New York Times News Service
WASHINGTON -- The Food and Drug Administration is moving against a widely used, powerful animal drug because it contaminates supplies of milk and pork.
The FDA could either ban the drug outright or impose dosage limits that would make it impractical to use.
The government's program to eliminate the drug, sulfamethazine, from the food supply is not yet official.
But it is prompted by new results from animal studies providing clearer evidence than ever before that sulfamethazine causes cancerous tumors of the thyroid glands of rats and mice.
The results come 14 years after scientists at the federal agency and the Department of Agriculture identified illegally high residues of the drug in pork as a threat to consumers.
``We are proceeding on the basis that this compound causes tumors,'' said Dr. Gerald B. Guest, director of the Center for Veterinary Medicine at the FDA.
``We have several steps to complete, but I expect that in six months we will propose to withdraw this drug from the market. You can hang out the black crepe for sulfamethazine.''
Manufacturers of sulfamethazine, which has annual sales in the United States of about $60 million, said they disagree with the government's interpretation of the animal studies and would oppose the change in policy when it is made official, probably by next spring.
Representatives of the milk and pork industries and most veterinarians say that sulfamethazine is safe at the levels now found in food.
``Sulfa drugs have been used in human medicine since the 1930s,'' said Fred Gutzmann, a marketing manager at American Cyanamid Co. in Wayne, N.J., one of the largest distributors of medicated mixtures for feed and water that contain sulfamethazine.
``The effect that is seen in the rodent studies is something that would not occur and does not occur in humans even with direct use under long-term therapeutic use. We don't think there is any science that justifies the FDA removing this product from the market.''
A ban would make livestock farming more difficult because the drug is effective in fighting disease, they said. They added that banning sulfamethazine would not cause food prices to rise, as alternate drugs are available.
Small amounts of sulfamethazine are mixed into feed or water in many of the 90 million pigs raised each year in the United States to speed growth and prevent respiratory diseases.
Dairy farmers most often administer the drug to cows in the form of a large pill, or bolus, to treat diseases of the lungs and cure hoof infections.
The government bars sulfamethazine residues in pork products from exceeding 100 parts per billion and prohibits any level of the drug in milk.
Sulfamethazine, an anti-bacterial compound, was one of a handful of animal drugs introduced in the late 1940s and 1950s that helped to radically alter the structure of American livestock farms.
Because it is inexpensive and effective against an array of diseases in hogs and cattle, sulfamethazine made it possible for farmers to improve the economic efficiency of raising meat and milk by closing down outdoor corrals and confining large numbers of infection-prone animals in production houses and feeding pens.
The issue raised by the animal drug industry is whether sulfamethazine is itself a carcinogen or produces physiological changes in rodents that produce cancerous tumors.
Steve Kimbel, a spokesman for the Animal Health Institute, the trade group for the $2.5 billion-a-year animal drug industry, said that some scientists who have reviewed the new studies say that high levels of sulfamethazine administered to the rodents caused the pituitary glands to produce higher levels of a hormone that stimulated the thyroid, producing the cancerous lesions.
Kimbel said the industry was prepared to argue in federal hearings that rodents are prone to such lesions when exposed to high concentrations of sulfamethazine.
But when exposed to low levels of the drug, the lesions do not appear in the rodents and they would not appear in people, said Kimbel and other industry executives.
The long regulatory history of sulfamethazine is the latest illustration of the difficulty federal agencies are having in untangling the web of scientific, political and economic issues before toxic substances can be eliminated from the food supply.
In almost every case, the most difficult conflict to resolve is the actual hazard posed by the substance versus its usefulness to farmers.
The conflict is a little easier for the Food and Drug Administration because the Food, Drug and Cosmetic Act, the law that regulates animal drugs, directs that if a substances is determined to be unsafe and is found in food it must be withdrawn from use.
In 1975, the Department of Agriculture and the Food and Drug Administration discovered that 15 percent of all pork sold in the United States contained residues of sulfamethazine that exceeded the federal safety limit.
The government began a program to warn farmers they were in violation of the law and to teach others how to be more careful in using the drug.
The problem persisted in the 1980s, although violations found in pork samples that were tested were reduced to 1 percent this year from 5.3 percent in 1985, according to the FDA.
The declining levels, though, were not low enough to satisfy the Japanese, who threatened last year to halt purchases of American pork because of sulfamethazine residues.
Japan and the United States began a program to screen pork to be shipped to Asia.
The lower levels did not calm state authorities.
In Iowa, the center of the nation's $9.1 billion pork industry, 47 of the 41,000 farms that raise hogs were investigated by the State Department of Agriculture after the discovery of illegal amounts of the drug in pork.
In March 1988, the FDA was jolted again when it discovered sulfamethazine residues in five of 49 samples of milk collected from 10 cities.
Despite education campaigns and warnings from the government, some dairy farmers continue to sell the milk of cows treated with the drug even though it is illegal to have any level of sulfamethazine in dairy products.
The problem was halted after the farmers stopped adding the milk of the cows being treated to the regular supplies.
California, the nation's second largest milk producer behind Wisconsin, has been one of the most aggressive states in regulating violators.
On Oct. 10, state inspectors discovered contaminated milk on a large dairy farm in Riverside County near Los Angeles and ordered the farm to destroy 3,300 gallons.
It was the third time in a year that the California Department of Food and Agriculture had ordered a farm to dump contaminated milk, said Leon Jensen, the state's dairy program coordinator.
Health specialists said sulfamethazine can produce minor allergic reactions in a small percentage of the population exposed to it. The more serious problem are studies that indicate the drug could cause cancer, Guest said.
``I don't think there is a serious direct public health threat,'' Guest said.
``There has been a long history of use in man,'' Guest said. ``But there is a study of sulfamethazine that resulted in tumors and that's a problem. We have a law that says if you have a drug that causes tumors in animals you have to treat it ultraconservatively.''
The latest test results grew out of cancer studies begun by the FDA in 1980 at the National Center for Toxicological Research in Jefferson, Ark.
The long-term studies involved feeding rats and mice varying doses of sulfamethazine over two years to determine the drug's potential to cause tumors.
In 1981, the first studies were halted after researchers discovered errors in the dosages fed to the rodents.
A year later, new studies were begun that were completed and published in April and May.
Guest said the evidence is now strong enough to remove sulfamethazine from the market but government regulations require a series of hearings and public notices before that final step is taken.
The agency is waiting for the final assessment of the risk posed by the drug, which Guest said may be ready by February.