For the first time in a Utah river, toxic levels of a river-dwelling species of cyanobacteria have been detected after a dog suffered seizures and died soon after frolicking in Zion National Park’s Virgin River.

The discovery prompted public health officials to caution people against swimming or letting their pets or livestock enter the Virgin River’s North Fork, which flows out of the park and through Springdale, Rockville and Virgin.

Most communities along the river have stopped using it as a source of drinking water.

Known as benthic algae — the kind that grows on rocks and other substrate — this photosynthetic bacteria is different than the green slimes that form toxic blooms on Utah water bodies every summer. Thought to be connected to agricultural runoff and higher-than-normal temperatures, these algal blooms can fill afflicted lakes’ water columns with toxins, prompting warnings to stay out of the water and to not eat fish caught there.

A toxic bloom of river-dwelling benthic algae, in a national park no less, is a major cause of concern, according to Erica Gaddis, executive director of the Utah Division of Water Quality.

“It’s a complete mystery. It’s just bizarre, we are all stunned,” she said. “There’s a need to understand this.”

In response to the dog’s July 4 death, samples were taken along the Virgin River and brought to Salt Lake City for testing. The results came back Friday indicating concentrations of anatoxin-a, the harmful substance produced by cyanobacteria that affects the nervous system, at nearly four times the health threshold for recreational exposure, triggering the public health warnings.

The harmful algae was found in multiple spots in the river.

“Why is it showing up now? We can’t answer that,” Gaddis said. “It’s a new area for Utah. These sorts of blooms can be triggered by disturbances in the watersheds. It’s not tightly linked to nutrient loads like blooms in lakes.”

The state has previously issued health warnings for E. coli impairment in the North Fork.

This latest algal bloom comes at a time when the Utah Department of Environmental Quality sharply curtailed bloom monitoring on 60 water bodies after the Utah Legislature reallocated the program’s funding. Officials restored monitoring on 18 lakes this summer, thanks to a $92,000 federal grant, according to Gaddis. The scaled-back program focuses on lakes that have seen past blooms and see heavy recreational use, such as Deer Creek, Echo, East Canyon and Scofield reservoirs, Yuba Lake and Utah Lake. Eleven of the 18 lakes are state parks.

Currently, health warnings are in place on Utah Lake at the American Fork and Lindon marinas.

At Zion, the dog died an hour after playing in the Virgin River, “snapping” at the algae growing on the rock, at a spot near Canyon Junction where the North Fork exits Zion Canyon, according to park spokesman Jeff Axel.

“When the dog was pawing at the rocks, it freed up the toxins from the bacterial mat,” he said, before the animal apparently ingested it.

According to a report from the park, the dog soon lost the ability to walk, was in pain, and was having seizures — all symptoms consistent with exposure to cyanobacteria toxins.

Test results from water samples showed a concentration of anatoxin-a as high as 55 micrograms per liter in some samples; the threshold for primary recreation, recommended by the Division of Water Quality and the Utah Department of Health, is 15 micrograms per liter.

More samples will be collected this week, and DEQ will post updates on the testing at habs.utah.gov.

The Southwest Utah Public Health Department has issued a public health warning for the affected areas of the river. The regional health department will post signs warning people of the risks of exposure to the river water.

Zion has posted 50 warning signs, at nearly every accessible spot on both the North Fork and Pine Creek, Axel said. New water samples have been drawn from numerous spots from the Narrows to the park’s visitors center; results will be available on Tuesday.

Residents and visitors are advised not to swim in the river, drink the river water (even if using a purifier), or get near areas of algal scum. People who fish should clean their catch well, and properly dispose of the guts.

The health warnings do not apply downstream to Quail Creek Reservoir, Sand Hollow Reservoir or the Santa Clara River basin.

The Utah Division of Drinking Water is working with local utilities to ensure finished drinking water that originates from the river is free of cyanotoxins. So far, tests of Springdale’s drinking and agricultural water have not detected cyanotoxins. The town, the gateway to Utah’s most popular national park, will continue to test finished drinking water to ensure the water is safe.

The Washington County Water Conservancy District, Zion National Park and the towns of Virgin and Rockville are not using the North Fork of the Virgin River as a source of drinking water.

Officials have urged residents to keep use of culinary water to a manageable level, and to keep secondary water from being pulled into the drinking water system, causing potential cross-connection contamination.