(AP) -- An outbreak of listeria in cantaloupe that has killed as many as four people is a mystery to disease specialists who are used to seeing the pathogen in deli meats and soft cheeses.
About 800 cases of listeria are found in the United States each year, according to the federal Centers for Disease Control and Prevention, and there usually are three or four outbreaks. Produce has rarely been the culprit, but federal investigators say they have seen more produce-related listeria illnesses in the last two years. It was found in sprouts in 2009, celery in 2010 and now cantaloupe.
"There are a lot of very good questions about where listeria is in the environment and how it gets in the fruit, and we don't have all the answers," said Dr. Robert Tauxe of the CDC.
Tauxe said a likely scenario is that listeria - which often lives in wet, muddy conditions - from the farm or packing facility got on the outside of the fruit and then contaminated the edible portions when it was cut. Victims may have then kept the fruit in their refrigerator for some time, allowing the bacteria to grow. Unlike most pathogens, listeria will continue to grow when refrigerated.
He said that while rare, listeria can be deadly. On average, it can be fatal for one in five who fall ill.
Colorado officials said Friday that the contaminated melons were whole fruit from Jensen Farms in the Rocky Ford region of Colorado, and have been recalled. Twenty-two people so far have been sickened in seven states: Colorado, Indiana, Nebraska, New Mexico, Oklahoma, Texas and West Virginia. Two deaths have been confirmed by CDC, one each in Colorado and New Mexico, and two more in New Mexico are under investigation.
Colorado's chief medical officer, Chris Urbina, said listeria found in samples taken from Jensen Farms' cantaloupe match the strain of the bacteria found in those who fell ill in that state.
"I'm confident that it's the only farm," Urbina said.
Listeria is found in many places in the environment - soil, water, air - and can easily contaminate animals which can in turn contaminate a food processing facility and stay there for a long period of time. While most healthy adults can consume it with no ill effects, it can kill the elderly and those with compromised immune systems. It is also dangerous to pregnant women because it easily passes through to the fetus.
To avoid listeria, the government has long warned those at-risk populations to avoid the most common carriers of the pathogen - hot dogs, deli meats, unpasteurized milk and cheeses made with unpasteurized milk.
Now that listeria is showing up in produce, should consumers be concerned? No, say CDC and Food and Drug Administration officials.
"It's only when a strange alliance of the stars occurs you get an extraordinary event like this," says Jim Gorny, a produce safety expert at the FDA. "It's a surprise that we'd have an outbreak of this extent so we really want to understand what happened."
Gorny says the FDA, which investigates farms and food facilities to find the source of an outbreak, still is working to determine how the contamination occurred.
He says more listeria outbreaks may have been discovered from produce in recent years because more people are eating raw fruits and vegetables and government reporting of illnesses has become more efficient.
In Colorado, the outbreak's effects already have been felt among melon growers. In Rocky Ford, cantaloupe farmer Greg Smith this week had to lay off his lone employee in a farm stand because he said customers all but vanished with news of the listeria outbreak. Smith's operation is not affiliated with Jensen Farms.
Farther north in Colorado Springs, Tammie Palmer told The Associated Press Thursday her 71-year-old husband remains hospitalized after eating contaminated cantaloupe. She has filed a lawsuit against Jensen Farms and Wal Mart, where the family said they purchased the fruit.
"He wasn't able to talk to me for five days. When I talked to him, his eyes rolled into the back of his head. It's been a nightmare," she said.
Explore further: FDA, CDC investigate listeriosis outbreak's source