Wednesday, January 2, 2008

Food safety: Listeriosis

During pregnancy women are advised to be extra careful about ingesting particular foods. This advice stems from two main concerns: (1) The immune system of a pregnant woman is suppressed, making her more susceptible to illness including food-borne pathogens and (2) certain pathogens can cross the placenta and present a danger to the fetus. While the reasoning is sound, the data on the presence and effects of some important food-borne pathogens is scant and their translation into actual recommendations for the pregnant woman is puzzling.

For my first post on this topic, I've spent some time researching listeriosis, a disease caused by ingesting the bacterium Listeria monocytogenes. I don't want to discuss the details of listeriosis, suffice it to say that pregnant women, fetuses and newborns are particularly susceptible and many of the foods pregnant women are told to avoid are so categorized due to concerns about listeriosis. See the Center for Disease Control (CDC) factsheet on listeriosis for more general disease information. Let me just stress that listeriosis is highly fatal, and is fatal more often than any other pathogen. This is not a slight risk.

I became interested in this topic when I was pregnant during the fall of 2005 and told to avoid all "soft cheeses" because they could contain Listeria monocytogenes. Being a true lover of cheeses (from Wisconsin), I couldn't understand what ALL "soft cheeses" had in common that allowed for the growth of this bacterium. In fact, I knew (and verified) that Listeria is killed during pasteurization. When asked for more details, my doctor could only say that it was something to do with the way the cheese is processed. This was not helpful, as I couldn't be sure that some of the moderately soft cheeses, such as Havarti, were safe and my typical diet includes a wide variety of cheeses that weren't easily classified into "soft" or not. Although I did eliminate all "soft" and other interesting cheeses from my diet during my pregnancy I became convinced that a better recommendation would be to avoid cheeses made from unpasteurized or raw milk and maybe mold-ripened cheeses. The more I researched the topic, the more I realized that there hadn't been much research on the actual presence of Listeria in foodstuffs. In fact, it is only in the past two decades that Listeria has been known to be food-borne (it's ubiquitous in the soil and air).

So, how many people have had listeriosis recently and what was the source of their infection?
The CDC publishes weekly morbidity and mortality reports (MMWR) which are searchable at Pubmed and on the CDC website.

In the MMWRs since 1998 and other pubmed articles there are the following reported cases of Listeriosis:

  • 108 cases in 24 states (14 deaths and four miscarriages or stillbirths) during August -December 1998 attributed to hot dogs
  • 29 cases in 10 states during May 17-November 26, 2000 attributed to deli turkey meat
  • 3 cases in November 2000 in recent Mexican immigrants attributed to a fresh soft Mexican cheese from a local dairy farm
  • 46 culture-confirmed cases, seven deaths, and three stillbirths or miscarriages in eight states July 18-September 2002, attributed to eating sliceable turkey deli meat
  • Yearly reports since 2000 list approximately 2500 cases of listeriosis yearly in the U.S. causing 500 deaths
Note that none of these outbreaks are related to pasteurized cheese products.

Since we can't be sure that all cases of listeriosis are reported and not all reported cases have a determined cause, it would not be good to rely on this information alone. Thus, I also investigated how common Listeria monocytogenes is in the general food supply.

The most comprehensive study I could find tested 31,705 food samples in California and Maryland in the years 2000 and 2001 for Listeria monocytogenes (Gombas et al 2002).

In order of most often contaminated to least often:
Samples from CA:
  1. Deli salads (chicken salad, egg salad etc.; 99 samples contaminated)
  2. Smoked seafood (71)
  3. Luncheon meats (28)
  4. Seafood salads (27)
  5. Blue-veined cheeses (17)
  6. Bagged salads (14)
  7. Mold-ripened cheeses (13)
  8. Fresh soft cheeses (1)

Samples from MD:
  1. Deli salads (103 samples contaminated)
  2. Seafood salads (88)
  3. Luncheon meats (54)
  4. Smoked seafood (43)
  5. Bagged salads (8)
  6. Blue-veined cheeses (6)
  7. Fresh soft cheeses (4)
  8. Mold-ripened cheeses (1)

In order of highest level of contamination (i.e. greatest concentration of L. monocytogenes) which corresponds to greatest risk of exposure for humans eating the food.

CA, foods exceeding 100-1000 CFU/g
  • Smoked seafood (6 samples)
  • Luncheon meats (3 samples)
  • Deli salads (1 sample)

MD, foods exceeding 100-1000 CFU/g
  • Luncheon meats (5 samples)
  • Smoked seafood (3 samples)
  • Seafood salads (2 samples)
  • Bagged salads (1 sample)

At least one sample for each food category was found to contain >1-10 CFU/g except seafood salads in CA and mold-ripened cheeses in MD. Currently there is no legally acceptable level of Listeria in foods (i.e. any detectable Listeria renders that food in violation of FDA standards).

