Listeriosis is a rare foodborne illness caused by the bacterium Listeria monocytogenes (Listeria), commonly found in soil, vegetation, and animal feces. Listeria is known to disproportionately affect immunocompromised patients, elderly people, pregnant women, and newborns.
Listeria outbreaks and its general infections have become more common since the 1960s after the widespread use of refrigerators, processed foods, and the extended shelf-life of foods created better environments in which listeria could grow.
Listeria is highly resistant to standard antibacterial procedures within food preparation, such as cold temperatures, moisture removal, and high salt.
The first listeria outbreak was not identified until 1985; since then, science has learned more about the bacterium and how to identify and track it. Therefore, listeria was not a major consideration for prior generations of pregnant women because health organizations were not aware of it, its risk factors, its complications, or the foods/products that were likely to harbor it.
In the United States (U.S.), 50 to 100 pregnancy-associated listeria cases are reported annually among approximately 3.5 to 4 million births. Overall, including in the general population, the U.S. Centers for Disease Control and Prevention (CDC) estimates listeria is responsible for approximately 1,600 cases and 260 deaths per year. The European Union reported 2,502 confirmed cases in 2017.
The very commonly cited prevention measures of avoiding deli meat, "soft-ripened cheeses", and unpasteurized dairy originated after the 1985 dairy outbreak. That outbreak resulted in 142 infections – 65.5% of which were pregnant women (mostly Hispanic), leading to 20 fetal losses. The contaminated item was queso fresco style cheese and (crumbly) cotija in which raw milk was inadvertently introduced into the pasteurized product.
Pregnancy-associated cases are well documented to more commonly occur among ethnic minorities; this is likely due to this population’s higher consumption of certain cheeses that based on their manufacturing process, are more likely to contain listeria (see Causes).
Following the 1985 outbreak, the CDC began monitoring dairy products and sporadic cases of listeriosis, including voluntary recalls. The massive disproportionate adverse outcomes for pregnant women resulted in a “zero tolerance policy” for the foods most likely to be contaminated with listeria. This guidance is estimated to have cut the neonatal listeriosis incidence in half. If a manufacturer even suspects listeria infection, the entire product must be recalled.
Due to this tracking, researchers have learned even more about how listeria infects certain foods; therefore, complete avoidance of some foods is no longer necessary. However, habits regarding other foods still need to be implemented.
Infection occurs when a food product contains the presence of listeria, and that food product is then consumed. Although cold temperatures, moisture removal, and salt do not slow or kill listeria growth, cooking and pasteurization do kill its growth.
Most dairy outbreaks are due to the lack of pasteurization (i.e. raw), inadequate pasteurization, or contamination post pasteurization. Produce and ready-to-eat foods can be contaminated due to unhygienic processing facilities.
Therefore, it is recommended pregnant women do not consume any raw or unpasteurized milk or dairy products (or juices); this is the only standing rule necessary for all dairy products with one additional consideration:
Dairy products, as well as any food product, can become contaminated after pasteurization due to unhygienic practices at the manufacturing facility. This cannot be prevented through any action by the consumer. This also occurs through the handling of vegetables, ready-to-eat meals, and frozen foods.
This is where guidance regarding dairy products gets nuanced. It does appear, based on known outbreak data, that dairy facilities are more likely to be contaminated with listeria than other facilities that process vegetables or other non-dairy products. Further, these "higher-risk" facilities also usually process “soft-ripened” or queso-style cheeses.
The complete avoidance of both unpasteurized and pasteurized Hispanic queso-style cheeses is therefore based on data. Regardless, overall, the risk is still considered very, very low – but the consequences could be fatal for the fetus.
It is up to each individual pregnant woman to consume foods based on the amount of risk she is willing to accept, which is different for everyone.
Pasteurized cheeses that are not Hispanic-style cheeses can be safely eaten (again, contamination could occur post-manufacturing, but this risk could be considered negligible).
Specific Foods and Associated Risk
While listeria can be found on numerous different types of foods, some foods support its growth better than others – to include at potentially extremely high levels.
