Fish farmers may soon have a better way to treat a serious parasitic disease of catfish, known as ich or whitespot. Scientists found that potassium permanganate quickly stops the parasite in its tracks.

Ichthyophthirius multifiliis is a protozoan parasite. In its microscopic free-swimming stage called a theront, it burrows into skin or gills of fish to feed on mucus and tissue. It grows to the next stage, a trophont, and creates a white spot — pustule — resembling a salt grain. When the mature trophont falls from the fish, it forms a cyst where thousands of theronts develop. They burst out and resume the cycle — all in a week or less.

Agricultural Research Service aquatic toxicologist David L. Straus exterminated theronts in infested well water in less than an hour, using doses of potassium permanganate as small as 1 part per million. With 1.25 ppm he stopped an outbreak of ich — a serious disease of fish in farm ponds — on channel catfish.

He concluded that potassium permanganate, though more expensive than the favored copper sulfate, would be effective and less toxic to fish in soft water. Potassium permanganate may also work against other fish parasites.

U.S. fish farmers lose an estimated $50 million a year to diseases. Few drugs to fight fish diseases have been approved by the U.S. Food and Drug Administration. But experiments by Straus and others at ARS' Harry K. Dupree Stuttgart National Aquaculture Research Center, Stuttgart, Ark., provide information FDA needs to consider approval of potassium permanganate to treat fish.

After exposing catfish to doses up to 2 ppm for 12 weeks, the scientists found no elevated manganese in fish flesh or liver and concluded that such treatments would pose no hazard to people who eat catfish.

ARS scientists also provide FDA with information on copper sulfate as a therapeutic agent for farm-raised fish. FDA restricts use of these agents to diseases defined by an approved label claim and supported by animal, environmental and human food safety data.