Dog-killing worm found in California for the first time

As colossal sandworms race across the big screen in “Dune: Part Two,” a new kind of invertebrate has been documented for the first time in California – one that’s no larger than a penny but is potentially fatal to dogs and other animals, and that can lead to irritating outbreaks of swimmer’s itch in people.

Last spring, scientists with UC Riverside’s department of nematology discovered the parasite Heterobilharzia americana, a flatworm commonly known as a liver fluke, along the banks of the Colorado River in California, according to a Thursday news release from the university. The parasite has been documented almost exclusively in Texas and other Gulf Coast states, and “has never been reported this far west,” the release said, raising concerns for public health and the species that live along the river and its surrounding tributaries.

The flatworm is capable of infecting raccoons, marsh rabbits, horses, nutria, bobcats, mountain lions, opossums and other mammals, per the UC Riverside team’s study, published last week in the scientific journal Pathogens. As summer looms, scientists are warning people to stop their canine companions from going for a swim in the river before it’s too late.

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

Dillman and his team of researchers were prompted to conduct the survey after the Los Angeles County Department of Public Health issued an advisory confirming 11 cases of canine schistosomiasis in dogs from Los Angeles, Orange and Riverside counties between 2018 and 2023. One of the animals died from the illness, which causes their liver and intestines to deteriorate.

The team ventured down to Blythe, a town about 75 miles east of Joshua Tree National Park, where they discovered the culprit that had been transmitting the worms, and therefore infecting dogs that had entered the river: two species of snails, Galba humilis and Galba cubensis.

The team got to work, gathering more than 2,000 of the snails, in part by sweeping through the water with kitchen strainers. Through subsequent DNA testing in a lab, the team learned both species of snails were “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 host snails release the free-swimming flatworms into the water, where they can survive on their own for about 24 hours before they penetrate the skin of unsuspecting mammals as they wade in or drink. Once the parasitic worms have found a new host, they enter their juvenile form – a schistosomula – and migrate to an animal’s lungs, where they can cause hemorrhaging, according to the study. Then, they make their way to the liver and develop into male and female forms, which then mate and lay eggs. Those eggs get released into the veins of an animal's intestinal lining, and can travel to other parts of the body.

“The presence of the adults in the veins isn’t the problem,” Dillman said. “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.”

It can take several months for the worst symptoms to appear in dogs. Early signs include “vomiting, coughing, fever, bloody diarrhea, anorexia, weight loss, lethargy, polyruia [excessive urination] and polydipsia [excessive thirst], ultimately leading to collapse,” according to the study. These symptoms are similar to those caused by tumors and other diseases in dogs. A lengthy testing process and delayed diagnoses can cause veterinarians to resort to euthanasia, rather than the appropriate treatment of high-dose praziquantel or fenbendazole, medications for worm infections that can completely cure some animals.

“Veterinarians consistently emphasize that H. americana is frequently overlooked in diagnoses and is becoming a growing concern, particularly in dogs, horses and other mammals,” the study read.

The flatworm is not capable of infecting people, Dillman said, but can penetrate human skin and cause swimmer’s itch, resulting in a nasty red rash. Most cases do not require medical attention, but home remedies such as a cool compress, corticosteroid cream, or an Epsom salt bath can help, according to the California Department of Public Health. The flatworms are also unlikely to contaminate urban drinking water and can “easily be filtered out with common water purification strategies,” Dillman said. Still, “nobody should be drinking straight out of the river,” he noted. River water can be rife with viruses, bacteria and other parasites such as giardia that have nothing to do with H. americana.

If your dog is exposed to the water in the Colorado River and starts exhibiting any of the aforementioned symptoms that could be related to the parasite, “it’s a good precaution to ask your veterinarian for a simple fecal test,” Emily Beeler, a veterinarian with the Los Angeles County Department of Public Health, said in the release.

More research is needed to determine the extent of the parasite infestation and whether the flatworm could continue to expand throughout California. Further investigation is also required to understand its impact on the biodiversity of wildlife along the river, where “it poses a potentially severe threat,” the study read. But the findings so far are noteworthy, helping scientists identify that the parasite has a wider geographical distribution than previously thought, and allowing them to start to develop effective control strategies to prevent further spread of the disease. Researchers are also trying to figure out what conditions led to the presence of the snails, which could be the result of climate change, human activity or both.

“This could be a much bigger problem,” Dillman told Gizmodo. “Not just in Southern California, but Nevada, Utah, Colorado,” and parts of New Mexico bordering the river.