What is ‘liver fluke’, a dog-killing parasite that has spread to western US?

A dog-killing flatworm parasite has been discovered in southern California’s Colorado river, scientists from University of California, Riverside have confirmed in a new study published in journal Pathogens. The worm was previously believed to be largely confined to the southeastern United States but the new research shows it has spread to western parts of the country as well.

Heterobilharzia americana, commonly called the liver fluke parasite, was previously predominantly observed in Texas and other Gulf Coast states. However, it has never been documented to extend as far west as its current reported range.

Since 2019, a total of 11 dogs across three counties in California have been diagnosed with this disease, resulting in one fatality. Health officials are optimistic that by raising awareness, they can mitigate the risk of additional infections and fatalities.

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

Also known as a ‘liver fluke’, the parasite enters the dog through the skin when canines wade or swim in freshwater inhabited by infected snails, the intermediate host. The worm gets into the veins of the intestinal lining and that’s where it develops into an adult and mates. It then lays eggs that get into the lungs, spleen, liver and heart, causing canine schistosomiasis, which affects dogs’ livers and intestines.

This parasite is extremely dangerous, causing severe debilitation. Although treatments are available for infected dogs, these options do not always produce positive results and infections may eventually necessitate euthanasia.  Previously thought to be a rare occurrence, the new study shows that it is far more common than previously thought. 

Following the reports of infection in local dogs, scientists conducted an investigation of the waters in Riverside County. Given that the infection is typically spread through snails, the research team gathered over 2,000 snails from the banks of the Colorado river. They then analysed the DNA of these snails to determine the presence of both the snails and the flatworm.

The researchers found that two varieties of snails could transmit the parasite — 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’s statement added.

Following its transformation within a snail, the flatworm emerges with the objective of locating a mammal to infect. During this phase, it can only sustain itself independently for approximately 24 hours. If a dog or raccoon is in the water or consuming it, they become susceptible to infection.

H americana is not known to cause 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.