While the U.S. Centers for Disease Control and Prevention (CDC) have so far announced 55 illnesses and eight deaths linked to the Listeria monocytogenesis outbreak occurring across 14 states, the variable nature of Listeria infections — and foodborne illnesses in general — suggests that many illnesses and some deaths caused by this outbreak may go undiagnosed and unreported.


Among foodborne pathogens, Listeria has a reputation for being particularly deadly. According to CDC estimates, of 1,600 reported Listeria illnesses each year, 260 — 16 percent — result in death.


Almost any severe illness or death attributed to Listeria occurs in people with compromised or weak immune systems, such as developing fetuses, the elderly or otherwise immunocompromised individuals. In healthy adults, the worst cases display flu-like symptoms rarely serious enough to inspire a hospital visit, while many adults consume Listeria without ever feeling the effects.

But because severe Listeria infections share common symptoms with other diseases, and because detecting the bacteria requires a blood test, medical professionals may not properly diagnose listeriosis for each patient hospitalized with the disease.

Compared with other foodborne pathogens, however, the rate of detection for Listeria is actually quite high, according Robert Buchanan, Ph.D., director of the University of Maryland’s Center for Food Safety and Security Systems. He said that estimates suggest Listeria is accurately diagnosed in one of every two medical cases, whereas Salmonella diagnoses are likely closer to one in 30.

“If you get a severe case of listeriosis, it almost always involves hospitalization,” Buchanan said. “Once you’re in there and they start looking around, the probability of them finding it is pretty high.”

Buchanan said that despite the high detection rate, miscarriages likely account for the greatest number of undetected Listeria deaths. Pregnant women infected with Listeria can pass the bacteria onto their developing child without ever realizing they are infected, resulting in miscarriages or stillbirths that might not be accurately attributed to Listeria.

Women should consider testing for Listeria during their second or third trimesters of pregnancy if they have shown any signs of illness similar to listeriosis, particularly if they have eaten high-risk foods such as deli meats or unpasteurized dairy, said Jeff Duchin, M.D., chief of communicable disease control for King County and University of Washington epidemiology professor. If health care providers detect Listeria soon enough, a prompt intravenous treatment of antibiotics can prevent the bacteria from permanently harming the child.

More importantly, Duchin said, pregnant women and other people susceptible to Listeria should consider adjusting their diets.

“In my experience, many people who are at risk of serious Listeria infections need to be more conscious of foods to avoid: Unpasteurized milk and cheese, queso fresco, deli meats, hot dogs, unwashed produce, et cetera,” Duchin said. “Pregnant women and immunocompromised people in general really need to be aware of the risks they’re taking with certain foods and ask whether or not they need to be eating them.”

Among the elderly, some may contract listeriosis during bouts with other medical complications, leading to worsened illnesses without the health care providers realizing they’re infected.

Duchin attributed the relatively high death rate among listeriosis patients to two facts: First, the large majority of people who consume Listeria do not need to seek medical attention, so the statistics do not include them; second, once Listeria invades a certain type of white blood cell known as a macrophage, it appears to be a particularly difficult intruder for weak immune systems to combat.

While Buchanan said that medical professionals in states linked to the Listeria outbreak should be on alert for signs of the bacteria, he and Duchin both said that anyone pregnant or immunocompromised suspecting they could be infected should seek medical attention to get tested.

“If you’ve been exposed to the product in question, you live in the area of the outbreak and you’re experiencing any flu-like symptoms like a fever or joint ache or muscle ache, it’s a very good idea to mention to your healthcare provider that you’re worried about Listeria,” Duchin said.

Buchanan also commented on his surprise that the outbreak has developed in cantaloupe at such a wide scale.


“Usually, you have to have a pretty good slug of Listeria to get infected,” Buchanan said. “So, my question is: How did they get it from this cantaloupe? There’s no question Listeria will grow on cantaloupe, but are cantaloupes getting contaminated by people cutting into the interior without washing them, then leaving them out at room temperature? You need to be sure to wash the surface before you cut it, and then make sure you refrigerate it and eat it within a reasonable time.”