Listeria and Fear of Unpasteurized Dairy

I was at Whole Foods the other day, browsing their cheeses, when this woman next to me shrieked in horror upon reading that the Gruyere Special Reserve is made from UNPASTEURIZED milk. I turned to her and said that, after doing my own research on the topic, I now prefer unpasteurized dairy products over pasteurized dairy products. Here’s the conversation that followed:

    • HerGood for you, but I’m nursing a baby.
    • Me: And I’m 15 weeks pregnant.
    • Her: [eyeballs popping out of head] Well, I trust my OB/GYN, who warned me about Listeria.
    • Me: So did mine, and that’s why I’m no longer seeing her. Because after doing my own research, I’ve concluded that my chance of contracting listeriosis is much higher when eating mass-produced, processed foods, like deli meats, than eating these cheeses.
    • Her: [in a condescending tone] Well, good luck with that!

        Am I suggesting that she (or you) take my advice over a doctor’s? No. But I hope that I at least inspired her to do a little more research on her own. After all, not two days went by when I heard on NPR about yet another non-dairy related listeria outbreak – this time, tied to

cantaloupe. I wonder whether this woman also heard this story. If so, then did *that* finally inspire her to devote just 5 minutes to researching this topic on her own?

What is listeria?

When people talk about listeria, they’re usually referring to the bacterial strain listeria monocytogenes. What I found really surprising in my research on this “pathogen” was that, not only do healthy animals commonly carry this bacterium, but so do *many* people, including pregnant women who give birth to healthy babies! [1]

But once in a while, listeria causes an infection that could turn into meningitis, a disease of the nervous system. It’s extremely rare for this to happen to people (or animals) with strong immune systems, but for those who are immunocompromised, like the elderly or young children, it’s pretty scary because:

  1. Your chances of dying from listeriosis are relatively high compared to other pathogens. It can have up to a 25% fatality rate [2].  This is a lot higher than, say, salmonella, which has a fatality rate of less than 1%.
  2. The pathogen can be easily transmitted from a mother to her unborn fetus, which can cause a miscarriage or stillbirth.
  3. It’s a pretty hardy bacterium, meaning it can live a long time and withstand a pretty big temperature range (from refrigerator temperature to human body temperature). So it could be hanging out in the soil or your refrigerator for months, if not a year or more.

What causes listeriosis?

So why does listeria sometimes make people and animals sick and sometimes not? From what I can tell, scientists don’t really know. But a very clear pattern is known: animals that contract listeriosis are those that have been eating spoiled silage,  rotten vegetation, or feed that’s been infested by mice or tainted with fecal matter. In other words, good feeding practices and hygiene is key to preventing listeriosis in animals.

So how on earth does this relate to cantaloupe? As of this writing, the source of that outbreak hasn’t been determined, but an infected animal can transmit the pathogen through its milk and feces (manure). From there, it can get into the ground and water and then into fruits and vegetables that are grown in the affected soil or water.

So why risk it with unpasteurized milk?

Yes, you actually could contract listeria by drinking unpasteurized milk from an infected animal, but as you’ll see below, most dairy-related outbreaks come from contamination that occurs after pasteurization, but before packaging and distribution.  Despite what I hear in the mainstream media, the data I was able to find shows that most infections come from consuming other processed, mass-produced and mass-distributed foods that don’t contain any dairy products. Especially deli meats and hot dogs. Thus my comment to the Whole Foods shopper.

The data

My analysis is based on US data from this online outbreak database. Here’s my Foodborne Illness Analysis. And here’s how I aggregated it:


The data set, in terms of number of incidents, it so small, that statistical analysis of any kind doesn’t make a lot of sense. So in this case, I’m not going to talk in terms of averages or medians or correlations, but rather, just share some observations. I look forward to hearing others’ interpretations of this data, but here are my takeaways and a few comments:

