Twenty cattle likely died due to a harmful algae bloom at a Harney County, Ore., reservoir and an increase in toxic plant consumption, according to the state Department of Agriculture.

Toxic algae blooms are on the rise and the number of them this year is “concerning,” said an Oregon Health Authority spokesman.

The deaths occurred at Cottonwood Reservoir, and the ODA was notified Aug. 1.

The OHA issued a harmful algal bloom recreational advisory the same day, which was lifted Aug. 13 after monitoring.

The cattle belonged to one rancher and no other illnesses or deaths have been reported.

Increasing algal blooms

According to OHA spokesman Jonathan Modie, cyanobacteria’s toxic effects on livestock have been documented for more than 100 years.

Harmful algal blooms “are not going away any time soon as global climate change is increasing their frequency and duration through increased temperatures and sunlight, lower snowpack and decreasing water levels creating warmer water temperatures,” Modie added, in an email.

The 11 harmful algal bloom notices so far this year have already matched the figure for 2023. The state averaged 8.4 such notices a year in the past decade, and the highest number was 12 in 2017.

Those figures don’t include the South Umpqua River and the mainstream Umpqua River, where a permanent advisory has been posted since 2011. Bedrock pools on the bank warm water and make it favorable for blue-green algae growth.

Cyanotoxin exposure

Signs of cyanotoxin exposure from harmful algae blooms in animals include vomiting for companion animals, increased salivation, fatigue, stumbling, shortness of breath, weakness, seizure like activity, liver failure and eventually death, according to an ODA news release.

Animals can ingest the toxin through direct exposure to the infected water, eating floating mats or dried crust along the shore.

People — and especially small children and pets — should avoid areas where the water is foamy, scummy, thick like paint, pea-green or blue-green, or if thick brownish red mats are visible or bright green clumps are suspended in the water, according to the OHA.

Cyanotoxins can still exist in clear water, however.

As of Aug. 15, the OHA had seven active harmful algal bloom advisories, including those at:

• North Santiam River, Big Cliff Reservoir and Detroit Reservoir, issued July 16.

• Thief Valley Reservoir, issued Aug. 5.

• Brownlee Reservoir, issued Aug. 5.

• Hells Canyon Reservoir, issued Aug. 5.

• Wapato Marsh at Oaks Bottom Wildlife Refuge, issued Aug. 7.

• Lake Billy Chinook and Lake Simtustus, issued Aug. 8.

Due to limited resources, only a fraction of water bodies in Oregon are monitored or sampled.

“The actual number of blooms occurring around the state is probably much higher,” Modie said.

Cocklebur plant

Receding levels at Cottonwood Reservoir also has led to an increase in growth of the cocklebur plant, which grows in rich soils such as shorelines.

Livestock can be poisoned and killed when consuming the seed via pasture, feed contamination with bur seeds, in hay or while grazing crop residues.

Animals only have to consume about 0.75% of their body weight to cause them to die.

The most common signs of toxicity are depression, weakness, going off feed, refusal to move, severe spasticity of the muscles, hyperexcitability and an unusual walk with the ears erect and the head held high until they are down and no longer able to move.