Listeriosis outbreaks like the current one traced to Colorado cantaloupes are extremely rare, according to a farm food-safety expert in Penn State's College of Agricultural Sciences. But Luke LaBorde, associate professor of food science, worries that melons present a heightened risk for spreading food-borne illnesses.

Listeria normally is associated with animal products such as hotdogs, ready-to-eat deli meats and raw milk, although outbreaks linked to fruits and vegetables are not unheard of. The current listeria poisoning blamed on cantaloupe now ranks as the third deadliest outbreak in U.S. history.

LaBorde remembers one other similar case. "There was an outbreak associated with cabbage grown in Nova Scotia in 1981," he said. "It was blamed on contamination from sheep manure."

According to the U.S. Centers for Disease Control and Prevention, at least 13 people are dead and 72 sickened in 18 states in the current outbreak of listeriosis tied to contaminated cantaloupes. CDC officials said the totals are likely to rise in the still-widening outbreak, which is the deadliest in the United States in more than a decade.

Listeriosis, a serious infection caused by eating food contaminated with the bacterium Listeria monocytogenes, is an important public health problem in the United States. The disease primarily affects older adults, pregnant women, newborns and adults with weakened immune systems. However, in rare cases, persons without any of these risk factors also can be affected. The risk may be reduced by following recommendations for safe food preparation, consumption and storage.

All of the deaths and illnesses in the current outbreak are tied to whole cantaloupes grown and shipped by Jensen Farms of Holly, Colo. The U.S. Food and Drug Administration has traced the current listeriosis outbreak to Rocky Ford cantaloupes, which were packed and shipped to at least 25 states. Federal officials have detected evidence of the outbreak strains of listeria in packing houses and on equipment at the site.

Jensen Farms issued a voluntary recall of the whole fruit on Sept. 14. After that, Carol's Cuts LLC, a Kansas food processor, issued a recall for nearly 600 pounds of cut fresh cantaloupe and fruit medley containing cantaloupe. The Rocky Ford-brand cantaloupes from Jensen Farms were shipped from July 29 through Sept. 10 to Arkansas, Arizona, California, Colorado, Idaho, Illinois, Kansas, Minnesota, Missouri, Montana, Nebraska, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, South Dakota, Tennessee, Texas, Utah, Virginia and Wyoming.

While it's not yet clear exactly how the cantaloupes became contaminated, the fruit is especially susceptible because of its rough, porous skin and soft, succulent interior, LaBorde noted. "I have been saying the last few years that I worry melons are a risk for conveying disease, and I think cantaloupes, in particular, present a perfect storm for carrying pathogens that cause food-borne illness."