  • In the dairy category, more people were affected by listeria after consuming pasteurized dairy than unpasteurized dairy, both in absolute numbers and as a percentage of those who consume the product.
  • Far more people contract listeriosis from processed meats compared to dairy products of any kind.
  • The media’s standard practice of using unpasteurized milk as the “poster child” for listeriosis without mentioning processed meats is totally misleading.
  • The amount of recalled/tainted meats is just staggering! I wonder how many more people contracted listeriosis and didn’t know it and just thought they had the flu
  • 86 of the 199 affected by pasteurized dairy got it from a Jalisco cheese plant that normally produces only pasteurized milk, but the investigation found that some milk hadn’t made it through the pasteurizer and contaminated the rest of the batch. The outbreak database categorizes this as unpasteurized, but I chose to categorize it as “pasteurized” dairy, since that’s how the product is labeled and sold. Bear in mind that dairies that produce milk destined for mass production and commercial distribution don’t have to be as careful because any pathogens present in the milk will be killed through pasteurization and/or radiation.
  • Regarding the 2000 N. Carolina dairy outbreak (this is where several Mexican immigrants got listeriosis from making mexican cheese at home, using milk from a small dairy farm), I find it interesting that the CDC’s editorial comment is that the solution is to educate people on the dangers of consuming raw milk products as opposed to teaching people how to prevent contamination through proper hygiene. After all, as in almost all cases in this report (including this one), the pathogen strain was never detected in a single animal but rather, the contaminant was introduced after-the-fact [4].   In other words, the milk being unpasteurized had nothing to do with it.
  • 16 people contracting listeriosis from raw milk in 27 years, out of ~12 million raw milk drinkers[5], not to mention all the consumers of raw milk cheeses, puts the odds of contracting listeriosis from raw dairy products at less than 1 in tens of millions. In other words, you’re thousands of times more likely to be killed by a lightening strike or a dog attack than by contracting listeriosis from raw dairy products in the US. [6]

When I explain to people why I continue to consume raw dairy products, even now that I’m pregnant, their final words are always the same, “Well, I personally don’t think it’s worth the risk.” I should ask some of them why they own dogs then! Seriously though, I think that the consumption of ALL food carries risks of different kinds. Hopefully I’ve shown why the risk of contracting listeriosis from raw dairy products in particular is practically negligible. To me, it’s a question of weighing the relative risk against the reward. I believe that raw dairy products, which come from healthy, pasture-raised animals  and have all of their beneficial enzymes and probiotics intact, are some of the best superfoods on the planet. And my research on that topic will be the subject of another blog post.


[1] Farber, J., Peterkin, P. (Sept 1991). “listeria monocytogenes, a Food-Borne Pathogen“. American Society for Microbiology 55 (3): 481, 478.

[2] “Todar’s Online Textbook of Bacteriology”. Listeria monocytogenes and Listeriosis. Kenneth Todar University of Wisconsin-Madison Department of Biology. 2003.

[3] I made the following assumptions in these estimes: 1) 300M people in America, of which they all consume produce, 2) 85% of the population consumes processed meats, 3) 12M Americans consume raw milk + 15% of the population consumes raw milk cheeses, such as Gruyere, most blue cheeses, English Cheddar, Manchego, or Parmigiano Reggiano, 4) 85% of the population consumes pasteurized dairy products.

[4] CDC report (July 6, 2001). Outbreak of Listeriosis Associated with Mexican-Style Cheese – North Carolina, October 2000–January 2001, “Milk from each cow also was tested for presence of L. monocytogenes. Repeated testing did not identify any cow with milk confirmed positive for L. monocytogenes, suggesting that the cows were not infected and that L. monocytogenes may have originated from environmental contamination.

[5] This number is an estimate from the Farm to Consumer Legal Defense Fund, based on 2006/2007 CDC data, extrapolated to today.

[6] Odds of dying in an earthquake are 1-in-131,890. Odds of dying from a dog attack are 1-in-147,717.


10 comments on “Listeria and Fear of Unpasteurized Dairy

  1. Leigh Ann says:

    Well said, Nevra! This is the “stuff” I love talking about, but never have the time to lay it out or explain it like this. Now I will just direct people to your blog 🙂 Thanks for sharing!

  2. Arturo says:

    I like how you’re becoming a mini-agricultural economist!

    I think that your table could be appended to make the different “vehicles” more comparable by using a variable common to all of them. For instance, the volume of production per year, or the number of customers that consumed that product. That way you derive some type of “per capita”-like measure. For instance, a lot more people drink/eat pasteaurized products than non-pasteaurized products, so you need to adjust for this difference to better gauge the impact.