According to the CDC, the riskiest foods are raw sprouts, raw milk, cold deli meats and (uncooked) hot dogs, smoked seafood, and "soft-style" (see definition below) cheeses.
Ready-to-eat foods (meats, pastas, salads, and pre-cut produce) are considered higher-risk sources of potential listeria infection and have been implicated in several recent outbreaks. Ready-to-eat foods usually contain high salt and remain refrigerated until eaten – neither of which inhibit listeria growth. Unless a pregnant woman heats these products up until steaming, there is always a very small amount of risk present (in comparison to other foods).
“Deli meats” refers to cooked meats that have been sliced and prepared for sandwiches, wraps, etc. and are also called sandwich meat, lunch meat, cold cuts, or sliced meats. The U.S. Food and Drug Administration (FDA) estimates 1 case of listeriosis in 83,000 servings of deli meat consumed by pregnant women. Risk is eliminated when processed meats are cooked/heated to steaming prior to consumption.
Note: The likelihood of contamination is higher in deli meats sliced at a retail market than products sliced and packaged at a processing plant.
The “soft” cheese category is confusing for pregnant women attempting to determine if mozzarella, Parmesan, ricotta, cream cheese, mainstream queso dips, feta, or Brie are considered risky.
“Soft-ripened” is a category of a cheese based on its rind, texture, and style of ripening, which is usually “outside in”, starting from the rind. Commonly consumed cheeses in this category include Brie and Camembert. All soft-ripened cheeses made in the United States are pasteurized. Therefore, any contamination from this category occurs after pasteurization.
Of 58 listeriosis outbreaks reported during 1998 to 2014, a total of 17 (30%) were associated with soft-ripened cheeses, and resulted in 180 illnesses, 14 fetal losses, and 17 deaths.
Although this category is implicated more often than other dairy products, risk still remains extremely low if these soft-ripened cheeses are pasteurized; it is not possible for women to predict which manufacturing facilities may be more hygienic than others.
Feta, mozzarella, Parmesan, cream cheese, ricotta, and cottage cheese are not “soft-ripened” cheeses and carry less risk than “soft-ripened” (by outbreak data only). Although rarely implicated in listeria infection (but have been), it still possible after the manufacturing process, similar to any other food product, but again, risk is assessed to be extremely low.
Note: Although contaminated pasteurized dairy products is becoming a more common occurrence in the U.S. (even ice cream), it is not necessary to completely avoid dairy during pregnancy. Therefore, the recommendation stands to avoid the “riskiest” aspect of this category (pasteurized/unpasteurized Hispanic queso-style cheese), even though total risk is still considered very low.
Ice cream: In 2015, various name Blue Bell ice cream products were implicated in a listeria outbreak due to contamination of one of the production lines. In 2014 and 2015, three people contracted listeriosis from a milkshake machine at the University of Washington Medical Center. Bacteria were detected on several sites inside the ice-cream machine, including the dispenser nozzle.
Soft serve ice-cream can become contaminated due to the machinery not being cleaned adequately, rather than anything wrong with the ice cream itself. New models of machinery self-pasteurize, meaning that each night, the machine heats up to a temperature hot enough to kill bacteria that may be present in the holding tank.
However, pregnant women are generally not aware of the type of machinery used at a given location. Overall, risk associated with soft-serve ice cream is extremely low.
Raw sprouts, home-grown or purchased, need warm and humid conditions to sprout and grow – ideal conditions for bacteria. Bacteria easily gets into the seeds used to produce the sprouts and can only be eliminated through cooking, not rinsing – as opposed to other types of produce. Sprouts are also common harborers of E. coli.
The cold smoking process for seafood does not adequately kill listeria that may be present. Only cooking can kill any bacteria. However, canned (i.e. white tuna) or shelf-stable smoked seafood may be eaten (the refrigerated type is riskier). Read more about the safety of fish and other seafood.
