Flatworm Parasite In CA Portion Of Colorado River Causes Illness, Death In Dogs

Since 2019, 11 dogs in Los Angeles, Orange and Riverside Counties have have been sickened by a flatworm parasite, and one has died. University of California Riverside professor of nematology Adler Dillman and his team confirmed the flatworm's presence in a portion of the Colorado River that runs through the California city of Blythe in Riverside County. This is the first known instance of the parasite this far west.

Dillman received detailed information from veterinarians about where these dogs and their owners had been prior to the diagnosis of this disease, which led him and his team to sample the locations along the river.

The flatworm parasite, known scientifically as Heterobilharzia americana, was discovered after the UC Riverside team collected more than 2,000 snails from the river. The snails served as intermediate hosts for the parasite that would infect dogs by penetrating their skin.

“It (the parasite) can develop to adulthood in the mesenteric veins of the dog, those are the veins that feed that come off of the intestine,” he said. “These parasites begin to lay eggs, and it's very bad for the dog. They get quite sick, and it actually can be fatal.”

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Can the parasite spread to other parts of Southern California?

Dillman said he is concerned that the parasites might run the length of the Colorado River and he said there is no information yet on whether it is present in the river’s tributaries.

“We are interested in doing more surveys up and down the Colorado River and into some of the tributaries that go into Arizona, Utah, Colorado, and just to see what the full extent of their spread is,” he said. The primary thing we would look for is the presence of these snails. There's only three, maybe four snails that are known in the wild to be able to support this parasitic infection.”

But, these snails are very small — around 1 to 3 millimeters in length.

“They're going to be on the vegetation close to the bank or on rocks and things. It's not what you typically think of when you think of a snail. They're just so small,” Dillman said.

Can the parasite spread from dog to dog or dog to human?

Emily Beeler, a public health veterinarian with the Los Angeles County Department of Public Health, said dogs can only get the parasite from being in water where infected snails are present.

“It's not directly transmissible between dogs and from dogs to people,” she said.