The common advice to avoid certain foods during pregnancy, such as feta cheese, is a precautionary measure against a specific bacterial infection that poses a serious risk to the developing fetus. Feta’s traditional characteristics place it among foods that can harbor a dangerous microorganism, leading to the severe foodborne illness known as listeriosis. Understanding the science behind this restriction explains why certain types of feta are considered unsafe, while others are acceptable for consumption.
The Pathogen: Listeria Monocytogenes
The microorganism responsible for this concern is Listeria monocytogenes, a bacterium found naturally in the environment, including soil, water, and animal feces. Unlike most other foodborne pathogens, Listeria has the unusual ability to survive and even grow at refrigeration temperatures. This cold tolerance allows the bacteria to increase in number on contaminated foods over time while they are stored.
Pregnant individuals are uniquely susceptible to listeriosis due to natural changes in their immune system during gestation. Hormonal shifts decrease cell-mediated immunity, which is the body’s primary defense against pathogens like Listeria. This altered immune response makes pregnant women approximately 10 to 20 times more likely to contract listeriosis than the general adult population, meaning even a low dose can lead to a systemic infection.
Why Soft Cheeses Are High Risk
Feta and other soft cheeses are considered high-risk foods because their manufacturing and physical properties create an ideal environment for Listeria to flourish. The risk is significantly higher when the cheese is made from unpasteurized, or raw, milk, which may already contain the bacteria. Pasteurization, which involves heating the milk to eliminate harmful pathogens, is crucial for safety.
Beyond the milk source, the internal characteristics of soft cheeses promote bacterial growth. Feta has a high moisture content and relatively low acidity compared to hard, aged cheeses, which are drier and more acidic. These conditions support the multiplication of Listeria even during cold storage. Furthermore, soft cheeses like feta do not undergo a significant aging process, which, in hard cheeses, can help to inhibit bacterial survival.
Health Consequences of Listeriosis During Pregnancy
While listeriosis can be a relatively mild, flu-like illness for the mother, the consequences for the fetus can be devastating because the bacteria can cross the placental barrier. Maternal symptoms, such as fever, muscle aches, and gastrointestinal distress, are often nonspecific and may not appear for several weeks after exposure. Even a mild infection in the mother can lead to a severe infection in the unborn child.
Infection of the fetus can trigger a range of serious adverse outcomes, depending on the timing of the infection. Listeriosis contracted in the first trimester is associated with a high risk of miscarriage, while later infection increases the chance of stillbirth or preterm delivery. Newborns can develop severe conditions such as sepsis or meningitis, often resulting in long-term developmental issues.
Safe Consumption Guidelines for Feta and Other Soft Cheeses
The risk associated with feta cheese is not absolute, and consumption guidelines focus on eliminating the bacterial threat. The primary method for making feta safe is to ensure it has been made with pasteurized milk. When purchasing feta, it is essential to check the product label for the words “made with pasteurized milk.”
If the pasteurization status of a soft cheese is unknown, it can still be made safe through sufficient cooking. Heating feta and other soft cheeses until they are steaming hot, or reaching an internal temperature of 165°F, will destroy any present Listeria. Pregnant individuals should also avoid unpasteurized milk and be cautious of cross-contamination from high-risk foods.