water while swimming or fetching objects for long periods of time. It results from an imbalance of electrolytes in the body leading to a disturbance in brain function.

Additional water samples may further define the extent of the health advisory, officials said. The bloom and associated toxins may have originated upstream, spreading downstream beyond the area of Cathedral Park. OHA is awaiting results from samples taken at Ross Island Lagoon, Holgate Channel, Kelly Point Park, the Cathedral Park boat ramp, and Willamette Cove.

People should keep an eye out for visible signs of an algal bloom in other areas of the river and stay out of the water in locations with visible scum, the OHA recommended.

It's unknown when the advisory will be lifted because algal blooms may last for weeks or months, officials said. Blooms can last until weather or rain sufficiently cools water and algae can't grow.

For more information, or to report potential exposure, pet owners should visit healthoregon.org/hab, call 877-290-6767 or email This email address is being protected from spambots. You need JavaScript enabled to view it..

On Friday, Aug. 26, OHA officials expanded the health advisory to an area upstream. Read more here.

Editor's note: This story was updated Monday, Aug. 22, with additional information from OHA.


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