Officials are on alert in California and Oregon following recent reports of deadly parasites in popular rivers that are killing salmon.

According to the US Fish and Wildlife Service, there has been an increase in the presence of the Ceratonova shasta parasite in the well-known Klamath River, which flows into Southern Oregon and Northern California.

As a result, numerous Chinook salmon have turned up dead in large groups along the sides of the Scott and Trinity rivers, tributaries of the Klamath River.

The Klamath River is one of three rivers that bisect the Cascade Mountain Range, beginning in Oregon’s high desert interior before cutting through the Klamath Mountains and entering the Pacific Ocean in Northern California, according to the National Wild and Scenic Rivers System.

During the 2026 outmigration season, officials collected numerous salmon from March 17 through May 12 to measure the parasite load in the fish.

Gilbert Myers, a fisheries technician with the Yurok Fisheries Department, pulls fish traps out of the Klamath River on June 9, 2021, in Weitchpec, California. The Yurok Tribal Fisheries Department has been monitoring a drought-caused fish kill of juvenile salmon brought on by an outbreak of Ceratonova shasta. Getty Images

As a result, officials discovered that 46% (319) of the 696 fish collected tested positive for the deadly parasite.

This is a large jump from 2025, when only 22% (39) of salmon exhibited these conditions.

The parasite can affect several freshwater species and typically makes fish appear darker, with swollen abdomens.

Two dead juvenile salmon sit in a bucket after being removed from a Yurok Fisheries Department rotary screw trap on the Klamath River. Getty Images

This parasite is transmitted to fish through the gills by infectious actinosporean tetractinomyxon stages in the water, shed by parasitized freshwater polychaete worms, according to the Alaska Department of Fish and Game.

Despite the scary news, there are no human health concerns associated with the Ceratonova shasta parasite.