The U.S. Food and Drug Administration announced Thursday that it is banning the import of five species of fish from China until the shipments are proven to be free of contamination. Cited for residues from drugs that are not approved for use in farm-raised fish in the United States, the five species under alert are catfish, eel, shrimp, basa (similar to catfish) and dace (similar to carp).

The contaminants include the antimicrobials nitrofuran, malachite green, gentian violet and fluoroquinolone. With long-term exposure, nitrofuran, malachite green and gentian violet have been shown to cause cancer in lab animals; the use of fluoroquinolone in food-producing animals can result in antibiotic resistance. Chinese authorities also prohibit the use of nitrofurans and malachite green in aquaculture.

"The levels of contaminants are low, but could cause serious health problems if consumed over a long period of time," said Dr. David Acheson, the FDA's assistant commissioner for food protection, in a telephone press conference. "[The] FDA is taking this action to protect the public health of the American people." But, Acheson added, the low levels of contaminants found in the farm-raised fish the FDA tested means that there is no imminent threat to public health.

"We're not asking for this product to be withdrawn from the market or for people to take this out of their freezers and throw it away," Acheson said. "This is a long-term health concern. It is not an acute health concern."

China is the third largest exporter of farmed fish, accounting for 70 percent of the total produced, and it is the third-largest exporter of farmed fish to the United States, said Margaret O'K. Glavin, FDA's associate commissioner for regulatory affairs. The FDA currently inspects 5 percent of seafood from China, she said.