More than a dozen deaths have been reported in the national outbreak of listeriosis linked to cantaloupes from a farm in Colorado, making it the third deadliest recorded Listeria outbreak in U.S. history, after a 1985 outbreak linked to cheese, and a 1998 outbreak linked to Sara Lee deli meat and hot dogs.
Listeria has been described as having a “Jekyll & Hyde” personality. In the natural environment, it exists as a benign “saprophyte” living off decaying vegetation. In humans, though, it is one the deadliest bacterial infections currently known, with the potential to infect the brain and kill 1 in 6 people infected despite early antibiotic treatment.
What is responsible for this transformation from an environmental soil bacterium to public health hazard? One way new human infectious diseases emerge is through a process called “exaptation,” in which an adaptation in one context coincidentally predisposes evolutionary success in an unrelated context, as was the case with Legionnaire’s disease. Legionnaire’s is caused by bacteria whose primary niche was the scum lining the rocks of natural hot springs, a warm, moist environment not unlike the human lung. The invention of machines that could mist the air with water afforded an opportunity—as evidenced by the 1976 American Legion convention outbreak in Philadelphia—for ventilation systems to condition the air with bacteria now known as Legionella. In the environment of the human respiratory tract, Legionella’s prior adaptations proved lethal in 29 of 182 cases.
Similarly, passage though animals raised for meat has been blamed for changes in pathogens that enhance human infectivity and virulence. This “exaptive” cross-host adaptation may have played a role in the emergence of diseases such as SARS and highly pathogenic strains of bird flu. The toxin that makes E. coli O157:H7 so deadly in humans apparently evolved to facilitate benign colonization in cattle. Likewise, when Listeria are ingested by livestock, adaptation to their new environment commences with the activation of a set of stress genes associated with virulence in humans. This can lead to udder invasion in mammals, resulting in mastitic milk and fecal shedding for further transmission into meat and dairy products. Compared to strains found in the environment, Listeria adapted to growth in farm animals may be more often associated with large-scale human outbreaks.
This is evidently the first time cantaloupes have been considered a culprit. The FDA/USDA Listeria risk assessment identifies deli meats as the only “very high risk” food category, accounting for an estimated 85% of cases. On a per serving basis, deli meats were identified as the riskiest, followed by hot dogs, meat pâté, unpasteurized milk, and seafood. Other dairy products, including cheese and pasteurized milk, fell into the moderate risk category, while fruits and vegetables were classified as low risk, coming in at relative risk ranking of 14 and 18, respectively (the top 13 riskiest food groups were all meat and dairy).
Of 17 outbreaks noted during a 25-year period by CDC researchers, 8 were meat, 8 were dairy, and one was from coleslaw made from cabbage contaminated with infected sheep manure. It is unclear how the cantaloupes in this case were contaminated, though early reports of the use of human sewer sludge on an adjacent field is one possibility.
To protect oneself from more typical sources of Listeria, the Centers for Disease Control and Prevention recommends that those at high risk (such as people over 50 and pregnant women) avoid hot dogs, lunch meats, cold cuts, and other deli meats unless they are recooked until “steaming hot,” to kill off any accumulated bacteria. In other words, ready-to-eat meats may not really be ready to eat.
Cantaloupes, like alfalfa sprouts, have been associated with outbreaks of foodborne illness—especially Salmonella—but it’s important to put the risk in perspective. Approximately 50 Americans are sickened in melon-borne Salmonella outbreaks every year. At the same time, the FDA estimates that 142,000 Americans are sickened every year by Salmonella from eggs.
According to a 2011 report from the Emerging Pathogens Institute, the five food poisoning sources that cause the greatest disease burden in the United States (in terms of cost, illnesses, hospitalizations, and death) are Campylobacter from poultry, Toxoplasma from pork, Listeria from deli meats, Salmonella from poultry, and then Listeria from dairy products.
-Michael Greger, M.D.