Dog-killing flatworm parasite discovered in new state as scientists warn of spread West

A dog-killing parasite that was believed to only exist in Texas and other Gulf Coast states has been discovered as far west as California for the first time, scientists have warned.

Experts at the University of California Riverside found the Heterobilharzia americana parasite, a flatworm commonly known as a liver fluke, in spots along the Colorado River where it runs through Southern California. According to the university, the flatworm has never before been seen outside of Texas and surrounding areas, and other studies have found most infections occur in Texas and Louisiana, though some have occurred in North Carolina, Texas, and Kansas.

The worm can cause canine schistosomiasis, an illness that impacts the liver and intestines of dogs. While there are treatment options once a dog becomes infected, they are not always successful and such infections can lead to euthanasia.

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

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H. americana flatworm found in California

Scientists took to examining the waters in Riverside County after several cases of infection were reported in local dogs. Because the infection is generally transmitted via the presence of snails, the research team collected more than 2,000 along the banks of the Colorado River and examined DNA to identify the presence of the snails and the flatworm.

The team was surprised to find two species of snail that can carry the parasite.  “Not only was it a surprise to find H. americana, we also did not know that the snails were present here," Dillman said in a statement.

The parasite works by first making its way into the snail's body and traveling through its system. Once transformed, it ventures out to find a mammal host to infect within 24 hours before it is unable to survive further on its own. Dogs and raccoons are specifically susceptible to acquiring one of these worms via fresh water, whether it be from swimming in or drinking it.

A worm can enter into the veins of the intestinal lining, where it will mature, mate and lay eggs. The eggs then get into the lungs, spleen, liver, and heart, causing an autoimmune reaction, according to Dillman. The immune system responds by forming clusters of cells called granulomas, which eventually lead to organ tissue failure.

What canine infections look like

It may take several months before notable and serious symptoms appear following infection.

Symptoms tend to occur gradually, Emily Beeler, a veterinarian with the Los Angeles County Department of Public Health, said in a statement. Loss of appetite, vomiting, diarrhea, profound weight loss, and signs of liver disease are all symptoms associated with the progression of the 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 Beeler. “Treatment typically involves use of multiple medications and close monitoring of the dog by a veterinarian."

The parasite is not capable of causing such infection in humans. However, it has impacted 11 dogs in three California counties and killed one since 2019. With the new discovery, experts are warning dog owners to be vigilant and aware of the waters in which they take their pets to swim. Many freshwater sources such as rivers and swamps are capable of sustaining such parasites, even if humans have yet to identify them in those waters.