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.”
Because there are few symptoms that are unique to listeriosis, doctors must consider a variety of potential causes for infection, including viral infections [like flu], and other bacterial infections that may cause sepsis or meningitis.
Early diagnosis and treatment of listeriosis in high-risk patients is critical since the outcome of untreated infection can be devastating. This is especially true for pregnant women because of the increased risk of spontaneous abortion and preterm delivery. Depending on the risk group, rates of death from listeriosis range from 10% to 50%, with the highest rate among newborns in the first week of life.
Methods typically used to identify diarrhea-causing bacteria in stool cultures interfere or limit the growth of Listeria, making it less likely to be identified and isolated for further testing. On the other hand, routine methods are effective for isolating Listeria from spinal fluid, blood, and joint fluid. Magnetic-resonance imaging [MRI] is used to confirm or rule out brain or brain stem involvement.
This sheet talks about exposure to or having Listeria infection [listeriosis] in a pregnancy and while breastfeeding. This information should not take the place of medical care and advice from your healthcare provider.
What is listeriosis?
Listeriosis is an infection caused by Listeria monocytogenes bacteria, often called just Listeria. Listeriosis is typically caused by eating food that has been contaminated with Listeria. Listeria can be found in your home, in restaurants and other places such as the grocery store or food processing plants. Food with Listeria can introduce the infection into the refrigerator which can spread to other foods. Listeria can continue to live in cold temperatures, such as in the refrigerator, but the chance for spreading can be slowed if the refrigerator is kept at a temperature of 40 degrees Fahrenheit or lower.
Some of the foods that are more likely to be contaminated with Listeria include unpasteurized [raw] milk, uncooked meat and fish, uncooked vegetables, lunch meat and soft cheeses. Listeria outbreaks can also occur in cantaloupes.
Who is at risk for listeriosis?
The people most at risk for listeriosis include people who are pregnant, young children, adults over the age of 60, and people with weakened immune systems. Following some simple food safety guidelines can reduce the chance of listeriosis.
What precautions should I take to avoid the infection?
To decrease the risk of listeriosis and other food-borne illness in all individuals:
- Do not drink unpasteurized milk [also called raw milk] or eat any foods made with unpasteurized milk.
- Thoroughly cook raw foods from animal sources.
- Heat foods to at least 165°F [to steaming], to kill the bacterium.
- Wash raw vegetables and fruit, even if you plan to peel them [to remove skin].
- Separate uncooked meats from cooked meats and vegetables.
- Wash your hands, cutting boards, knives, counters, and sinks after contact with uncooked foods.
- Consume ready-to-eat foods as soon as possible.
- Keep your refrigerator at or below 40 degrees Fahrenheit [4 degrees Celsius].
- Clean your refrigerator regularly.
People who are pregnant should take additional precautions to decrease the risk of listeriosis:
- Do not eat soft cheeses [such as feta, Brie, Camembert, blue-veined cheeses and Mexican-style cheeses such as queso blanco, queso fresco and Panela] unless they have labels stating that they are made from pasteurized milk. However, it should be noted that some Mexican style cheeses made from pasteurized milk have been a source of Listeria infections, possibly due to the cheese making process.
- Reheat, to at least 165°F / to steaming, any leftovers, ready-to-eat foods, hot dogs, cold cuts, deli meat, frozen vegetables, and frozen prepared foods.
- Take care to not get the juice of deli meats and hot dogs on other foods/surfaces and wash your hands after handling deli meats and hot dogs.
- Do not eat refrigerated hummus, pâté, meat spreads or refrigerated smoked seafood, unless it is an ingredient in a fully cooked dish [like a casserole].
- Avoid ready to eat salads.
How do I know if I’ve been infected with Listeria?
Not everyone affected with Listeria will develop symptoms. Symptoms of listeriosis range from showing no symptoms to having diarrhea, fever, muscle pain, joint pain, headache, stiff neck, backache, chills, sensitivity to bright light, and/or sore throat with fever and swollen glands. These symptoms can begin days to weeks after eating contaminated food. A blood test can confirm whether you have been infected with listeriosis. If you have eaten contaminated food and do not have symptoms, some experts feel no special testing or treatment is needed. Be sure to discuss this with your healthcare provider.
How can I find out if my baby has been infected with Listeria during my pregnancy?
An ultrasound to look at the baby can be used to check for an enlarged heart, thickened bowel, and increased thickness of the stomach walls, which may occur in some babies infected with Listeria. A blood test can also be performed on the baby after birth to detect whether the baby has been infected with Listeria.
Does having/getting listeriosis increase the chance for miscarriage?
Miscarriage can occur in any pregnancy. There is an increased chance of miscarriage when infection occurs in the first trimester of pregnancy. Infections after the first trimester are associated with a lower chance of pregnancy loss.
Does having/getting listeriosis increase the chance of birth defects?
Every pregnancy starts with a 3-5% chance of having a birth defect. This is called the background risk. Listeriosis infection has not been linked to an increased chance of birth defects.
Does Listeria cause other complications for pregnancy?
Pregnancies affected by or exposed to Listeria can have an increased chance of infection in the uterus, preterm delivery [delivery before 37 weeks of pregnancy] and in the most severe infection, the possibility of stillbirth.
Does having/getting listeriosis in pregnancy cause long-term problems?
Listeriosis during pregnancy can also increase the chance for serious health problems for the newborn. Newborn babies infected with Listeria can develop either early onset or late onset listeriosis. Early onset listeriosis develops 1-2 days after birth, and the baby often has signs of a serious bacterial infection. Late onset listeriosis occurs 1-2 weeks after birth, and usually includes symptoms of meningitis [ a condition where there are areas of swelling around the brain and spinal cord in the baby]. If not treated quickly with antibiotics, the effects of meningitis can lead to long-term problems for some children. Late onset listeriosis is most likely related to Listeria present in the mother’s birth canal.
Not all babies who are exposed to listeriosis during pregnancy will have problems. Early diagnosis and treatment with high doses of antibiotics might prevent infection of the fetus.
Are there any treatments for listeriosis during pregnancy?
Large doses of antibiotics have been recommended to treat listeriosis during pregnancy. Such therapy has been successful, leading to lower incidences of preterm deliveries and stillbirths. Your healthcare providers will talk with you about the right treatment for your pregnancy.
Can I breastfeed with listeriosis?
It is not clear if Listeria passes through breastmilk. It appears that if there is a chance, it is considered to be low. If you have been diagnosed with listeria and are breastfeeding, talk to your healthcare provider or your child’s pediatrician about this and all your breastfeeding questions.
I was exposed to Listeria. Can it make it harder for me to get my partner pregnant or increase the chance of birth defects?
There is no evidence linking a father’s or sperm donor’s exposure to Listeria with a higher chance of infection during pregnancy. In general, exposures that fathers and sperm donors have are unlikely to increase risks to a pregnancy. For more information, please see the MotherToBaby fact sheet on Paternal Exposures at //mothertobaby.org/fact-sheets/paternal-exposures-pregnancy/pdf/.
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OTIS/MotherToBaby recognizes that not all people identify as “men” or “women.” When using the term “mother,” we mean the source of the egg and/or uterus and by “father,” we mean the source of the sperm, regardless of the person’s gender identity.
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