Occasionally, major outbreaks occur in more widely distributed foods, leading to larger numbers of infections, such as produce (i.e. lettuce). However, the avoidance of produce is not feasible, based on the significant health benefits offered through the consumption of fruits and vegetables, when compared to the extremely low risk of listeria contamination. Regardless, all fruits and vegetables should be "washed" (no soap, water only) prior to consumption.
Maternal infection can present with or without symptoms, but most pregnant women appear to be asymptomatic. However, some women may experience “flu-like” symptoms (fever, chills) or headache, which makes diagnosis difficult and often late, especially due to the rarity of listeriosis.
Additionally, most pregnant women with listeria infection have no underlying conditions. It is estimated that this delay in diagnosis (and treatment) could be a major contributing factor to the high morbidity and mortality rates among pregnant women and their babies.
Further, it is assessed that listeriosis in pregnant women is underreported, due to asymptomatic infection. It is therefore theorized that listeriosis could be a missed cause of early miscarriage, especially in single cases not tied to an outbreak.
Note: Pregnant women can contract listeriosis when there are no known outbreaks. Listeria is present in many foods; most non-pregnant individuals will not be affected, but pregnant woman (and the elderly) can be, which is documented as a single case.
Single-cases of listeria are very difficult to diagnose and track back to a source. Listeriosis does not normally present with nausea and/or vomiting such as other foodborne illnesses (but can), and other individuals in the household who ate the same product may not become ill.
The estimated additional risk for listeriosis infection (or invasive infection) in pregnant women ranges from 10 to 100 times higher than the general population. It is estimated that 16% to 27% of all infections with listeria occur in pregnant women.
This increased risk may be skewed, however. It is not disputed that pregnant women have higher complications than non-pregnant individuals with a listeria infection. What is questionable is the susceptibility of pregnant women, and the reasons in which pregnant women are disproportionately affected and experience severe complications.
First, those included in outbreak data are sick enough to show symptoms and be tested. Not all non-pregnant individuals who contract the infection show symptoms and are therefore not included in outbreak or case totals, which can make pregnant women a larger proportion of the known infected. Further, the number of pregnant women who contract listeria and recover without treatment is not known.
Based on recent information regarding pregnant women’s immune systems, it is more likely that pregnant women just become sicker after contracting the infection, rather than more susceptible in the first place (read Immune System). It also depends on severity of infection and stage of pregnancy.
However, since many pregnant women are assessed to be asymptomatic, this indicates not all pregnant women will experience severe infections (although their infants may). In these cases, infection is usually noted because pregnancy-related complications occur such as preterm delivery, neonatal illness, or stillbirth that warrants investigation. This is in reverse to other illnesses that usually cause complications in the mother first.
Additionally, the delay in diagnosis described above can significantly alter the course of the infection and increase mortality and morbidity rates. If earlier detection and testing was available, mortality and morbidity could drop significantly in pregnant women (see Diagnosis and Management).
Researchers have advised that to avoid this delay, any pregnant woman who presents with a fever and flu-like or gastrointestinal symptoms should be considered for listeria infection when other, more common illnesses have been ruled out.
Listeriosis can cross the placenta and infect the fetus, with a prevalence estimated to be 8.6 in 100,000 live births. Infection of the mother is passed to the fetus/newborn in 20% to 60% of cases, with a neonatal fatality rate between 20% and 35%.
Congenital infection (during pregnancy) can lead to miscarriage, stillbirth, preterm labor, and neonatal meningitis. Infection earlier in pregnancy carries a higher chance of severe complications.
A study of 70,131 deliveries published in October 2021 determined 29 pregnant women had listeriosis (China). The main symptoms were fever (21/29), increased white blood cells (26/29), abdominal pain (16/29), and decreased or vanished fetal movements (7/29).
Inflammation was detected in pathological examination of placenta in all subjects. Among 33 fetuses carried by 29 subjects, fetal outcomes include six miscarriages, nine stillbirths, four newborn deaths immediately after birth and four after treatment discontinuation, nine discharges after successful treatment in hospital, and one death after treatment. As for maternal outcomes, 29 pregnant women all recovered after delivery.
