Deadly dog parasite found in Southern California in a 1st

small curly haired dog wearing a collar and standing on a river bank as it shakes water from its fur
A parasitic worm that can cause a dangerous disease in dogs has been found on the banks of the Colorado River. (Image credit: LorenzoPatoia via Getty Images)

A parasitic worm that infects and kills dogs has been found in California for the first time, a new study reports. The worm was discovered near the Arizona border on the banks of the Colorado River.

The discovery marks the furthest west this dog-infecting worm has been found in the U.S. The parasite, called Heterobilharzia americana, is mainly found in Gulf Coast and South Atlantic states but appears to have expanded its range into additional regions in recent years.

In dogs, the parasite causes a potentially fatal disease called canine schistosomiasis. If diagnosed promptly, the infection can be successfully treated with deworming drugs, such as praziquantel, said senior study author Adler Dillman, a professor of parasitology and chair of the department of nematology at the University of California, Riverside. The worm cannot cause schistosomiasis in humans but can sometimes irritate people's skin, causing "swimmer's itch."

Dillman and colleagues began their survey of the Colorado River, which was described March 13 in the journal Pathogens, after LA County veterinarian Dr. Emily Beeler reached out to them about a local cluster of dog illnesses. 

Related: Parasitic worms found in man's brain after he likely ate undercooked bacon 

Between 2018 and 2023, 11 dogs in three Southern California counties caught canine schistosomiasis, and one ultimately died of the infection. All had gone swimming in the Colorado River near the California-Arizona border prior to falling ill; their travel histories suggested that the river might be the common source of infection, despite H. americana not being reported in Southern California before.

"They were concerned that there might be something local," Dillman said of Beeler and her fellow veterinarians.

So Dillman and colleagues organized several trips to Blythe, a border town about an hour east of Joshua Tree National Park. The team searched for small, freshwater snails that serve as hosts for the parasite. H. americana worms infect these snails shortly after hatching, mature a bit and then get released into the surrounding water. 

Dillman and his team scooped up snails from the banks of the Colorado River. (Image credit: Adler Dillman/UCR)

The free-swimming worms then bore through the skin of a mammal host, such as a raccoon, bobcat, opossum or dog. The parasites swim through the mammal's bloodstream as they continue to mature. Eventually, they make their way to a major vein of the intestines, where they mate and lay eggs that are later shed in the animal's poop. 

However, some of the eggs end up drifting through the bloodstream and getting lodged in major organs, such as the lungs, liver or heart. There, they create hard lumps called granulomas that the immune system attacks, and if untreated, this can lead to organ dysfunction and death. 

The team collected around 2,000 snails in all, and through DNA analysis, they found that a large percentage belonged to two species known to be worm hosts: Galba humilis and Galba cubensis.

They found that a handful of the snails contained DNA from the parasitic worms — specifically, that of worms in their swimming life stage. But because only four out of hundreds of snails tested positive for this swimming stage of the parasite, it's tricky to estimate how many infected snails could be in the river overall, Dillman said. More snails in their samples may have been infected, but the worms in them may have not  yet been mature enough to be spotted in their analyses. 

"But what it does confirm for us is that they [the snails] can actually get infected in this area," he said.

Although long considered endemic to states like Florida and Texas, H. americana has more recently been found in Indiana, Tennessee, Oklahoma, Arkansas, Kansas and Utah. The new study suggests that the worm is also in California and may also be living in other Colorado River tributaries in nearby states. This warrants further investigation, Dillman said. 

In the meantime, the researchers behind the study want Calfornians to be aware of the potential risks of canine schistosomiasis and be on the lookout for its early symptoms, such as lethargy, loss of appetite, weight loss, vomiting and bloody diarrhea.

"My message is not, 'It's there, and no one should go anywhere near it,'" Dillman said. Rather, pet owners should be aware that their dog could potentially get infected by swimming in this stretch of the Colorado River near the California-Arizona border, Dillman said. 

This article is for informational purposes only and is not meant to offer medical advice.

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Nicoletta Lanese
Channel Editor, Health

Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She holds a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Her work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains heavily involved in dance and performs in local choreographers' work.