Highly pathogenic virus detected in fish for the first time in California

: A deep-sea barileye fish (Macropinna microstoma) observed with MBARI’s remotely operated vehicle (ROV) Doc Ricketts in the outer Monterey Canyon at a depth of approximately 600 meters during an expedition aboard MBARI’s flagship research vessel David Packard. The sequence opens with research specialist Shannon Johnson excitedly calling out her observation of a barreleye fish to MBARI submersible pilots during the ROV’s ascent to the surface. Credit: © 2026 MBARI

NEVADA COUNTY, CA– A highly pathogenic virus that infects fish was discovered in a large private lake in Nevada County, California, after more than 200 fish died in September.

The virus, called largemouth bass virus, was identified in sunfish and largemouth bass during necropsies performed by the California Department of Fish and Wildlife (CDFW) Fish Health Laboratory.

The source of the infection has not been found, but CDFW said the virus can be introduced through contaminated equipment, infected baitfish, stocking of infected largemouth bass and introduction of other infected carrier species.

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Kayak anglers Nick Raymond, left, of Colonie and Joe Villani of Albany chase largemouth bass at Six Mile Waterworks on Tuesday, May 29, 2018 in Albany, NY.

(John Carl D’Annibale/Albany Times Union/Getty Images)

This is the first time largemouth bass virus has been detected in California.

The largemouth bass virus poses no risk to humans or warm-blooded animals, so anyone who eats an infected fish or swims in waters infected with the virus cannot be affected.

The CDFW recommends that fish be properly prepared to prevent other foodborne illnesses.

“This is a sobering finding given the extent of largemouth bass’s distribution in California, its popularity as a sport fish and its economic importance to our rural communities,” said Jay Rowan, CDFW Fisheries Branch Chief. “We ask all Californians to help prevent further spread by not moving fish, live or dead, from one body of water to another and by cleaning, emptying and drying their boats and fishing equipment after use.”

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The virus was first detected in Florida in the 1990s. Since then, it has spread throughout the country, the CDFW said.

Largemouth bass.

(Dennis Anderson/Star Tribune/Getty Images)

Largemouth bass virus is very difficult to eradicate once a population of largemouth bass is infected.

Outbreaks can last days or weeks before resolving on their own. There is no treatment or vaccine available to prevent an outbreak.

“Typically, an outbreak will result in the mortality of a high percentage of larger largemouth bass in the infected body of water,” CDFW said. “Surviving fish eventually develop resistance to the virus over time and populations can recover. However, larger fish populations can have significant impacts for years after an initial outbreak.”

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Signs of the virus in infected fish include: skin sores or ulcers, dark skin, distended body cavity, bulging eyes, redness, lethargy, irregular swimming behavior, surface swimming, and difficulty submerging.

09/16/04: A largemouth bass fish samples a lure on the end of a line in the Bass Reservoir by Lee Morris at the Charles County Fair Thursday.

(Mark Gail/The Washington Post/Getty Images)

The CDFW said stressful events and environmental conditions such as high summer water temperatures, poor water quality and the intensity of fishing in a body of water can activate the disease in infected fish.

Mysterious strange-looking deep-sea fish caught on camera during research expedition

Anglers and people using bodies of water recreationally should always assume the virus is present and follow these instructions:

Never touch or move a live or dead fish unless it is to be eaten immediately.
If you clean a fish, do it at the body of water where it was caught or at home and throw all parts of the fish into the trash.
Place leftover bait in the trash.
Any boat or boat used must be emptied of all water before leaving the area. This includes water from bait tanks, holds, livewells, wash lines, etc.
Handle the fish you intend to release with care. Stress appears to increase mortality of fish infected with the virus.
Any fishing or recreational equipment that comes into contact with water or mud after use should be rinsed, cleaned or dried before leaving the area.
Wash the equipment in hot water and allow it to dry completely before using it in another body of water.