Fish tissue samples showed the parasite that caused a die-off of whitefish in the Yellowstone River this summer also exists in the Jefferson River, indicating that the parasite is already present in at least one place outside of the Yellowstone drainage.

“It means that it’s probably spread farther than we knew, which doesn’t surprise me,” said Travis Horton, regional fisheries manager for Montana Fish, Wildlife and Parks.

FWP plans to test fish tissue from several rivers across the state to learn as much as they can about the reach of the parasite. Horton said crews are gathering samples from the Madison, Gallatin, East Gallatin and Big Horn rivers. The Great Falls Tribune reported that tests are also being done on tissue samples from the Missouri and Smith rivers.

These tests come after FWP documented more than 4,000 dead whitefish in the Yellowstone River. Tissue samples sent to a federal fish health lab in Bozeman confirmed that the parasite Tetracalsula bryosalmonae was the culprit.

It can cause proliferative kidney disease, which is known to be one of the most devastating diseases for trout and whitefish. Idaho’s Snake River system has seen whitefish die-offs in recent years because of the parasite, and it has been found twice before in Montana — in the Madison River tributary Cherry Creek and a reservoir in the Smith River drainage. Horton said no fish deaths were documented in either case.

Fish are more susceptible to the parasite when rivers run low or hot, conditions the Yellowstone saw this summer and that the Jefferson River sees almost annually.

Horton said dead whitefish were seen in the Jefferson River near Sappington Bridge in the summer of 2015, but not at the scale of what was seen on the Yellowstone this year. Because whitefish are more susceptible to poor river conditions than trout, they didn’t think anything strange was going on and didn’t test fish for the parasite.

“That reach of river is chronically low and hot,” Horton said. “At the time we just deduced it was temperature and stress.”

After the die-off in the Yellowstone, officials decided to test fish in the Jefferson. Results confirmed the presence of the parasite in at least one whitefish and two rainbow trout sampled from the Jefferson River in the same area.

That doesn’t necessarily mean the fish in 2015 died because of the parasite, but it does confirm that it is present in the river system.

No more dead fish have been seen in the river this year, Horton said, which might be a sign that the fish there are building some sort of immunity to it.

“Every river is going to act differently,” Horton said.

Patrick Byorth, the director of Trout Unlimited’s Montana Water Project, said he wasn’t surprised the parasite had been found outside of the Yellowstone River system. It finds easy transportation between river systems on gear worn by anglers or boats who hop between rivers, which is why FWP urges people to clean and dry their equipment. But it can also spread via wildlife.

“I think waterfowl spread this particular parasite fairly easily,” Byorth said.

In response to the fish kill on the Yellowstone, FWP closed 183 miles of the rivers plus its tributaries to all recreation on Aug. 19. Officials said it was necessary to reduce stress on the fish and reduce the risk of spreading the parasite. The upper and lower sections of the closure have been lifted, but a large section of the river that flows through the Paradise Valley and the length of the Shields River are still closed to all recreation.

The Jefferson River has been closed to fishing since early August because of low flows and high water temperatures.

Michael Wright can be reached at mwright@dailychronicle.com or at 582-2638. Follow him on Twitter @mj_wright1.