WASHINGTON, April 1, 2015 - The last arsenic-based animal drug is being pulled from the market under pressure from the Food and Drug Administration.

In a March 25 letter to the agency, Zoetis Animal Health said it would suspend sales of the drug nitarsone, trade name Histostat, by this fall. The drug is primarily used in turkeys for treatment of blackhead disease, or histomoniasis, a major killer of the birds.

The company said it was acting at FDA's request and that the timetable would give the turkey industry time to prepare “for a future in which no therapeutic product will be available for management” of the disease.

FDA has been working to get arsenic-based animal drugs off the market because of concerns to human health. The concern is that organic arsenic, the less toxic form of arsenic that is used in the drugs, can turn into inorganic arsenic, a known carcinogen, according to scientific reports.

FDA withdrew approvals for three other arsenic-based animal drugs -- roxarsone, arsanilic acid and carbasone -- in February 2014. Roxarsone was pulled from the market in 2011 after an FDA study found higher levels of inorganic arsenic in chickens fed the drug than in other chickens.

The National Turkey Federation expressed disappointment at the Zoetis decision, which the group said would “lead to additional suffering in flocks afflicted with the disease."

“Our members are particularly disturbed by the fact that this decision was made even though there are no studies that demonstrate harmful residues in turkeys from use of this product. The decision was based solely on a study of a similar, but chemically different, drug conducted in a different species,” the group said.

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In 2013, FDA denied a petition from the Center for Food Safety and the Institute for Agriculture and Trade Policy to ban all four drugs, saying it had not completed several studies and finished evaluating other information.

Last fall, Food Animal Concerns Trust and other advocacy groups appealed to FDA to either ban nitarsone or prove that it wasn’t causing dangerous residues in turkeys.

“Intentionally adding likely carcinogens to your food supply is never a smart idea, so we are happy to see the FDA and Zoetis taking this step although we would have liked to see it much sooner,” FACT said Wednesday. “After the end of the year, people enjoying a turkey sandwich or sitting down to a family gathering will no longer have to worry whether they are consuming arsenic with their meal.”