FRIDAY, Feb. 16 (HealthDay News) -- The number of Americans sickened by salmonella-tainted peanut butter rose to 290 across 39 states, the U.S. Centers for Disease Control and Prevention reported late Thursday, even as government scientists labored to detect the exact source of the contamination.

Health officials have warned consumers to discard certain jars of Peter Pan peanut butter or Great Value peanut butter that may be contaminated with Salmonella Tennessee, a rare but potentially fatal form of the food-poisoning bacteria. The affected jars have a product code -- located on the lid -- that starts with the number 2111. Both products were made by the ConAgra Inc. food company in a single plant in Sylvester, Ga., according to the U.S. Food and Drug Administration.

The FDA is recommending that all affected jars of Peter Pan or Great Value peanut butter purchased since May 2006 be discarded.

How salmonella -- which typically comes from animal feces -- got into the processed peanut butter remains a mystery. According to a report from the Associated Press, rodents and birds sometimes make their way into peanut storage bins at the Sylvester plant, but any salmonella would be killed during the peanut roasting process, when temperatures exceed the 165 degrees needed to destroy the bacteria.

Peanuts are heated beyond that point once more during processing, during the step in which they are ground into a paste and mixed with other ingredients prior to being sealed into jars.

"The heating process is sufficient to kill salmonella, should it be present," Mike Doyle, director of the University of Georgia's Center for Food Safety, told the AP.

That means the only stage where the bacteria might get a foothold would be during the brief cool-down process, just before the peanut butter is sealed into individual jars. Still, "there's a lot that happens after that heat step before it's put in jars," Dr. David Acheson, chief medical officer for the FDA's Center for Food Safety and Applied Nutrition told the AP. "So there's definitely an opportunity for contamination after the roasting."

ConAgra spokesman Chris Kirchner told the AP that his company randomly tests 60 to 80 jars of peanut butter daily at the Sylvester plant for salmonella and other contamination. "We've had no positive hits on that going back for years," he said. The plant was last visited by FDA inspectors in 2005, the AP said.

The plant has been temporarily shut down, and FDA inspectors visited Wednesday and Thursday to investigate the outbreak. Testing is also being done on jars of peanut butter in the homes of those sickened, the agency said.

The outbreak appears to have started in August 2006, according to the FDA, with the CDC reporting the highest numbers of cases in New York, Pennsylvania, Virginia, Tennessee and Missouri. About 20 percent of those sickened were hospitalized, and there have been no deaths.

According to CDC epidemiologist Dr. Mike Lynch, 85 percent of those who fell ill with Salmonella Tennessee said they had eaten peanut butter, and about a quarter said they ate it daily.

ConAgra, based in Nebraska, announced Thursday that it was recalling all varieties of Peter Pan and Great Value peanut butter with the product code 2111. While company officials are mum on exactly how much peanut butter is being recalled, the AP said more than 974 million pounds of peanut butter are sold in the United States each year, and Peter Pan remains one of the country's top three brands. The Great Value brand, which is also made by other companies, is a Wal-Mart brand.

Responding to the problem, ConAgra announced that it has added extra capacity to its toll-free Consumer Affairs hotline (866-344-6970), which is set up to respond to consumer questions and concerns. Callers can also apply for refunds for products covered by the recall.

According to information on the company's Web site, heavy call volumes are causing long delays. Consumers are asked to "please bear with us, [and] wait 30-60 minutes and try again."

Meanwhile, supermarket chains across the country scrambled to pull the products off their shelves.

Stop & Shop and Shaw's, two of New England's largest grocery chains, ordered their stores to remove all the Peter Pan peanut butter, the Boston Globe reported.

In Texas, H-E-B, which operates 300 stores in the state as well as northern Mexico, removed the entire Peter Pan line from its shelves early this morning, Leslie Lockett, a spokeswoman for the company in Austin, told the AP.

And in U.S. military commissaries around the world, workers were ordered to remove all Peter Pan peanut butter jars, according to Stars and Stripes. Officials from the Defense Commissary Agency, which provides groceries for the military, sent out the order on Thursday morning, the newspaper reported.

Symptoms of salmonella poisoning include fever, diarrhea and abdominal cramps. Salmonella can cause life-threatening infections in people in poor health or who have weakened immune systems.

Any such illnesses should be reported to state or local health authorities, the FDA said.

Other states reporting peanut butter-linked cases include Alaska, Alabama, Arkansas, Arizona, California, Colorado, Connecticut, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Massachusetts, Maryland, Maine, Michigan, Minnesota, Mississippi, Montana, North Carolina, Nebraska, New Jersey, New Mexico, Ohio, Oklahoma, Oregon, South Carolina, South Dakota, Texas, Vermont, Washington, Wisconsin, and West Virginia.

This outbreak follows major food-borne illness scares in 2006 involving spinach, tomatoes and iceberg lettuce.

One expert said he was not surprised that such outbreaks continue to plague U.S. consumers.

"This is not an unusual event," said Dr. Philip Tierno, the director of clinical microbiology and immunology at New York University Medical Center and author of the The Secret Life of Germs and Protect Yourself Against Bioterrorism.

"Salmonella is a very prevalent organism," he added.

Tierno said these contamination problems arise because the FDA does not regulate the safety of produce -- and he doubts that the food industry can monitor itself.

"I think the government is moving toward making a change," he said. "Outside monitoring is the way to go."

According to the CDC, there are an estimated 76 million cases of food-borne illness each year in the United States, the vast majority of which are mild and cause symptoms that last a day or two. Some cases are more serious, leading to 325,000 hospitalizations and 5,000 deaths annually. The most severe cases tend to occur in the very old, the very young, and those with weakened immune systems.