Another 63,000 Lahontan cutthroat trout were destroyed at a federal fish hatchery Saturday after a new outbreak of the disease furunculosis, prompted last month's killing of 350,000 of the endangered game fish there. The latest action came after attempts to treat the cutthroat with antibiotics failed at the Lahontan National Fish Hatchery south of Gardnerville.

In early February, the hatchery destroyed 350,000 of the fish in an effort to stop the spread of the disease. Another 80,000 fish had died before the action.

Biologists had been struggling since November to treat the fish afflicted with furunculosis, a bacteria [infection caused by bacteria] found naturally in northern Nevada's watersheds. "We thought we had the disease handled in February, but apparently some fish didn't take to the antibiotics the first time and the disease stayed," Randi Thompson, spokeswoman with the U.S. Fish and Wildlife Service, said.

Coupled with the earlier deaths, the disease claimed nearly 500,000 of the hatchery's fingerling cutthroat in recent months. The action left the hatchery with virtually no cutthroat until next month's arrival of 1 million eggs from Pyramid Lake, 30 miles northeast of Reno. The hatchery raises about 600,000 cutthroat for annual release into the region's Pyramid and Walker lakes and Truckee River.

Saturday's action was taken to help save a crucial brood stock of about 100 Pilot Peak cutthroat. They are kept in separate ponds and have not been affected by the disease, Thompson said. In all, more than 400,000 cutthroat were suffocated in recent weeks by an infusion of carbon dioxide pumped into shallow water-holding tanks. Biologists theorize the bacteria destroying the fish's organs was carried to the hatchery by birds from the nearby east fork of the Carson River. Officials plan to begin sterilizing the hatchery this week to prevent another outbreak of the disease.

The 63,000 carcasses will be taken Monday by Reno Rendering to be turned into fertilizer and feed, Thompson said. Lahontan cutthroat are listed as a threatened species under the Endangered Species Act. Though protected, they are allowed to be caught by anglers because of hatchery production programs.

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Marjorie P. Pollack, M.D.

[Furunculosis is caused by Aeromonas salmonicida, a nonmotile, pigment-producing rod originally described as the cause of a septicemic disease, potentially producing high mortality. In the acute form, hemorrhages are found in the fins, tail muscles, gills, and internal organs. In more chronic forms focal areas of swelling, hemorrhage and tissue necrosis develop in the muscles. These lesions progress to crateriform abscesses discharging from the skin surface (furuncles). Liquefactive necrosis occurs in the spleen and kidney. Diagnosis is made by isolating a pure culture of the organism. Avoidance is the most effective prevention because A. salmonicida is an obligatory fish pathogen. - Mod.TG]

[The use of fish contaminated with A. salmonicida as feed is of interest. There is a published report (Frye, F. L., An unusual epizootic of anuran aeromoniasis, JAVMA 187:1223-4, 1985) of an epizootic of A. salmonicida, normally a pathogen of teleosts, in a colony of African clawed frogs (Xenopus laevis). In this report, the frog colony had been fed several kilograms of fresh-caught salmon that had been added to the beef heart-vitamin-mineral mix that was usually fed to the frogs. The salmon was postulated to be the source of the epizootic. Frogs are commonly infected with A. hydrophila, not A. salmonicida. -Man. Ed. DS]

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