    • Nevra says:

      Arturo, you asked the exact same question as another economist I know – you all must think alike! I just added it. The reason I originally decided not to is because I figured the absolute probability matters a lot more than the relative probability – especially in this case where the absolute probability is so low. But I hope the additional data helps.

  3. Amanda Aller says:

    As a woman desiring to be pregnant in the next year, this column is a godsend! I have been grappling with the issue of eating raw milk and cheese during pregnancy, not because I do not trust it (we eat exclusively raw dairy at home and I promote it to my health coaching clients, peers, and friends, and on my Facebook page), but, because of the intense emotions connected with taking any chances with the health of an unborn baby. I have had clients insinuate I was crazy to support their use of raw milk/cheese during pregnancy, like I’m a closet baby-murderer, and, it can be hard to stay strong in the face of such stinging personal criticism. After reading this, I am so comforted to have scientific proof of what I have always *known* to be true in my heart, that it is ludicrous to avoid these nutrient dense foods at any stage of life due to baseless fear-mongering and uneducated critics. Thank you, thank you, thank you! You can be sure I will be reading and sharing this wonderful post many times in the future. You have my profound gratitude for it : )

    Amanda Aller
    Owner, The Health You Want (find The Health You Want on Facebook, too)

    • Nevra says:

      So glad to hear it Amanda. It’s amazing how strong peer pressure can be, but in the end, I think you have to do what *you* are most comfortable with, even if it goes against the “conventional wisdom.”

  4. I am stealing your table to publish on my facebook page, and I am guessing your are totally okay with that 😉

    • Nevra says:

      Steal away! But please keep in mind that on the raw data table (the one you have to click the link to get to) I’ve changed one data point, which I noted. It’s on row 30 of the Excel spreadsheet, which is the Jalisco Cheese Plant outbreak from 1985. CDC’s database categorizes this as an unpasteurized milk outbreak, which I feel is incorrect because Jalisco is in the business of producing only pasteurized products. It just so happens that in this case, some of the milk didn’t get adequately pasteurized. Therefore, this outbreak should have been categorized as a PASTEURIZED product contamination because that’s how the product is intended to be consumed, labeled and sold. As you might imagine, milk that is produced with the intent of being pasteurized is produced with lower standards than milk that is intended to be consumed raw and therefore MUST be pasteurized. Just wanted to make sure that’s clear.

  5. Yorktown mom of 2 says:

    I agree with you. I also drank raw milk throughout pregnancy and fed raw milk formula (nourishing Traditions recipe) to both of my preemies (3.0 lb and 4 lb 3 oz) with zero problems. In fact at 30 and 11 months, the only illness they’ve ever had have been 2 colds.

    This is just aother example of the Feds trying to step in on people and dictate health regs, despite their total ignorance of wellness and nutrition.

    Thanks for writing the post, I’ve shared it with many others as rebuttal to the CDC “study” if yiu can even call it that. Scary that some people actually want these peopl in charge of our nation’s “health”care.

  6. Beth Ann says:

    Hi Everyone,
    I happened upon this blog somehow and thought I’d chime in. I’m a medical student and mom of three (yup, and crazy!) in the middle of studying microbiology and immunology and thought I’d share some additional info about listeria. It’s true, the odds of getting listeriosis from anything is minute. Unfortunately, when we are pregnant that minute risk is increased anywhere from 5-20 times depending on the research you look at, since our immune system is somewhat suppressed, especially in the 3rd trimester. The chances of getting it are small, but the risk of harming your baby if you do get it are huge, since listeria can cross the placenta. Oftentimes this can result in death of the fetus. I decided that even though the chances were small, the risk was so great that I could live without soft cheese, unpasteurized dairy, deli meat, and hot dogs for 9 months. But it’s a personal decision for all of us. Great blog BTW!

    • Nevra says:

      Hi Beth Ann – thanks for your feedback! Yes, I’m a big supporter of personal choice. You clearly have a good understanding of the risks to make the right choice for you and your family. And good luck with the medical studies and kids – wow, surprised you found the time to even read my post!!

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