Health officials warn of little-known danger to people and pets on the Columbia River

For decades, public health messaging around toxic algae blooms in Oregon and Washington have painted a clear picture of what to avoid: warm, stagnant lakes and ponds where planktonic algae colonies can grow and accumulate.

But Clark County health officials are shifting to track a previously-unmonitored form of toxic growths called benthic algae that can grow in faster-moving waterways — and can be just as deadly if ingested by people or pets.

At a Clark County Board of Public Health meeting Wednesday in Vancouver, county public health specialist Maggie Palomaki explained what sets toxic benthic algae apart from the type of algae blooms people are familiar with.

“Benthic algae mats, rather than being dispersed through the water column, they’re actually attached to the substrate at the bottom of a body of water,” Palomaki said, adding that their fixed state allows them to grow in fast-moving waterways. “When there’s an overgrowth, they can detach and float to the surface.”

And when they do, the results can be deadly.

Palomaki said that officials started tracking the threat in October 2024 when a Camas-area resident reported that their dog had died from cyanotoxins, a type of poison commonly produced by various species of toxic algae. She added that some of the species that form toxic planktonic blooms can also grow as benthic colonies.

The dog’s owner was well-versed on the risks of toxic algae in bodies of stagnant water and had taken their dog to the Columbia River near Ackerman Island instead. But the dog apparently ate a piece of benthic algae that had drifted from the river’s depths and rapidly experienced symptoms such as stumbling, vomiting and tremors before it died, Palomaki said. Prior to the October 2024 poisoning, Clark County officials weren’t aware of toxic benthic algae in the County’s waterways, Palomaki said.

Palomaki said another dog died in August after spending time on the Columbia River.

“Up to this point, our monitoring and education has been focused on the planktonic algae blooms that we see on the surface of the water,” Palomaki said. “So we’re now asking ourselves, ‘What can we do differently?’”

Currently, there are few options for testing, she said — and the state of Washington doesn’t even have toxicity thresholds for benthic mats. That makes it harder to accurately test waters for the presence of benthic algae and issue public safety warnings, she said.

Instead, Palomaki said, Clark County health officials are taking an educational tack.

They have updated their messaging to include the dangers of algae mats and to encourage people to be vigilant about faster-moving bodies of water that formerly wouldn’t have been cause for suspicion. She also said that Clark County is coordinating with local agencies in Oregon to ensure consistent messaging on both sides of the Columbia River.

“Ultimately, we’re just trying to be flexible since this is new to us,” she said, “We’re just being open to evolving and adjusting as we learn more about this topic.”

Staying Safe

Officials caution that dogs can ingest the toxins produced by benthic algae if they eat pieces of algae mats, drink water that contains algae or lick it off their fur. People can be exposed to algae toxins through skin contact, ingestion or even inhalation of contaminated water.

Symptoms of toxin exposure include vomiting, tingling, salivation and loss of motor control. Anyone experiencing symptoms is advised to contact poison control immediately, or an emergency veterinarian in dog poisoning cases. You can reach a trained healthcare provider 24 hours a day at the Oregon Poison Center at 1-800-222-1222.

Officials have also released a guide for spotting and avoiding benthic mats that can be viewed here.

— Tatum Todd is a breaking news reporter who covers public safety, crime and community news. Reach them at ttodd@oregonian.com or 503-221-4313.