• Posted March 15, 2024

Waterborne Parasite That Can Kill Dogs Is Spreading to U.S. West

A parasite that can kill dogs has been found in a portion of the Colorado River that runs through Southern California.

Heterobilharzia americana is a flatworm usually referred as liver fluke. It has never been reported this far west, according to scientists at UCRiverside, who are warning pet owners of the danger. 

The worm can cause canine schistosomiasis, which affects dogs' liver and intestines. It can be fatal.

"If you're swimming in the Colorado River with them, your pets are in peril," said nematologist Adler Dillman, a professor of parasitology at UCRiverside.

Researchers recently converged on Blythe, Calif., east of Joshua Tree National Park, after learning about cases of the infection there. All of the infected dogs had been swimming in the Colorado River.

A snail transmits the deadly worm, and Dillman's team collected more than 2,000 snails from the riverbanks. 

"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 in a university news release. "Not only was it a surprise to find H. americana, we also did not know that the snails were present here." 

His team described in the March 13 issue of the journal Pathogens how they used DNA to confirm the identity of both the snails and the worm.

The worm can survive only about 24 hours once it leaves the snail. However, if a dog or raccoon is in the water where it is found or drinking it, it can get infected.

The worm gets into the dog's intestinal lining, matures and reproduces. Its eggs get into the dog's lungs, spleen, liver and heart, Dillman explained. 

"The immune system tries to deal with it, and hard clusters of immune cells called granulomas form," he said. "Eventually the organ tissues stop functioning."

The worst symptoms may take months to appear.

Since 2019 in California, three counties have confirmed infections in 11 dogs, and one dog has died. Health officials hope getting the word out about the parasite will prevent further infections and deaths.

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

Infected animals will need to be closely monitored by a veterinarian and treated with multiple medications, she said.

H. americana is not known to infect humans, Dillman said. It can, however, cause an outbreak on the skin called "swimmer's rash."

He said urban drinking water is not contaminated, because the worms can be easily filtered out with common water purification equipment. However, people and pets should not drink water directly from the river.

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

More information

The U.S. Department of Agriculture has more about intestinal parasites in dogs.

SOURCE: UC Riverside, news release, March 14, 2024