Dog-killing worms found in California for the first time: Scientists discover hundreds of snails in Colorado River harboring the parasite that has killed at least one pet

Scientists have discovered dog-killing worms in Southern California for the first time, which have infected at least 11 pets - one of which died.

A team at the University of California Riverside (UCR) found Heterobilharzia americana, a flatworm commonly referred to as liver fluke, are being transmitted by snails living along the Colorado Riverbed.

The worms can cause canine schistosomiasis, an illness that impacts the liver and intestines of dogs, and were previously only found in Texas, Louisiana and Florida. 

The findings have suggested that the tiny flatworms could be spreading throughout the Colorado River and smaller rivers that feed into it, providing a broader expansion than previously known. 

A team at the University of California Riverside (UCR) found Heterobilharzia americana, a flatworm commonly referred to as liver fluke, were being transmitted by snails along the Colorado Riverbed

UCR nematology professor Adler Dillman said: 'Dogs can die from this infection, so we are hoping to raise public awareness that it's there.

'If you're swimming in the Colorado River with them, your pets are in peril.'

Los Angeles County of Public Health issued an advisory in 2023 confirming 11 cases of canine schistosomiasis that spanned from 2018 to 2023.

Dillman and his team caught wind of the local infections and headed to a region of the river where all 11 dogs spent time swimming.

The team collected more than 2,000 snails, finding two species harboring flatworms - Galba cubensis and Galba humilis.

'Not only was it a surprise to find H. americana, we also did not know that the snails were present here,' Dillman said

This discovery was made at Mulberry Grove Pond, which receives water from Mill Creek, one of the tributaries of the Colorado River

After transforming itself inside one of the snails, the worm ventures out to find a mammal to infect. In this stage, it can only survive on its own for about 24 hours. If a dog or a raccoon is in the water, or drinking, then it gets infected

Galba cubensis was previously believed to be the only natural host of the worm, with G. humilis only showing marginal susceptibility.

Both snails primarily come from the Americas, but have invaded Asia, Europe and Africa. 

'This discovery was made at Mulberry Grove Pond, which receives water from Mill Creek, one of the tributaries of the Colorado River,' the researchers shared in the study.

'This pond is approximately 280 feet from Mill Creek, and the shore of the Colorado River is approximately 2.17 miles away from the pond.'

The team, however, believes the parasite could be spreading throughout the Colorado River and taking advantage of the rivers that feed into it to find new hosts. 

'In-depth studies are required to investigate the extent of parasite infestation along the banks of the Colorado River and its tributaries in the United States and Mexico to gain a clear understanding of the parasite’s infection,' reads the study. 

The team collected more than 2,000 snails, finding two species harboring flatworms - Galba cubensis and Galba humilis

After transforming itself inside one of the snails, the worm ventures out to find a mammal to infect. 

In this stage, it can only survive on its own for about 24 hours. If a dog or a raccoon is in the water, or drinking, then it gets infected.

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'It gets into the veins of the intestinal lining, and that's where it 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 infected, it can be several months before the worst symptoms of the illness appear. 

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

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

Researchers emphasized that H. americana is not known to be capable of causing 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, while also noting that the worms can be easily filtrated out of water systems.