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Parasite that’s deadly for dogs now confirmed in California

For five years, an elusive tormentor of animals hid from authorities in east Riverside County as 10 dogs were injured and another killed.

The victims hailed from Los Angeles, Riverside and Orange counties, while the suspect maintained a strong alibi: It had never visited California.

It took authorities years to discover the one key detail linking the abuser with those abused: time spent at the Colorado River at or near the California-Arizona border town of Blythe.

That’s where professor Adler R. Dillman, chair of UC Riverside’s Department of Nematology (study of roundworms) and an expert on parasitology, and a team of researchers and students recently unmasked their culprit.

A parasite called Heterobilharzia americana, a flatworm commonly referred to as liver fluke, was behind the illness of the 11 dogs.

The parasite normally makes its home in Texas and in the South. Its discovery in California, in the Colorado River, marked its first recorded appearance in the area.

Dillman said he was part of a team that made four trips to the Colorado River between March and August of 2023 to collect more than 2,000 snails of two distinct species known to transmit the worms. They used DNA to identify both snails, Galba cubensis and Galba humilis, and the flatworm.

That squad included UC Riverside postdoctoral researcher Anil B. Baniya and graduate students Connor Goldy and Perla Achi. The trio, Dillman and others published their findings Wednesday in the scientific journal Pathogens.

“I was super excited to finally provide an answer as to what was happening with those dogs,” Dillman said in a phone interview on Wednesday evening. “We suspected it was this parasite, but once we finally confirmed it, I was jumping up and down with excitement.”

Dillman and various public health agencies dedicated to humans or animals have been trying to raise awareness of the parasite and its harm to dogs.

The parasite can cause canine schistosomiasis, an illness that affects the liver and intestines of dogs, according to Dillman.

Los Angeles County Department of Public Heath veterinarian Emily Beeler said symptoms include:

  • loss of appetite
  • vomiting
  • diarrhea
  • profound weight loss
  • signs of liver disease, which can include the above symptoms and, among others, increased thirst, the need to pee and yellowish eyes

“If your dog has these symptoms after swimming in the Colorado River,” Beeler said, “it’s a good precaution to ask your veterinarian for a simple fecal test.”

The health department notes that the disease in dogs has a very gradual, “insidious onset.”

The department issued one of its first warnings in March 2019 after two South Bay dogs tested positive for the parasite after returning from a trip to the Colorado River in 2018.

Eventually, the number grew to 11 dogs from five households across three counties infected between 2018 and 2023 with canine schistosomiasis, with seven from Orange County.

Some of the dogs had shown signs of illness, others were tested as a precaution after their owners realized they also had taken their pets to the river. In total, nine were tested through fecal exams, while two underwent liver biopsies, according to the L.A. County Department of Health.

Although six dogs were asymptomatic, five were stricken by vomiting, diarrhea and weight loss, according to the department.

One of those five dogs died, while the other four recovered thanks to treatment that included antiparasitic drugs.

It was discovered through interviews that all of the infected dogs had visited the Colorado River near Blythe.

In January 2023, Dillman said Beeler called and asked him to test those waters for the parasite.

Dog owners told him they had visited two primary places, Quechan Park and Hidden Beaches Resort.

Dillman and his team’s first discovery was that the two species of snails that carry the parasite had migrated to California.

How they arrived remains a mystery.

“We don’t know how these snails reached California,” Dillman said. “It’s a bit complex.”

The professor noted that there was an outbreak of canine schistosomiasis in Moab, Utah, in 2018. Ultimately, 12 dogs living near a man-made pond with those snails present tested positive. Dillman said the location of that pond was only a couple of miles from the Colorado River.

“Researchers didn’t check to see if the snails were in the river,” he said. “It’s possible they’re there.”

Along with dogs, the parasite has been known to infect raccoons and horses.

The disease cannot be transmitted between dogs and humans, though people may develop a self-limiting rash (meaning it goes away on its own without treatment) after swimming or contacting contaminated waters, according to Sara Strongin, chief veterinarian at the Riverside County Department of Animal Services.

The snail excretes a juvenile form of the worm, which then has 24 hours to find a mammal host to infect or die, according to Dillman.

The goal of the worm is to reach the intestines of a host, where it continues to develop into an adult. There it mates and releases thousands of eggs, Dillman said. Those eggs clog the lungs, spleen, liver and hearts of hosts along with immune cells called granulomas that fight them.

Eventually, organ tissue stops functioning, Dillman said.

If found in time, however, antiparasitics like praziquantel and fenbendazole can help fight off an infection.

“The goal here,” Dillman said, “is to get awareness out and make sure dog owners know what signs to look for.”

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