In the summer of 2020, a husky puppy named Keanna was on a hike on the Pa’rus trail in Zion National Park and spent some time splashing in the Virgin River. Thirty minutes later, the dog had trouble walking, began to have seizures and soon was dead.
Officials took water samples from the river and discovered the culprit: toxic cyanobacteria, a toxic form of bacteria.
Since then, the waterborne bacteria hasn’t caused any other pet deaths or harm to visitors, but it can still grow in the river, officials said. If blooms of the bacteria grow, they could affect fall recreation and tourism near the river, and could cause health risks if disturbed.
In November, much of the Virgin River was under a warning advisory due to bloom risks. The Utah Department of Environmental Quality put a new health watch on the lower Virgin River in July, alerting visitors that harmful algae mats may be present at multiple locations between Springdale and Bloomington. DEQ posts updates on its testing at habs.utah.gov.
Recent dry months may contribute to the growth of the dangerous blooms, according to Hannah Bonner, an environmental scientist with DEQ and its a recreational health advisory program coordinator.
The best way to keep pets and people safe, Zion rangers said, is for visitors to know how to spot the blooms and how to avoid them.
When Keanna died in 2020, the region had gone three years without a strong monsoon pattern; that year was the second driest monsoon in Zion National Park history. The driest year was 2019, which may have accelerated the bacteria’s growth a year later.
Usually, intense rainfall “cleans” the river, with flash floods flushing out the blooms. The lack of normal monsoons meant the blooms were able to reach a dangerous level.
The fact the bacteria was found in running water at all was surprising to Zion officials and scientists. Cyanobacteria generally grow in still water, but this bacteria was able to attach to rocks and branches in the river, avoiding notice until its toxins killed Keanna.
This type of bacteria can use photosynthesis, like plants, and produces a toxin called anatoxin-a, or as scientists call it, the very fast death factor. This toxin is dangerous to both humans and animals and can damage a person’s nervous system in minutes.
Though this toxin has not caused harm to any humans at the park, it has been known to harm people elsewhere and has had other impacts. A severe Florida outbreak in 2018 discouraged tourism and cost the state’s economy an estimated $2.7 billion; another outbreak killed nearly a thousand sea animals off California’s coast in 2023.
Current solutions include applying chemicals to the affected water, which may not work in Zion National Park. That’s a “type of treatment that tends to be very difficult to employ in such a complicated system, and the effects on the water ecosystem are unknown,” said Robyn Hendricks, the park’s physical science manager.
“Rivers pose a unique challenge [to treat] as they are an open system, particularly [in] Zion National Park,” she said.
Usually, eliminating these bacteria requires reducing their supply of nutrients from outside sources, such as controlling fertilizer that flows into water from farms or agricultural company sites.
But Zion’s cyanobacteria can survive in low-nutrient environments such as the Virgin River, according to Hendricks. It can get nutrients such as nitrogen and phosphorus from the atmosphere and the water.
Instead, Zion National Park is focusing on making sure visitors know how to stay safe while touring the river. Signs around higher risk areas have warnings and images of the blooms. Zion rangers also tell guests about the possible health threat.
Dogs are especially vulnerable to toxin exposure and are drawn to the blooms because of their earthy smell, according to Hendricks. Dogs must be kept on a leash and out of the water in the park. Pets also are not permitted on any trails aside from the Pa’rus trail, in wilderness areas or in the park’s public buildings, although they are welcome on public roads, parking areas, campground and picnic areas, and the park lodge.
“People knowing what to look for is the best prevention and response,” Bonner said. “If everyone knew how to identify them [blooms], we wouldn’t have any issues.”
