What I want all my students to know about Listeria


The FDA guidance on Listeria is now out in draft form. I have known about Listeria since I was in grad school at the University of Wisconsin-Madison in 1985. That is when I learned about Listeria, but farmers and vets had known much longer. I now know that many of the miscarriages in friends, aunts and grandmothers were caused by the presence of Listeria in raw milk and from exposure in the Wisconsin farm environment. When I teach the workshop on Preventive Controls for Human Food or in other work, this is what I tell my students.

Listeriosis is an infection by Listeria monocytogenes. There is a primary and potential secondary infection. We have all consumed Listeria. Most Listeria passes through us without causing an infection. When the primary infection develops, most of us experience flu-like symptoms and recover easily. You may have had listeriosis and not even known it.

Some of us cannot fight the primary infection, because the immune system is already weakened or compromised. This includes adults who are highly stressed, undergoing chemotherapy to fight cancer, with suppressed immune systems for bone marrow or organ transplant, or who have immune-suppressing conditions like HIV, auto-immune diseases, and pregnancy.

After Listeria colonizes the intestine, cells will transfer through the lining of the intestine and into the bloodstream. This is sepsis, and a healthy body may still fight off the infection. Once in the bloodstream, Listeria has access to all parts of the body. Listeria has the unique ability to cross membranes that other bacteria do not. Listeria can cross the barrier of the brain, the meninges. The infection is called meningitis and known as circling disease in cattle. Listeria can infect the eye and cause conjunctivitis. Listeria can cross the heart lining and cause endocarditis. Listeria can set up shop in the vagina, such that when an infant is born, the infant is exposed to the pathogen. Post-partum infant deaths may be caused by Listeria.

The most impressive feat of Listeria is its ability to cross the placental barrier. I am not aware of any other bacterium capable of this. Let me know, if you know of one. Depending on the age of the fetus, Listeria infection causes a miscarriage or death shortly after birth. This is heart-breaking, and most of us know a woman who has suffered a miscarriage. Not all miscarriages are due to Listeria.

When the CDC tracks a Listeria outbreak, the number of fetal deaths are counted along with the infant, child and adult deaths. The number of deaths from Listeria is staggering and can be as high as 40% in an outbreak. The average is 20% mortality rate from listeriosis. While the sheer number of Listeria cases, i.e. individuals, is nowhere near the annual one million cases of Salmonella, it is the number of deaths that makes Listeria a pathogen of concern.

If you need help understanding the FDA Listeria guidance or the requirements for your food safety plan, please contact the friendly folks at ConnectFood.

Dr. Kathy Knutson has food safety expertise in microbiology, hazard analysis, and risk assessment. As a recovering academic, she resides in Green Bay home-of-the-Packers, Wisconsin with her brilliant husband and two handsome sons. Learn more about her consulting services at https://www.linkedin.com/in/kathyknutsonphd.

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