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Nova Scotia Duck Tolling Retriever wading in water.

Ticks and fleas aren’t the only parasites that dog owners have to worry about in southern California. A new study from the University of California, Riverside, reports that a deadly flatworm species has invaded parts of the Colorado River, putting dogs’ lives at risk.

Since 2019, the infestation has spread to 11 dogs across three counties and led to one death. Through education, researchers hope to empower dog owners with the knowledge they need to keep their pets safe. Recognizing symptoms, closely watching your dog’s activity, and understanding flatworm infestations could save your pet’s life.

Mystery Illness Led to Flatworms Discovery

Dr. Adler Dillman, the department chair of nematology at UC Riverside, first started his research on the spread of flatworms in early 2023. The discovery of flatworms in the Colorado River has been in the making since 2018, thanks to researchers, local officials, and everyday dog owners.

About five years ago, Dr. Lauren Norby, a veterinarian at Harbor Animal Hospital in Torrance, California, examined a dog struggling with weight loss and liver disease. Upon discovering flatworms, she did another exam on the other dog in the household. Sure enough, it also had flatworms, just no outward symptoms.

Dr. Norby reported her findings to Los Angeles County, which requires healthcare providers to report communicable diseases. An alert about flatworms made it to the public, which a dog owner in a neighboring county saw. Her dog was also suffering from flatworms, and her insights would later help Dr. Dillman’s research.

Labrador retriever laying down on the couch.
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“That one dog owner in Orange County was amazing,” says Dr. Emily Beeler, a veterinarian at the Los Angeles County Department of Public Health. “She had done so much work herself in talking to dog owners in the same area and was spreading the word. She helped us find the additional cases and huge credit to her as an owner.”

The dog owner brought to light a consistency that changed the direction of the research. For the past 20 years, she’d taken her dogs to a specific spot in the Colorado River. Until now, they had never contracted flatworms. This breakthrough led to researchers collecting and examining 2,000 snails near Blythe, a border town in Riverside County. The results revealed that flatworms had spread to the area, and potentially beyond.

Deadly Flatworms Have Made Their Way West

Flatworms (specifically “H. americana”) were thought to be exclusive to the American South and in Gulf Coast states. However, Dr. Dillman’s study revealed that flatworms have spread further west than previously realized.

Flatworms have always posed a threat to dogs, but not previously in this region. These flukes start their lifecycle in a host’s feces, with raccoons, horses, and dogs among the most common hosts. Once in the water, the eggs hatch and the larvae infect snails.

“[These snails] are small, like 1/10 of an inch. They’re tiny,” Dr. Dillman says. “If I told you to go to the river and look for snails, you would miss them unless you knew exactly what you were looking for.”

Eventually, the snails will shed the adult fluke worms in the water, where they look for a host to continue the cycle. Your dog doesn’t have to drink water to contract these worms; they can burrow through the skin while your dog swims or wades near certain bodies of water. Once your dog digests the worm, it will attach to their intestinal veins, where it feeds and continues its reproduction cycle.

Golden Retriever standing on a paddle board in the water.
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Dr. Beeler notes that while any breed can contract flatworms, water-loving dogs are most at risk. These can include Labrador Retrievers, sporting group dogs, and any canine that loves a good swim. Even dogs standing on the riverside’s muddy flats can contract flatworms, so it’s even more important to monitor your dog’s activity and symptoms in the following weeks.

Symptoms of Flatworms in Dogs

If caught early, you can treat flatworms with deworming medications. However, if they go undetected for too long, they could damage the dog’s internal organs, including their liver and intestines.

“Flatworms could create a different range of symptoms,” Dr. Beeler says, “but gradual weight loss, coupled with vomiting and diarrhea, are what is seen.”

Both Dr. Dillman and Dr. Beeler bring to light a unique, human-specific symptom: swimmer’s itch. This skin condition is a reaction to flatworms (sometimes called “blood flukes”) burrowing into the skin. Symptoms include a rash that looks like a blanket of pimples and blisters, but it generally goes away within a few days. Swimmer’s itch isn’t just a human problem: if you have swimmer’s itch after swimming in any body of water, whether freshwater or saltwater, your dog could’ve also come into contact with flatworms.

How Flatworms Are Diagnosed in Dogs

“I get a lot of questions like, ‘We were just in the river today. Should I take my dog to the vet today to see if it got infected?'” Dr. Dillman says. “That’s not how the test works. It takes around 12 weeks for the parasite to be shedding eggs in your dog. So if you go to the river, I wouldn’t take your dog the next week to the vet as a preemptive measure because the test will come up negative, even if the dog is infected.”

Golden Retriever at the vet getting tests.
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Dr. Beeler notes that a veterinarian can best detect H. americana in one of two ways. One method involves sending a fecal sample to Texas A&M University, where it will go for PCR testing. This exam detects genetic material from a specific organism in a given sample.

The other method involves conducting a test called “fecal sedimentation.” This examination can sometimes detect the parasite’s eggs in the dog’s feces, but it’s not always accurate.

Are The Flatworms Dangerous to People?

While H. Americana spreading to southern California is certainly alarming, there’s good news that comes with the bad. For starters, flatworms do not attach to humans’ intestinal walls the same way they do to dog’s intestinal walls. While they can burrow into a person’s body and cause swimmer’s itch, they can’t establish the same kind of parasitic relationship.

“I just want to emphasize that, as far as we know, people are safe,” Dr. Dillman says. “I mean, this parasite is considered endemic to the United States and has been around for a very long time. We’ve never had a report or a case of a person being infected, and so, as far as we know, people are totally safe.”

Dr. Dillman also notes that there don’t seem to be other new parasites in the region, according to his research. “I’m very interested in this particular parasite,” he says, “and I will likely continue my research in this area. But no, I’m not aware of other emerging parasites of dogs in this area.”

Wirehaired Pointing Griffon wading in water outdoors.
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How to Help Your Vet Diagnose Flatworms

To completely avoid flatworms, steer clear of bodies of water containing infected snails. You and your dog don’t have to completely write off swimming in natural bodies of water, as long as you monitor their symptoms in the weeks afterward.

Dr. Beeler recommends keeping a travel log. “It’s so important to share with your veterinarian if your dog has been spending time in lakes, ponds, or rivers where this parasite has been found,” she says. “Nothing extensive, but you know, ‘took a trip up to the lake, dog swimming in lake.’ The parasite might not be there, and you might not need to worry about it at all. But if your dog later develops an illness, it is so helpful and important to know the timing and the location.”

If you’re getting a dog, try to find out where they’ve been as well. “It’s important to find out all the places that dog has traveled — and even if you don’t know, then share that information with your veterinarian.” Even one detail regarding your dog’s health and travel history can help your veterinarian diagnose potentially life-threatening conditions, including flatworms.
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