Parasite in the Colorado River is deadly to dogs, researchers say. Here's what to know

Scientists for the first time have discovered a parasite potentially fatal to dogs in a portion of the Colorado River near the California-Arizona border.

After multiple dogs were infected near Blythe, Calif., University of California, Riverside, scientists collected about 2,000 snails from the river bank to look for the species known to carry the parasite Heterobilharzia americana, also known as liver fluke.

A large percentage of snails collected were the two species that spread the parasite. Researchers confirmed through DNA testing some snails had previously transmitted the parasite.

All of the infected dogs had recently swum in the Colorado River. Scientists warn pet owners to reconsider swimming with their dogs as warm weather approaches.

“Dogs can die from this infection, so we are hoping to raise public awareness that it’s there,” said Adler Dillman, a UCR nematology professor. “If you’re swimming in the Colorado River with them, your pets are in peril.”

This is the first time H. americana has been reported in the West; it was exclusively in Texas and other Gulf Coast states previously.

Here’s what to know about this dangerous dog parasite:

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How is the parasite transmitted?

H. americana relies on freshwater snails as intermediate hosts after hatching, growing inside a snail until it leaves to find a mammal to infect. The parasite can survive on its own in water for about 24 hours.

Dogs can contract the parasite that causes canine schistosomiasis by swimming in or drinking contaminated water.

Parasites travel through the body into a vein in the intestines to mate and lay eggs. Typically eggs are shed in the animal’s feces, but some can remain in the bloodstream.

“The presence of the adult in the veins isn’t the problem,” Dillman said. “It’s the eggs that get into the lungs, spleen, liver and heart.”

They can stick to organs, creating hard lumps called granulomas. The immune system tries to attack the clusters, but organ failure can eventually occur.

What are symptoms to look out for?

After a dog contracts the parasite, it can take several months for symptoms to appear.

Symptoms are gradual but often include loss of appetite, vomiting, diarrhea, significant weight loss and signs of liver disease.

“If your dog has these symptoms after swimming in the Colorado River, it’s a good precaution to ask your veterinarian for a simple fecal test,” said Emily Beeler, a Los Angeles County Department of Public Health veterinarian.

With prompt diagnosis, dogs can be treated with deworming drugs like praziquantel and require close monitoring.

From 2018 to 2023, some 11 dogs in three Southern California counties were infected, and one died.

Can the parasite infect humans?

Although the parasite can be deadly to dogs and other mammals like raccoons, horses, and rabbits, it is not known to infect humans.

If a human is exposed and the parasite penetrates the skin, it can cause a red rash called swimmer’s itch.

The parasite does not jeopardize drinking water from the Colorado River, as it is a large worm that is easily filtered out during water purification processes.

Hayleigh Evans covers environmental issues for The Arizona Republic and azcentral. Send tips or questions tohayleigh.evans@arizonarepublic.com.

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. 

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This article originally appeared on Arizona Republic: Parasite Heterobilharzia americana found in Colorado River