Why are pregnant women so at risk for Listeria monocytogenes infections? This population group can suffer miscarriage, stillbirth, and premature labor if this infection is contracted. And the babies can be born with listeriosis, which is life-threatening.
One theory is that in pregnancy, the immune system is suppressed so the body doesn’t reject the embryo and fetus. This reduction in immunity may be why these women are so susceptible to listeriosis and why the complications can be so severe. Researchers at Berkeley, however, discovered that Listeria monocytogenes bacteria use the immune system suppression to invade the placenta where they are protected from the mother’s immune system.
Once the bacteria are in the placenta, they can grow rapidly, then emerge to infect the maternal liver and spleen. One theory is that miscarriage occurs as a defense mechanism so the body is protected from the source of the bacteria’s growth. It is difficult for any bacteria to cross the placental barrier, but once they do, complications can be very serious. Scientists think that the bacteria can cross the endothelium of the maternal blood vessels, and get into the fetal circulatory system of the placental villi.
In most people, most Listeria bacteria that get into the body are destroyed in the liver. But some can hide out in cells called hepatocytes in the liver. This leads to “hepatocyte lysis” and the bacteria are then released in large numbers into the bloodstream. The bacteria can also hide out in macrophages, a type of white blood cell, where they are protected against immune response and some antibiotics.
Who Gets Sick?
While the actual number of people who get sick from this infection iOS small, about 1,600 a year, the outcomes are usually bleak. About 260 people die every year. And most people who contract this infection are hospitalized simply because symptoms are so severe.
Pregnant women, children, the elderly, and those with chronic illness or compromised immune systems are also more susceptible to Listeria infections because of suppressed immunity and other problems with the liver and other organs. People who have undergone organ transplants, those with liver and kidney disease, diabetes, and AIDS are also at greater risk for complications from a Listeria infection.
The time it takes for the bacteria to reproduce in hepatocytes and macrophages, and the time it takes to get into the placenta, reproduce, and emerge into the blood stream may be why the incubation period between exposure to Listeria monocytogenes and the onset of symptoms is so long. The incubation period can range from three days to 70 days. Most pregnant women become ill about six weeks after infection. Knowing this time frame will help doctors trace back the food or water that is the source of the bacteria.
List of Listeria Monocytogenes Outbreaks
Unfortunately, the list of multistate Listeria monocytogenes outbreaks is long and tragic. This list does not include individual cases of listeriosis.
- The biggest multistate listeriosis outbreak in the U.S. was linked to Jensen Farms cantaloupes in 2011. One hundred forty seven people were sick with any of five subtypes of the pathogen. Thirty-three people died. Seven pregnant women were sickened; one woman suffered a miscarriage. Three infants were born with listeriosis.
- Imported Frescolina Marte Brand ricotta salata cheese was linked to 22 illnesses in 2012. Twenty people were hospitalized, four deaths were reported, and there was one fetal loss.
- In 2013, an outbreak linked to pasteurized cheese from Crave Brothers Farmstead in Wisconsin sickened six people. One person died and one woman suffered a miscarriage.
- In 2014, a Listeria outbreak linked to Roos cheeses sickened eight people; one person died.
- Also in 2014, a listeriosis outbreak associated with Oasis brands soft cheeses sickened five people; one person died. Three illnesses were related to a pregnancy; one newborn was diagnosed with listeriosis.
- Also in 2014, a deadly listeriosis outbreak was linked to Miller’s Organic Farm raw milk. Two people were sick; one died. This outbreak was not reported until 2016.
- An outbreak in late 2014 linked to Wholesome Soy bean sprouts sickened five people; two or three people died, depending on how the deaths were categorized.
- An outbreak linked to Bidart Bros. caramel apples sickened 35 people. Thirty-four people were hospitalized, and seven people died; listeriosis contributed to three of those deaths. Eleven illnesses were pregnancy-related. There were three pediatric cases of Listeria meningitis, three premature births, and one fetal loss.
- Blue Bell ice cream was linked to a 2015 outbreak that sickened 10 people; three people died.
- An outbreak in 2015 linked to soft cheeses produced by Karoun Dairies sickened 30 people. Three people died, and one woman suffered a miscarriage.
- In 2016, a Listeria monocytogenes outbreak linked to Dole packaged salads sickened 19 people in the U.S. and 14 in Canada. One person in the U.S. died.
- A listeriosis outbreak in 2016 was likely linked to frozen vegetables produced by CRF Frozen Foods. Nine people were sickened. All were hospitalized, and three people died, although two of the deaths were not caused by this illness.
- In 2017, an outbreak linked to Vulto Creamery soft raw milk cheese sickened eight people. One illness was reported in a newborn, and two people died.
- An outbreak in 2017 sickened two people in Rhode Island; the illnesses were associated with unpasteurized queso fresco.
- In 2018, a Listeria monocytogenes outbreak linked to recalled deli Johnston County Hams sickened at least four people; one person died.
- In late 2018, an outbreak linked to Long Phung pork products sickened four people in four states.
- In early 2019, a deadly Listeria outbreak linked to unnamed deli meats and cheeses sickened at least eight people in four states; one person died.
Foods to Avoid
Pregnant women should always be careful about what they eat to avoid any food poisoning, especially listeriosis. Common advice is to avoid eating deli meats, smoked meats, undercooked and uncooked eggs, undercooked and raw meat and seafood, raw milk, sprouts, raw milk cheeses, soft cheeses, and unpasteurized cider. These products are all susceptible to bacterial contamination, especially Listeria bacteria. And since Listeria bacteria can grow at refrigerator temperatures, all of these foods are risky. Wash all produce thoroughly, and avoid cross-contamination between uncooked meats, seafood, and eggs and foods eaten raw.
Pay attention to recall notices and outbreak investigations. Staying informed is one of the best things you can do.
It’s important to monitor your health when you are pregnant, especially with these symptoms. Symptoms of listeriosis include high fever, severe headache, muscle aches, confusion, diarrhea, nausea, and stomach and abdominal cramps. Pregnant women, unfortunately, may only have a very mild flu-like illness, but listeriosis can cause stillbirth, miscarriage, and infection in the newborn baby that can be very serious. And some women don’t have any symptoms at all.
If you do get ill, treatment is available. It’s important to get to your doctor as soon as you suspect something is wrong to receive antibiotic treatment.
