Could it have been the irrigation water? Or a heavy rain storm? How could cantaloupes become contaminated with Listeria to cause the outbreak that, as of Thursday, had sickened at least 76 people and claimed 14 lives in 18 states?

As the number of deaths and illnesses linked to tainted Colorado-grown cantaloupes continues to grow, so does speculation about what caused the contamination at Jensen Farms.

The U.S. Food and Drug Administration has said only that it found Listeria on melons, and on unspecified equipment in the production area of the packing operation in eastern Colorado. FDA investigators are working with state health authorities to determine what happened, and say they’ll use that information to find ways to prevent it from happening again.

But since 1990, cantaloupes have been the culprit in at least 36 outbreaks. This one is unusual only because it is the first attributed to Listeria, which has such a high mortality rate, relative to other foodborne pathogens. What’s not surprising is that cantaloupe, once again, is a source of foodborne illness.      

Roy Costa, president of Environ Health Associates and a registered sanitarian, says that in the case of Jensen Farms, one theory may be that the bacteria was introduced into the packing plant via incoming produce.

“Given the wide spread of cases over time, this points to pre-harvest contamination as opposed to post-harvest contamination,” he wrote on his Food Safety & Environmental Health Blog.

Costa asks if the problem could have been contaminated irrigation water, given that the Arkansas River, the source of water for this melon-growing region, has been low due to drought, “and the access of the water source to all sorts of animal vectors is a very likely exposure pathway for Listeria monocytogenes.”

In an article in Food Safety News in March, Costa explained that because the cantaloupe’s rough skin can trap and hold bacteria, there are hazard points at every step of the supply chain, from growing, harvesting, packing, storage, transport, distribution and processing, to the consumer’s final cut into the fleshy fruit.

The FDA’s draft guidance on cantaloupes spells out the many ways cantaloupes can become contaminated.

For example, heavy rains can splash Listeria-containing soil onto cantaloupes in the field, where they also can come into contact with wildlife feces, which can carry Listeria or pathogens like Salmonella. Workers hand-turning melons can introduce pathogens and rind punctures from mechanical damage can be entry points for harmful bacteria.

Leaving culled melons in the field from prior harvests may attract wildlife and insects and result in soil contamination. So-called ground spots, where cantaloupes come in contact with soil, typically have significantly greater microbial populations that other parts of the rind.

Once harvested, cantaloupes are also susceptible to contamination in the packinghouse. The FDA guidance notes that soaking, spraying and icing cantaloupes to cool them can spread bacteria.

But the FDA’s recommended pathogen-prevention measures remain voluntary. Earlier this week, Caroline Smith DeWaal, food safety director of the Center for Science in the Public Interest, called the current outbreak “an urgent reminder that consumers are waiting for the FDA to release guidelines and regulations” to help keep pathogens out of produce.

She said the FDA should move rapidly to release the new guidelines and regulations, currently due for release in January 2012 and January 2013, respectively, and said Congress should fully fund the FDA to implement the Food Safety Modernization Act, to ensure that outbreaks like this stop breaking records.

Already the country’s deadliest foodborne illness incident in more than a decade, the cantaloupe Listeria outbreak toll count will likely continue through October, because people who ate contaminated melon last week may not get sick for some time. On Thursday, New Mexico confirmed a fifth death and Arkansas reported an illness.

Meanwhile, public health officials are still trying to track where all the recalled cantaloupes went. An earlier report that some of the cantaloupes were exported was incorrect.

And not all the cantaloupes were sold in stores. On Thursday, the Idaho Department of Health and Welfare, which has confirmed one outbreak-related illness, said 43,000 pounds of cantaloupes shipped from Jensen Farms to a company in Aberdeen, Idaho, were too ripe, so the melons were donated to the public between Aug. 28 and Sept. 2.