Largemouth Bass Virus update

HARRISBURG, Pa. — Largemouth Bass Virus (LMBV) never really went away. As kills related to its presence diminished, biologists predicted that it would persist, and it has. From time to time during the past decade or so, one state or another has reported a fishery testing positive, but since 2005, rarely were mortalities involved. A notable exception occurred at Kerr Lake on the Virginia-North Carolina border in 2010.

But in 2018 all that began to change. Call it LMBV 2.0, with the headlines being that it recently contributed to a smallmouth bass kill in Michigan and likely played a major role in long-term decline of the bronzeback fishery in Pennsylvania's famed Susquehanna River.

Previously, researchers knew that multiple species of fish could be infected, but they had seen little evidence it could be lethal to them.

"For years, smallmouth bass were thought to be carriers of the virus but failed to show the signs characteristic of the disease observed in largemouth bass," said Geoff Smith, Susquehanna River biologist for the Pennsylvania Fish and Boat Commission (PFBC). "However, this study determined that exposing juvenile smallmouth bass to LMBV caused high mortality, with different signs of disease than those observed in largemouth bass."

Previously, scientists hadn't even considered LMBV as a possible contributor to the faltering smallmouth fishery. They focused on environmental factors, including pollution and diminished water quality.

But then Smith and others in multiple agencies decided to go in a new direction.

"Juvenile smallmouth bass experimentally infected with the largemouth bass virus exhibited internal and external clinical signs and mortality consistent with those observed during die-offs," their report revealed.

"Microscopically, infected fish developed multifocal necrosis in the mesenteric fat, liver, spleen and kidneys. Fish challenged by immersion also developed severe ulcerative dermatitis and necrotizing myositis and rarely panuveitis and keratitis."

Rising water temperatures is what triggered lethal consequences for young-of-the-year smallmouth carrying the virus. At 52 degrees, they showed no signs of distress. At 73 degrees, 10 percent of the fish died. At 82, half died.

They didn't die of the disease itself, but rather from secondary infections manifested by such visual signs as lesions and sores.

"Basically, this is the smoking gun," said Leroy Young, who served as Director of PFBC's Bureau of Fisheries until 2016. "This is the Eureka moment for this problem."

In Michigan, meanwhile, LMBV was linked to a smallmouth bass kill during 2018 in the northern portion of the Lower Peninsula.

“The Largemouth Bass Virus likely compromised the immune system of smallmouth bass in Beaver Lake, causing secondary bacterial infections to become more lethal and allowed the virus to be a direct factor in the fish kill,” said Gary Whelan, Fisheries Division research manager for the Michigan Department of Natural Resources.        

“Because these latest detections are at the northern edge of where LMBV has been found, we may see different responses than what was documented in southern Michigan.”

The agency also confirmed presence of the virus in three more northern lakes, Beaver, Avalon, and Cedar, with a largemouth die-off in the latter attributable to it. Prior to that, the disease had been documented only in the southern part of the state, where it contributed to several kills of adult largemouth during the early 2000s.

As biologists began to research LMBV about 20 years ago, they initially thought it was confined to fisheries in the South and lower Midwest, with warm water a key contributor to the virus turning lethal. But then fisheries in Indiana and lower Michigan tested positive just as the disease's national impact began to diminish.

Coincidentally, warm water in spring and summer led to the Susquehanna River's world-class productivity. Now it appears that it also contributed to its decline, as lab tests revealed. Fortunately, though, the fishery has begun to rebound, just as many did from Texas to Indiana following LMBV outbreaks during the late 1990s and early 2000s.

Sadly for anglers, some exceptions do exist, including Illinois' Mill Creek Lake, once noted for big fish. "We sampled it a couple of years ago, and it's still positive," said Mike Mounce, a fisheries biologist with the Illinois Department of Natural Resources. "The fishery never has recovered. We've got the numbers, but not the size."

And with these recent discoveries, should we now be concerned about LMBV's impact on other smallmouth fisheries, especially since climate is moderating and waters warming?

"This increases the opportunity of exposure of naïve populations of both smallmouth bass and largemouth bass to LMBV and subsequent disease episodes," Smith said.

"Fisheries managers and anglers in areas that were not subject to LMBV in the past need to be mindful of the presence of LMBV in their regions or areas they visit, to limit inadvertent spread to previously unaffected populations and waterbodies."

Anglers can help minimize the spread of LMBV virus and its activation into a lethal disease by doing the following:

  • Clean-Drain-Dry boats, trailers and other equipment thoroughly between fishing trips to keep from transporting LMBV and other undesirable pathogens and organisms from one water body to another. Research has determined that the virus can live for several hours in water in livewells, bilge or drains.
  • Never move fish or fish parts from one body of water to another.
  • Never release live bait into a fishery.
  • Handle bass as gently a possible if you intend to release them.
  • Stage tournaments during cooler weather, so fish caught will not be so stressed.
  • Report dead or dying fish to state wildlife agencies.