Risk of infant, foetal mortality from Listeria higher than believed

By Rory Harrington

- Last updated on GMT

Related tags Pregnancy Listeriosis

Listeria monocytogenes could present a high risk to unborn babies and infants at significantly lower levels than previously believed, new research has suggested.

The study from the University of Georgia, in the United States, found the risk of foetal or infant mortality among pregnant woman who eat food containing one million cells of the food-borne pathogen Listeria monocytogenes in soft cheeses and other foods is estimated at about 50 per cent. This suggests that five stillbirths could occur when ten pregnant women are exposed to that amount of the bacteria.

Risks at lower levels

Previous assessments of the hazard estimated that such a miscarriage rate would likely only be reached by exposure to more than 10 trillion Listeria cells. This means that such rates of infants and foetus’ deaths could occur at levels 10 million times less than previously thought, according to the results.

“We’re not saying there’s a new epidemic here, we’re suggesting we’ve come up with a more accurate method of measuring the risk and how this deadly bacteria impacts humans, especially the most medically vulnerable among us,”​ said study co-author Mary Alice Smith, Ph.D.

The researchers declared their estimates by extrapolating from test results on laboratory animals, such as guinea pigs, to conclude “Listeriosis is likely occurring from exposure to lower doses than previously estimated”.

The analysis also “shows studies using animal test subjects with physiologies more comparable to humans are more promising for future pathogen research endeavours”,​ they added.

Listeria monocytogenes is a bacterium sometimes found in soft cheeses made from unpasteurised milk and in processed, ready-to-eat foods such as deli meats, smoked seafood, and raw foods. The pathogen also has been found in pasteurised and refrigerated foods, such as pasteurised fluid milk and soft-ripened cheeses. Listeriosis rarely results in sickness among healthy groups buts can have serious consequences for the medically vulnerable such as the elderly, foetus, infants and those with a weakened immune system.

Pregnant women at another at risk group, with most cases reported during the third trimester. While they may no or only mild symptoms, the women can pas the bacteria to their foetus. In the first trimester, Listeriosis may result in spontaneous abortion. In later stages of pregnancy, the result may be stillbirth, premature delivery or birth of a critically ill newborn, explained the researchers.


The study carried out a risk assessment on consuming contaminated Mexican-style soft cheese during a pregnant woman’s third trimester; Using non-human primates and guinea pigs as models, the researchers determined the level at which 50 per cent had stillbirths. This data was then compared with estimates for human exposure to soft cheese.

Ten of 33 pregnant primates exposed to a single dose of Listeria experienced stillbirths. Using this primate data, the new model predicted 50 percent stillborn births among pregnant women, at a dose similar to that estimated from an outbreak of Listeriosis among pregnant women.

Exposures were measured using the FDA’s estimates for the number of Listeria-contaminated servings consumed by pregnant women. The dose of Listeria required for infection and onset of adverse effects was found to be more similar between humans, primates and guinea pigs than previous dosages used in government research that were adjusted from a mouse study.

Related topics Food Safety & Quality

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