The study also found that foods were more likely to be contaminated with Listeria if they were packaged in-store rather than by the manufacturer.

What really surprised me about these results was the prevalence of Listeria in deli salads. I have never seen a recommendation against deli salads during pregnancy. I also know a number of pregnant women who diligently avoid soft cheeses but are unaware that they should really be avoiding cold lunch meats, suggesting that OBs may not be stressing the foods to avoid in order of their risk. This suspicion was supported by a study done by the USDA on food safety/handling by pregnant women in which none of the participants re-heated their lunch meats to steaming hot temperatures, many were unfamiliar with Listeria as a pathogen and many did not know that pregnant women are at increased risk of food-borne diseases.

Now, let's take a look at how this data is reflected in the recommendations for food consumption in pregnancy. Here are the recommendations that the American Pregnancy Association has posted on their website for foods to avoid during pregnancy because of the risk of listeriosis. I chose this site's recommendations because they are reflective of the list I received from my doctor and the recommendations posted on many other commonly visited pregnancy websites (such as and and they specifically mention why each category of food is a risk (I'm only showing foods they list because of listeriosis).

APAs list of foods to avoid during pregnancy:
  • Deli Meat: If you are pregnant and you are considering eating deli meats, make certain that you reheat the meat until it is steaming.
  • Smoked Seafood -Refrigerated, smoked seafood often labeled as lox, nova style, kippered, or jerky (These are safe to eat when they are in an ingredient in a meal that has been cooked, like a casserole.)
  • Soft Cheeses: Imported soft cheeses and other soft cheeses such as: Brie, Camembert, Roquefort, Feta, Gorgonzola and Mexican style cheeses that include queso blanco and queso fresco, unless they clearly state that they are made from pasteurized milk. All soft non-imported cheeses made with pasteurized milk are safe to eat.
  • Unpasteurized Milk
  • Pate: Refrigerated pate or meat spreads. Canned pate, or shelf-safe meat spreads can be eaten.

As you can see, there is no mention of avoiding deli and seafood salads. A gross oversight according to the evidence I summarized above from Gombas et al (2002) which is the data forming the basis of a proposal currently under consideration by the FDA for reducing restrictions on the acceptable levels of listeriosis in ready-to-eat foods. Furthermore, the cheese recommendation is still listed as "soft cheeses" despite the recent clarification made by the FDA to avoid cheese made from raw or unpasteurized milk rather than just "soft cheeses." It also doesn't mention that cooking the cheeses can remove the threat of listeriosis by killing the pathogen.

In searching for the most recent cases of listeriosis or Listeria-contaminated foods (i.e. those occurring in the last few weeks of December 2007, see links below) I found a list of recommendations for those people susceptible to Listeriosis on the USDA website that very closely reflects the data that I presented here. This is the only list I've found anywhere that doesn't sound like what I pasted above from the APA. Hopefully, the new USDA list will be distributed by more OBs and popular pregnancy sites will start to use it as well. The Gombas et al. study has been out for 5 years, it's about time their findings are put to use in saving lives.

Current listeriosis news:

Listeriosis breakout in Massachusetts maybe linked to pasteurized milk:

Recall of beef patties sent to New Jersey schools due to high levels of Listeria contamination

Thanks for your patience with my first post. I hope that this information will help pregnant women reduce the risk of illness for themselves and their babies. I also hope it will inspire pregnant women to take a more critical view of the information they are given (or not given) by their medical providers. In future posts I will cover topics that are less discussed in popular pregnancy media, but first I'd like to address some very prevalent but often mistaken advice.

I'd also like to collect a bit of data on what kind of care pregnant women are getting. In particular, I'd like a list of the tests women have been offered or given during pregnancy (e.g. Rh factor blood test, toxoplasmosis titers) the number of routine OB visits, the number and timing of ultrasounds and notes on any discussion of due dates and induction at term. You can e-mail these to me at

--Allan's mom

Tuesday, January 1, 2008


What does a scientist do when she becomes pregnant? She reads about pregnancy in every place she can including websites, books and scientific journals. She is exuberant but careful and informed.

And what does she do when after an easy complication-free pregnancy her child is stillborn at 40 weeks and 5 days gestation with no obvious cause? She continues to mother her child in the ways that are left to her which include continuing her study of the scientific material relating to pregnancy but with an increased focus on stillbirth.

In this blog I share with you my quest for a deeper understanding of pregnancy. In particular, I will describe the research I find most compelling for a sage way to approach pregnancy. I will also discuss pregnancy management, particularly preventative measures to improve pregnancy outcomes. Sadly, there is not a great wealth of well-researched information on these topics and much available information about pregnancy is riddled with well-meaning supposition.

I hope this blog will be a resource for sage parents as they embark on the joys and trials of pregnancy and childbirth. I am not a medical doctor, but an overly critical evolutionary biologist. What I post here is food for your pregnancy thoughts and does not constitute medical advice.