California Dog Owners Warned of Lethal River Parasite

A parasite that could potentially kill your pet dog has been discovered lurking in a section of the Colorado River that runs through Southern California.

This parasite is a flatworm named Heterobilharzia americana, also known as a liver fluke, and has previously only been found in Texas and other states on the Gulf Coast.

However, a new paper in the journal Pathogens reports that it has been discovered in California for the first time.

The liver fluke is a parasitic flatworm that infects mammals—mostly dogs—and is most commonly encountered in regions such as Texas, Louisiana and Florida. When it infects dogs, it can cause a condition known as schistosomiasis, which can damage a dog's liver and intestines and potentially be fatal.

"Dogs can die from this infection, so we are hoping to raise public awareness that it's there," paper co-author Adler Dillman, a nematology professor at the University of California, Riverside, said in a statement. "If you're swimming in the Colorado River with them, your pets are in peril."

Eleven dogs across three California counties have been infected with this parasite since 2019, one of which has died. This drove the researchers to investigate a river in Blythe, a town on the Arizona Border east of Joshua Tree National Park in Riverside County, where all the infected dogs had swum. In the river—which is a part of the Colorado River—they collected over 2,000 snails and found that several were host species of the liver fluke.

"We actually found two species of snails that can support H. americana in the river in Blythe, and we found both snails actively shedding this worm," Dillman said. "Not only was it a surprise to find H. americana, we also did not know that the snails were present here."

The life cycle of these parasitic worms involves several stages. Adult worms live in the veins surrounding the intestine of their definitive host, usually a mammal. The eggs produced by the adult worms are passed out in the host's feces into the water. Once in the water, the eggs hatch, releasing larvae called miracidia.

These miracidia then infect snails, where they undergo further development. The snail releases cercariae, the next larval stage, which swims in water and infects drinking or swimming mammals via the skin.

"Dogs and other animals are exposed to the parasite when wading or swimming in fresh water. The immature stage of the parasite penetrates through the skin and ends up in the veins in the intestine. There, the adult parasites release eggs many of which end up in the liver, spleen, or other organs and cause significant damage. Dogs can die from organ failure and other complications of the disease," Dillman told Newsweek.

Months may pass between infection and symptoms appearing, the researchers explain.

"Symptoms start gradually with a loss of appetite, and eventually include vomiting, diarrhea, profound 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," Emily Beeler, a veterinarian with the Los Angeles County Department of Public Health, said in the statement.

"Treatment typically involves use of multiple medications and close monitoring of the dog by a veterinarian."

The discovery of these liver flukes and their host snails in the Colorado River in California is concerning, the researchers say. It "emphasizes the persistence and potential expansion of this parasitic threat," they wrote in the paper.

While this parasite can cause a skin rash in humans, it's not able to enter our bodies and cause major infections.

"It can cause swimmer's itch, a red rash where it penetrates human skin. But it's not able to cause infection," Dillman said.

Despite the Colorado River providing drinking water to millions of people across the Southwest, there is also no risk of it ending up in our drinking water, the researchers stress.

"Compared to other pathogens these worms are fairly large. They can easily be filtered out with common water purification strategies," Dillman said. Though there is no cause for concern about contamination of water sources, drinking the water directly is still inadvisable.

"You have viruses, bacteria, and other parasites such as Giardia in rivers," he said. "Nobody should be drinking straight out of the river, and that has nothing to do with this particular parasite."

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Update: 3/15/24, 1:37 p.m. ET: This article has been updated with comment from Dillman.