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With the aid of a flow cytometer, microbiologist Thomas Stabel and technician Ruth Willson check immune cell populations from stressed and nonstressed Salmonella-infected pigs. (K7070-7)

Keeping Off the Farm

Salmonella, Escherichia coli, Listeria, Campylobacter—the Latin names may have a lyrical sound, but for anyone who's ever suffered the intestinal distress and diarrhea these bacteria can bring via contaminated or improperly prepared food, they are words to fear.

It's been estimated that the United States alone has 6.5 million to 33 million cases of human illness from foodborne pathogens every year—and perhaps as many as 9,000 death.

Among the microorganisms that threaten consumers, the "big four" mentioned above cause the most cases of illness reported to the Centers For Disease Control and Prevention in Atlanta, Georgia.

Federal agencies and the U.S. meat, dairy, egg, and poultry industries hope to see the number of foodborne illnesses reduced substantially by the year 2000.

Salmonella is the pathogen associated most frequently with foodborne disease in both the United States and Canada. Animals can carry Salmonella species until they go to slaughter, or animal products may be contaminated during processing and preparation. But vegetables, too, can harbor the microbe.

Listeria monocytogenes has also been found in a wide range of foods—from dairy products, liquid whole eggs, red meat, poultry, and seafood to vegetables. Meats, especially poultry, and raw milk have been sources of Campylobacter jejuni. And E. coli O157:H7 has been linked to undercooked hamburger in the 1993 outbreak in the Pacific Northwest that killed four children.

Salmonella-free pigs in a "clean room" at the National Animal Disease Center in Ames, Iowa. (K7076-13)

At the ARS National Animal Disease Center (NARC) at Ames, Iowa, researchers are battling these pathogens by providing detection methods and prevention strategies that can be used all along the production and marketing chain, including on the farm.

ARS microbiologist Paula J. Cray has shown that by weaning piglets at between 10 and 18 days of age, they could be taken from a contaminated environment, placed in a rigorously clean environment with good ventilation and limited human access, and raised without detectable levels of Salmonella.

Joining Cray in these studies were Hank Harris, professor and chairman of Iowa Stale University's (ISU) Department of Microbiology, Immunology, and Preventive Medicine, and James McKean, ISU swine extension veterinarian.

"Both consumers and producers benefit from pigs having a reduced pathogen level when they reach the slaughterhouse," says Cray. "Pigs with reduced pathogen levels are healthier and gain more weight than their counterparts living in contaminated surroundings.

Microbiologist Karen Harmon prepares an electrophoresis gel that will be used in assessing the prevalence of Campylobacter in animal carriers. (K7071-16)

"Every farmer can make changes with existing facilities," she concludes. "We know that flies, for instance, can carry Salmonella, so it's no surprise that good fly control helps reduce disease."

In another approach, researchers have long suspected that stress is linked to increased shedding of Salmonella bacteria in pigs' feces. ARS microbiologist Tom Stabel has developed and is testing a stress model that makes it possible to more accurately predict and measure bacterial shedding by stressed animals.

"Only one pig has to be stressed to transmit Salmonella to other pigs, as well as to possibly contaminate carcasses at the slaughterhouse," says Stabel. "We're leaving behind traditional methods for measuring stress, which are less reproducible and often unreliable.

"This new stress model could be used to study E. coli, Campylobacter, and Listeria as well, which are also believed to be intensified by stress in the animal."

Stabel and Cray will use the same model to study the transmission of S. choleraesuis and S. typhimurium from pig to pig. The U.S. hog industry loses more than $100 million annually because of illness and medication costs related to S. choleraesuis alone.

Curbing Campylobacter

Second on the scientists' hit list of pathogens, Campylobacter jejuni is a major cause of human intestinal inflammation. This bacterial infection is most often transmitted to humans by way of undercooked poultry and unpasteurized milk products.

Laboratory technician Deborah Lebo isolates E. coli bacteria from the stomach contents of cattle fed special diets. (K7069-1)

"When cases of human illness occur or when a pathogen is suspected, time is the most critical element," says Irene V. Wesley, an ARS microbiologist who has studied these bacteria for 10 years. "Action has to be taken to get possibly contaminated food products off the market so more people aren't infected."

