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Meat inspectors "just say no " to drugs residues.

Meat Inspectors "Just Say No" to Drug Residues

New high-tech probes may speed up drug testing of meats and poultry we eat. Called monoclonal antibodies, the probes seek out and bind to residues from a type of drug used to treat farm animals. Ranchers rely on the drugs, known as benzimidazoles, to protect their cattle, sheep, pigs, chickens, and goats from parasitic worms. If unchecked, the worms will damage an animal's lungs, liver, or gastro-intestinal tract.

To make sure that these potent medicines don't end up in meat and poultry, USDA's Food Safety and Inspection Service (FSIS) randomly samples slaughterhouse carcasses, looking for benzimidazole residues.

At meatpacking plants around the country, FSIS inspectors collect specimens, then ship them to chemists at the agency's St. Louis laboratory for analysis.

Once at the lab, the specimens are scrutinized using a high-performance liquid chromatograph - standard equipment in many laboratories. But tomorrow the chemists might complement that approach with monoclonal antibody assays.

Agricultural Research Service chemist David L. Brandon says the new assays hold the promise of "dramatically speeding up testing without compromising accuracy."

Brandon and colleagues Anne H. Bates, Ronald G. Binder, William C. Montague, Jr., Elizabeth H. Sweet, and Robert E. Wilson at Albany, California, have developed four distinct monoclonal antibodies. Each will rapidly and inexpensively detect a specific benzimidazole, such as fenbendazole, in concentrations as low as 1 part per billion. That's equivalent to 1 second in 32 years. They aim for antibodies to four additional benzimidazoles this year.

The potential ease and convenience of the new assays might pave the way for boosting the number of samples checked each day by FSIS chemists in the laboratory.

Once individual tests are complete, Brandon envisions reworking them into a multi-drug screening procedure, capable of detecting all key benzimidazole compounds at once. With further work, this drug-screening kit of the future might prove rugged and convenient enough for packinghouse inspectors to check carcasses on the spot. For that to happen, however, the test would have to be exceptionally quick, so that it won't stall inspection and grading.

The agency's 1989 laboratory screening of nearly 2,000 samples of domestically produced meat and poultry and 1,173 imported animal carcasses revealed no violations of federally approved safe limits for benzimidazoles. Despite this clean record, FSIS keeps benzimidazoles on the list of chemicals it monitors nationwide because of the potential side effects that high doses could cause in humans.

Those problems show up in people - notably in developing countries - who sometimes take benzimidazoles in high doses as a prescription drug to kill internal worms. Medicinal doses for those patients can cause dizziness, nausea, vomiting, anorexia, diarrhea, drowsiness, and headaches.

But FSIS screening makes it unlikely that anything near those quantities of the drugs would end up in the meat and poultry we buy at the supermarket or butcher shop, or order at a restaurant. FSIS' William T. Hubbert, who oversees the agency's drug monitoring of meat and poultry, says he knows of no instance of benzimidazole-related side effects occurring in people as a result of eating meat.

Even so, the agency adheres to a strict safety margin in its drug residue testing, according to Hubbert in Washington, D.C. That means the maximum allowable benzimidazole drug residue in an animal sample is many times less than likely to produce any sort of measurable side effect in people.

The benzimidazole drugs that the antibodies detect can kill more than two dozen different kinds of debilitating worms, including stomach worms such as the barberpole worm, tapeworms and other intestinal worms, lungworms, and liver flukes.

Many of these parasites are pervasive; in fact, any place in the United States that has livestock will also have parasitic worms.

To keep cattle healthy, many ranchers routinely deworm their herds. Commonly, they use a long tube to squirt a liquid or toothpaste-like formulation of benzimidazole into each animal's mouth.

After treatment, ranchers must allow a few days to a few weeks to elapse before slaughter. The withdrawal time ensures that their livestock have used up the medicine. Each drug's label spells out withdrawal time; albendazole, for example, requires a 27-day wait.

The monoclonal antibodies that ferret out traces of these drugs are the product of man and mouse. Scientists like Brandon rely on the mouse's immune system to produce antibodies, a type of protein.

In this case, the scientists trick the mouse's immune system into forming antibodies to a benzimidazole deworming drug.

To make a monoclonal antibody, the Albany team injects laboratory mice with one of the benzimidazoles. The mouse spleen cells respond by making a flurry of antibodies - including some that specifically target the incoming drug.

The researchers then remove these spleen cells and fuse them with mouse cancer cells - harmless to humans - in a test tube. The result: new, hybrid tumor cells, or hybridomas. In the laboratory flasks, these hardworking hybridomas become tiny factories, churning out antibodies. The supply is plentiful because hybridomas reproduce rapidly - a characteristic of cancer cells.

The researchers repeat the process for each drug, making separate monoclonal antibodies for each. The new antibodies will bind readily to drug residues in a liquefied sample. Later, when exposed to a special solution, samples with drug residues will take on a faint green tinge. Drug-free samples, in contrast, turn a distinctive blue-green in about 15 minutes.

Sound simple? It isn't. "There can be lots of pitfalls," says Brandon. "It can be difficult to get the mouse to make an antibody to only the compound you want. Or, sometimes you can't get the hybridoma cells to keep producing antibodies.

"Making monoclonal antibodies," he says, "is widely regarded as a standard procedure. But it's really a combination of art, science, and luck."

PHOTO : Chemist Anne Bates works on an antibody test for a benzimidazole - a livestock deworming drug. (K-4002-1)

PHOTO : Using monoclonal antibodies, chemist Anne Bates tests for benzimidazole residue. (K-4003-1)

PHOTO : Chemist David Brandon evaluates a screening assay for benzimidazole residue at ARS' Food Safety Unit in Albany, California. (K-4001-1)
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Author:Wood, Marcia
Publication:Agricultural Research
Date:Mar 1, 1991
Words:1001
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