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; and signs of liver disease, which can include the other symptoms and 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.

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