In general, the high fetal fatality rate is likely due to asymptomatic cases in the mother. The lack of symptoms other than fever provides no warning that would lead to immediate treatment, and the infection can continue to progress in the fetus.
Listeriosis is documented to occur most often in the third trimester; however, this likely ought to be phrased “recognized more often in the third trimester”.
Listeria infection in the first trimester likely leads to miscarriage, but miscarriage is common, and usually does not lead to additional testing. However, fetal loss in the second and third trimesters almost always requires further investigation, leading to the finding of listeriosis.
Diagnosis and Management
Listeriosis is diagnosed through a culture in a laboratory setting of blood, spinal fluid, or placenta.
Immediate diagnosis and treatment are critical to avoid severe complications. The first-line treatment is amoxicillin or ampicillin alone or in combination with gentamicin, followed by trimethoprim/sulfamethoxazole (all antibiotics). Antibiotics may be administered intravenously for two to three weeks.
No randomized controlled trials have been conducted to assess most optimal type of antibiotic or duration of treatment during pregnancy.
Improved standards and surveillance have reduced the prevalence of contaminated foods at grocery stores, and outbreaks are caught much earlier than decades ago.
Listeriosis is almost entirely preventable through risk-based choices and proper food handling, especially when individuals eat and cook in their own home and follow proper food safety guidelines (read Safe Food Handling).
Based on the potential consequences of listeria infection, and data identified through outbreak tracking, it is routinely advised pregnant women avoid the foods most at risk for listeria contamination, which includes raw sprouts, raw and unpasteurized dairy products and juices, cold smoked seafood, and Hispanic queso-style cheeses.
Women should also cook/re-heat hot dogs and deli meats until steaming (prior to consumption), wash all produce, and practice proper food handling techniques.
All pasteurized products are considered safe during pregnancy; however, the only added risk is the small possibility of infection during the manufacturing process after pasteurization, which may be higher in certain categories of food production. However, there is no way to know which facilities are more hygienic that others, and overall, risk remains extremely low.
It is up to each individual pregnant woman to choose her living/eating habits based on the amount of risk she is willing to accept, based on current evidence.
Women should also consider sharing and submitting their experience below regarding listeria during pregnancy, to include whether they avoided certain food groups or even contracted the infection during pregnancy.
As of July 2021, the U.S. Food and Drug Administration, along with U.S. Centers for Disease Control and Prevention, and state and local partners, were investigating a multistate outbreak linked to frozen, fully cooked chicken products, such as chicken strips and diced chicken, and products made with fully cooked chicken, supplied by Tyson Foods Inc (Illnesses: 3; Hospitalizations: 3; Deaths: 1):
Shipped nationwide to retailers and institutions including hospitals, nursing facilities, restaurants, schools, and Department of Defense locations
Products include chicken strips, pulled chicken, diced chicken, chicken wing sections, fully cooked pizza with chicken, chicken salad sandwiches, chicken wraps, and salads with chicken
Brands include Tyson, Jet’s Pizza, Casey’s General Store, Marco’s Pizza, Little Caesars, and Circle K
Many of the recalled products have the establishment number “EST. P-7089” on the product bag or inside the USDA mark of inspection
Management of Pregnant Women With Presumptive Exposure to Listeria monocytogenes: Committee Opinion 614 (American College of Obstetricians and Gynecologists; Reaffirmed 2019)
Listeria Outbreaks (U.S. Centers for Disease Control and Prevention)
Listeria (Listeriosis) (U.S. Food and Drug Administration)
Timeline for Linking a Case of Listeria Infection to an Outbreak (U.S. Centers for Disease Control and Prevention)
Get the Facts about Listeria! (U.S. Food and Drug Administration)
Listeria from Food Safety for Moms to Be (U.S. Food and Drug Administration)
Listeriosis (World Health Organization)
Listeria and Pregnancy (American College of Obstetricians and Gynecologists; June 2018)