* Outbreak already deadliest in a decade
* Bacteria can incubate up to 2 months after infection
* CDC sees possible increase in deaths (Adds link to graphic in second paragraph)
By Julie Steenhuysen
CHICAGO, Sept 28 (Reuters) - Cases of illness in the U.S. listeria outbreak linked to tainted cantaloupes -- already the deadliest in a decade -- likely will rise in the next month as more people who have been infected with the bacteria begin to develop symptoms, health officials said on Wednesday.
To date, 13 people have died and 72 people have been infected in the outbreak in 18 states, including two pregnant women, according to the U.S. Centers for Disease Control and Prevention. (For a graphic on the outbreak, click here: http://link.reuters.com/nax93s)
Unlike E. coli and salmonella, two common causes of foodborne disease, listeria bacteria can cause illness as long as two months after a person has consumed contaminated food, making these outbreaks especially vexing.
"We will see more cases likely through October," U.S. Food and Drug Administration Commissioner Margaret Hamburg said in a telephone briefing.
Dr. Barbara Mahon of the CDC said the agency is only reporting on laboratory-confirmed illnesses and deaths.
"We do expect the number of cases will increase and the number of deaths may well increase," Mahon told the briefing.
Health officials have traced the source of the outbreak to cantaloupes grown by Jensen Farms in Colorado shipped between July 29 and Sept. 10. Such outbreaks are far more common in processed meats and cheeses and it is not yet clear how listeria bacteria got into the fruit.
"Listeria is a very common organism, which means it is very easily introduced into food at any point in a food chain -- in the field, at home and anywhere in between," said Martin Wiedmann, a professor of food science and a listeria expert at Cornell University in Ithaca, New York.
After consumption, symptoms of E. coli and salmonella usually take one to three days to emerge but listeria's symptoms are not noticed for one to eight weeks.
"That also makes it very difficult to trace back the source of outbreaks, particularly if they are small," Wiedmann said. "Most people don't remember what they ate in the last two months."
HIGH LEVELS OF CONSUMPTION
Listeria rarely causes serious illness. For listeria to do so, it needs to get onto the food and grow to levels where it can cause disease. Because it can grow at low temperatures, that can happen anywhere along the food chain.
"You need very high levels of listeria to get sick," Wiedmann said. "With listeria, we're talking about people having to ingest in excess of a million organisms."
Wiedmann said the reason there are not more outbreaks is that even when people are exposed to the bacteria, the numbers of organisms are usually too low to cause illness.
Dr. Thomas Frieden, director of the CDC, said the infected cantaloupe is being pulled off store shelves but there is still a risk of infection from fruit in people's refrigerators.
He said people should check with their grocery store if they are not sure of the source of their cantaloupe, and if they are in doubt, "throw it out."
Frieden said those most at risk are the elderly, the pregnant and people with a weakened immune system, such as people who have had an organ transplant or cancer.
Gene Grabowski, a food crisis expert at Levick Strategic Communications who handled an outbreak of tainted spinach in 2006-2007, said fresh produce is increasingly becoming a source of foodborne disease because more people are eating fruits and vegetables at every meal.
"People were not eating cantaloupes or blueberries for breakfast 10 to 15 years ago," Grabowski said.
Frieden said multi-state foodborne illnesses have increased in recent years. So far, there have been 12 in 2011.
"It is not that food is getting riskier but we are getting better at identifying problems," he said.
Grabowski said the cantaloupe outbreak underscores the challenges for all distributors of fresh produce.
And it is likely to renew perennial fears about the safety of the food supply, said Chris Waldrop, director of the food policy institute at the Consumer Federation of America.
Waldrop said some of these concerns should be addressed by the Food Safety Modernization Act, which gives the FDA more authority to regulate outbreaks. Hamburg said the FDA is working to release guidelines and regulations for the production of safe produce. (Additional reporting by Anna Yukhananov in Washington; Editing by Michele Gershberg, Eric Beech and Bill Trott)
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