To help beat the clock on Campylobacter outbreaks, microbiologist Karen Harmon uses a molecular technique called polymerase chain reaction (PCR) to detect the culprit bacteria. Harmon says PCR can recognize pathogens in animals and food samples in as little as 8 hours, compared with up to a month to grow and identify pathogens taken from fecal samples.

"With PCR, we can look for differences between the suspect organism and any other organism that might be in the sample," says Harrison. "The faster organisms can be detected and identified, the sooner we can get contaminated foods off supermarket shelves."

Harmon has used PCR to detect Campylobacter jejuni, and a new bacterium called Arcobacter in pigs. Closely related to Campylobacter, Arcobacter was identified and named in 1992 by Belgian scientist Peter Vandamme. It has been found in chickens, turkeys, pigs, and water.

In 1995, ISU graduate student Cliff Collins studied pork products from three Iowa meat processing plants. He had been trained by Wesley to identify and distinguish various species of Acrobacter.

Light dots (on screen) in a colony hybridization being studied by veterinarians Steve Bolin (left) and Brad Bosworth contain E. coli with specific genes associated with human disease. (K7074-1)

Collins' survey of pork products showed that one plant had no Arcobacter, a second plant had the organism in 25 percent of its products, and a third plant contained Arcobacter in 89 percent of its products.

Harmon has also used PCR to distinguish Listeria monocytogenes from a relatively harmless species called L. innocua.

"Identifying L. monocytogenes quickly is important because ready-to-eat products contaminated with this agent can be recalled," says Wesley, "Since PCR doesn’t give false positive results, this sensitive technique can be used for fast, unequivocal identification of L. monocytogenes."

Another frequently reported bacterial pathogen is the potentially deadly E. coli O157:H7. A team of NADC researchers—microbiologist Mark A. Rasmussen, veterinary medical officer Brad T. Bosworth, and microbiologists William C. Cray, Jr., and Thomas Casey—has found that pre-slaughter fasting induces stress in cattle, which in turn can affect the incidence of E. coli in the animals.

"Cattle that are subjected to dietary stress during transport and marketing are a high-risk group and may carry an unusually high number of O157:H7," says Rasmussen. [See "Grappling With E. coli," Agricultural Research, July 1995, p. 9.]

Wesley and microbiologist Albert L. Baetz are tackling the problem of reducing L. monocytogenes, the organism behind listeriosis in cattle and humans. They have developed a sensitive, accurate serological lest to detect the disease in cattle. Usually, L. monocytogenes is destroyed during milk pasteurization or cooking.

The U.S. dairy industry has adopted new pasteurization requirements, raising the temperature to which milk should he heated but reducing the treatment time.

To detect with presence of antibodies against Listeria in cattle and sheep, chemist Albert Baetz monitors a bioreactor for growth of L. monocytogenes. (K7068-10)

In conjunction with these changes, ARS microbiologist Judith R. Stabel is studying the effectiveness of a variety of pasteurization temperature-and-time combinations for killing Mycobacterium paratuberculosis, another potentially harmful microbe.

In 1994, she and fellow ARS researchers at NADC developed an improved test to identity livestock that might be carriers of this microorganism. The improved test accurately pinpointed 90 percent of known infected animals in trials at NADC, even though the animals had shown no signs of disease.

Brucellosis, a bane of U.S. cattle producers since at least the 1840's, is now on the decline in this country thanks to vigorous eradication efforts. But this bacterial disease still exists, and it can infect humans, usually when workers come into contact with infected animals. Occasionally, people contract brucellosis called undulant fever in humans—from eating unpasteurized goat cheeses.

Microbiologist Betsy J. Bricker has developed and patented a PCR method to detect the presence of Brucella species in cattle, including the organism responsible for brucellosis. She is adapting PCR to detect Brucella in soft cheeses, which would allow laboratory microbiologists to determine if a product had ever been exposed to the bacterium, before going on the market.

"We can't stop studying tuberculosis and brucellosis just because they're not major foodborne diseases now," says NADC director Thomas E. Walton. "We need to keep up-to-date on these diseases, because they are still occupational diseases for many people working with animals. And continued research and improvements to keep pathogens off the farm will hasten eradication and keep down the cost of control programs." — By Linda Cooke, ARS.

Scientists mentioned in this story are at the USDA-ARS National Animal Disease Center, P.O. Box 70, Ames, Iowa

"Keeping Off the Farm" was published in the February 1996 issue of Agricultural Research magazine.