Two years ago a parasite that can potentially kill dogs was discovered for the first time on the shores of the Colorado River in Southern California. Now that same parasite has likely made its way to Lake Mead and Lake Mohave in Nevada.
Fortunately for dog-owners, the risk of dogs contracting the parasite in Nevada is likely very low, according to a survey of more than two dozen sites by the National Park Service.
In 2024, researchers found that a dozen dogs that had fallen ill after swimming in the Colorado River near Blythe, California had contracted Canine schistosomiasis, an infection caused by a freshwater parasite called Heterobilharzia americana which is primarily found in southern and in the Gulf Coast states.
The parasite, which can be fatal to dogs in extreme cases, had never been documented in Colorado River’s waterways before. However, in 2018, the parasite was identified in a dozen dogs near a man-made pond in Moab, Utah, marking its first documented appearance in the West.
“It had never been seen in California before. So we started getting phone calls saying, ‘Hey, a dog died in the Colorado River. Is Lake Mead safe?’,” said Riley Rackliffe, an aquatic ecologist at the National Park Service.
That question led Rackliffe to launch a survey across Lake Mead and Lake Mohave in search of the semi-aquatic freshwater snails that act as intermediate hosts to the parasite before finding a mammal to infect.
“We were looking at beaches, any place where you have vegetation that’s coming out of the water, so the snail can crawl up and down the stem and get wet and dry and wet and dry right in there,” Rackliffe said.
Over two years, Rackliffe and a team of researchers and volunteers combed 28 sites across Lake Mead and Lake Mohave, collecting and testing about 1,300 snails in search of the parasite. They also collected and tested raccoon fecal samples near the lake shores, as raccoons are commonly final hosts for the parasite.
The survey was also conducted in partnership with the University of California, Riverside, which ran the Blythe, California study that first identified the parasite on the Colorado River.
The team of researchers found three species of snails in Lake Mead and Lake Mohave that were potential viable hosts for the parasite. Fortunately, said Rackliffe, the species of snails that could host the parasite were rare in both lakes.
“We did find these in just a few places, and mostly in Lake Mojave. We did find some at the confluence of the Virgin River with Lake Mead, but they were in the river habitat, not in the lake habitat,” Rackliffe said.
After surveying the snails, researchers were not able to find the parasite that causes Canine schistosomiasis, but they did find the parasite’s eggs in the raccoon fecal samples.
“That was found all over the place, even in Lake Mead, and in most of the marinas we were able to find infected raccoon poop. That means that the parasite is around and it’s infected the raccoons,” Rackliffe said.
While the snails that host the parasite are present in both Lake Mead and Lake Mohave and the parasite is likely regularly completing its life cycle through raccoons, Lake Mead and Lake Mohave are not the most ideal environments for the parasite to thrive, said Rackliffe.
“There isn’t much river habitat for the host snail to spread. They need emergent vegetation, like cattails, which is a rare habitat type. They seem to like that better,” Rackliffe said. “It’s not as abundant as the other species of snails that we already have.”
“We think the odds of infections are really low. We think the risk is really low because we haven’t seen infections, and the snail is pretty rare,” Rackliffe said. “The trick is that veterinarians in Vegas aren’t looking for it because it’s new. We didn’t used to have it.”
During the survey, Rackliffe said researchers also found the parasite eggs in raccoon scat around Lake Las Vegas, a habitat with more emergent vegetation. He suggested more surveys should be completed in urban parks that connect to Lake Mead as a water source.
“One of the places we found it was Lake Las Vegas, which is outside of our territory, and basically an urban park. There’s a lot of dogs that go there. So is that snail completing its life cycle in urban parks in Las Vegas? It’s entirely possible,” Rackliffe said.
“At the moment, we’ve never had a dog reported to us that was infected with this in the Las Vegas-Lake Mead area. That doesn’t mean they’ve never been infected, but that suggests that probably infection rates are low, or we would have heard about it,” Rackliffe said.
While the parasite is potentially fatal for dogs, it is not dangerous to humans. In humans, the parasite can penetrate the skin, but symptoms are limited to a red rash called swimmer’s itch.
Dogs can become infected by swimming in or drinking freshwater contaminated with the parasite, which can penetrate the skin. The parasites then travel through blood vessels where they lay eggs, causing severe, chronic inflammation in the liver and intestines.
After a dog contracts the parasite, it can take weeks or months for symptoms to appear. Symptoms are gradual but often include loss of appetite, vomiting, diarrhea, weight loss and blood in stool. If infection is diagnosed early and treatment is successful, dogs can quickly recover with no longstanding negative effects.
date: 2026-02-10 13:59:00
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