Earlier this month, the Carnation Co. issued a voluntary recall of the country's entire supply of Bon Bons brand ice cream and frozen Fruit Scoops. The reason: evidence of contamination by listeria monocytogenes, a potentially deadly bacterium.

Carnation is not alone. More than 500 listeria-contaminated products have been voluntarily recalled over the past two years, at a cost to the dairy industry of almost $70 million. And more recently, the bug has been detected in shellfish and there have been reports of it in some processed meats as well.

Listeria is not a new bacterium. What is new is that it is a significant cause of foodborne disease. It also has turned into a political, economic and health-policy nightmare. As government officials continue to look for listeria in foods, they are finding it, and that has led to confusion and controversy over how to rapidly detect it, regulate it and get rid of it.

"We regard listeria monocytogenes contamination of food to be a serious threat to public health," wrote Dr. Otis Bowen, secretary of the Department of Health and Human Services and Richard Lyng, secretary of the Department of Agriculture, in a recent letter to a dairy processor.

The majority of us are constantly exposed to the bacteria, which are even more widespread in the environment than salmonella, according to Lester Crawford, associate administrator of the USDA's Food Safety and Inspection Service. Yet while most of us are resistant to infection from listeria, it is more likely to be a danger to the elderly, chronically ill and pregnant women and their fetuses. They are susceptible to listeriosis, a disease with a mortality rate of about 30 percent, far more fatal than salmonella. Symptoms of the disease range from a mild flu-like illness to manifestation as acute meningitis, meningo-encephalitis or septicemia. More than 50 percent of the cases worldwide affect fetuses, resulting in spontaneous abortions.

What is baffling about the bacterium is that it is so hardy: it grows well in refrigerated temperatures, is more resistant to heat and salt than other foodborne organisms and can recontaminate processed meat and poultry or pasteurized products such as ice cream. Even in factories that are apparently clean, it insidiously creeps into air ducts, sponges and floor drains. What's more, research methods to detect listeria are extremely slow, and they cannot quantify the amount of bacteria in a contaminated sample.

What is baffling about the disease is why only certain populations are susceptible to it and to what extent contaminated food is the cause of it. A lot is still unknown, too, about the infectious dose required to cause the disease and the virulence of different strains of the bacteria.

The Centers for Disease Control made listeriosis a reportable disease last year, and has recently instituted a hospital-based surveillance program. According to preliminary, minimum estimates from CDC, the disease strikes seven Americans per million, or approximately 1,600 cases per year. Dr. Claire Broome, chief of CDC's Meningitis and Special Pathogens Branch, said that puts the disease in about the same incidence category as toxic shock syndrome.

There have been three reported foodborne disease outbreaks from listeria, the most publicized being the 1985 outbreak in southern California in which a Mexican-style soft cheese made with unpasteurized milk was implicated in the 142 cases, 28 deaths and 19 stillbirths.

There have been no reports of illness from any of Carnation's products or from any of the other dairy products that were recalled by food companies over the past two years. The products were recalled as a result of findings from the Food and Drug Administration's Dairy Safety Initiative, in which agency microbiologists have been surveying products in every dairy plant in the country for the presence of disease-causing bacteria. The Dairy Safety Initiative, begun after the 1985 salmonella outbreak in Chicago, has turned up positive findings of listeria in 2.9 percent of the factories surveyed, or 29 out of 1,016. The initiative is about two-thirds completed.

And therein lies the first source of controversy. "The approach being taken by FDA is having devastating effects on the industry," said Glenn Witte, vice president of the International Ice Cream Assn. Witte said that several small and medium-sized ice cream companies are close to going out of business as a result of the cost of voluntarily recalling products. (The agency does not have the authority to order a recall; it can only request that a company do so. Its only immediate power -- aside from a complicated and lengthy seizure process -- is to issue adverse publicity should a company decide not to recall its products.)

While Witte agreed that listeria should not be present in ice cream, "no one is getting sick. We're not saying stop {to the FDA}. Just relax. It's not the threat to the consumer that you're proceeding on," he said.

Douglas Archer, FDA's director of microbiology, disagrees. "We didn't go out in the dairy industry with a witch hunt in mind. We have justified ad nauseam what we are doing and we think we are right."

CDC's Broome concurs. "It's hard to prove that no one is getting sick {from listeria-contaminated products}," she said. In addition, Broome said, "it's hard to imagine that there really is a safe amount of listeria that will not cause the disease in particularly susceptible hosts."

Another source of controversy has been USDA's monitoring program for listeria in processed meats, implemented earlier this month. Critics of the new program are pitting the department against the FDA, charging that the two agencies have policies that are significantly different from one another in regulating industry and protecting the public health.

Unlike FDA's program, in which the agency requests a recall immediately after a plant sample tests positive for listeria, USDA will not request a recall until a follow-up sample reconfirms the results. In the meantime, potentially contaminated products are allowed in the marketplace.

The USDA "always tries to find a way to protect the processor," said Rodney Leonard, director of the Community Nutrition Institute, an advocacy group. "USDA procedures are not an impetus for the industry to clean itself up; FDA procedures are."

From the industry viewpoint, Witte, of the International Ice Cream Assn., believes that USDA's program has "more regulatory compassion" and that unlike the FDA, the USDA recognizes that "simply finding it {listeria} isn't a threat."

In the letter to the dairy processor and Indiana state senator, Morris Mills, secretaries Bowen and Lyng, wrote that "while some differences exist in the programs used by USDA and FDA, the net results of these programs -- both in terms of impact on public health and in terms of impact on industry -- are the same."

A position paper accompanying the letter explains that USDA takes samples by slicing meats on the production line, packaging them and placing them in a plant's freezer. They are then shipped to a USDA lab. As a result of these procedures, there is a "remote chance" that the pieces could be contaminated as they are being sampled, the paper states. If a positive sample is found, subsequent samples are taken under more rigid conditions to assure that a recall will withstand legal scrutiny.

Archer said that FDA microbiologists, who are specifically trained in aseptic sampling, do not deal with open samples, but take sealed ice cream cartons back to field office labs where they are then analyzed.

Critics are also charging leniency on the part of USDA because its sample size is far smaller than FDA's. USDA's Crawford argues that "a lot of people don't understand the methods" and are erroneously "mouthing off about it."

Crawford contends that the USDA sample size is smaller simply because the media used to detect the bacteria in meat are far more sensitive than FDA's, so not as much sample material is needed. There is no statistical difference between the two methods, Crawford emphasized.

Thus far, listeria has not been found in any of the samples of roast beef (cooked for deli counters) that USDA has tested as part of its new program. The bacteria have, however, been found in environmental samples taken at meat processing plants, and there have been reports of it in meats themselves, according to George Wilson, vice president of scientific affairs for the American Meat Institute.

According to Wilson, the trade association has data on at least 10,000 environmental samples (swabs from surfaces of equipment, floors and so on) taken in meat processing plants. Of those samples, about 15 to 20 percent were positive for the presence of the bacteria, Wilson said. Some subgroups within those samples had high incidences, however, Wilson added; if a plant had 10 environmental samples taken, it wasn't "surprising" to find half of them to contain listeria.

Two weeks ago, the FDA initiated a general surveillance program on shellfish and has found the bacteria in a limited number of imported products, according to Archer. In that time, no domestic seafood samples have tested positive for listeria, he said.

Speaking at a listeria workshop recently, according to a report in Food Chemical News, Joseph Lovett of the FDA's Cincinnati lab said, "If the USDA thinks they've got problems with meat, wait until we turn the rock over and look at the seafood industry."