Dog-Killing Worm Found In Riverside County's Piece Of Colorado River

BLYTHE, CA—A rarely seen parasite in California was recently found in the Colorado River in eastern Riverside County, UC Riverside researchers confirmed Thursday. They are raising awareness because the parasite is potentially deadly to dogs.

Previously found almost exclusively in Texas and other Gulf Coast states, the parasite, Heterobilharzia americana, was discovered after the researchers learned about dogs that became sick after swimming in the Colorado River in Blythe.

H. americana is a flatworm commonly referred to as "liver fluke." It can cause schistosomiasis, an illness that affects dogs' livers and intestines.

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

H. americana has never been reported this far west, according to Dillman, who said he assembled a research team and went to Blythe to investigate the sickened dogs and the parasite.

H. americana canine infection starts with snails and their worms. The UCR research team collected more than 2,000 snails from the banks of the Colorado River in Blythe. A paper published Thursday in the journal Pathogens describes how the team used DNA to confirm the identity of the snails and the flatworm.

"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 worms venture out of the snails to find "host" mammals to live inside. At this stage, the worms can only survive outside a host for about 24 hours. According to the researchers, if a dog is in water where the snails and their worms are present, or is drinking it, the pooch risks infection. Raccoons are also susceptible.

"It gets into the veins of the intestinal lining, and that’s where [the worm] develops into an adult and mates," Dillman said. "The presence of the adults in the veins isn’t the problem. It’s the eggs that get into the lungs, spleen, liver, and heart. The immune system tries to deal with it, and hard clusters of immune cells called granulomas form. Eventually the organ tissues stop functioning."

Once a dog is infected, the worst symptoms can take several months to appear. Since 2019, 11 dogs in three California counties have been confirmed with canine schistosomiasis. According to the researchers, one dog died.

"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," said Emily Beeler, a veterinarian with the Los Angeles County Department of Public Health.

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

H. americana is not known to cause disease in humans.

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

Dillman allayed concerns that the parasite could be contaminating urban drinking water.

"Compared to other pathogens these worms are fairly large. They can easily be filtered out with common water purification strategies," he said.

Though there is little concern about contamination of water sources for humans, drinking river water directly is still inadvisable.

"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."