Monday, March 26, 2007

Ask a Scientist: Listeria and Pregnancy

I know these are overwhelmingly about pregnancy. This is because my pregnant friends and acquaintances (now great, mighty and numerous) and my girlfriends who may one day be pregnant, keep asking things. And I believe that risks in pregnancy are, as Mark Twain said, greatly exaggerated.

Listeria! Not something you hear about a lot, because the CDC's most recently reported incidence- in 2005- was 0.3 per 100,000 (3 in a million).

A friend's midwife ('Not the kind with little bells and a faint smell of patchouli. These midwives are all mean and brisk and no-nonsense') told her that a pregnant woman's risk of listeriosis was 20 times the basal risk. Really?

Listeria is, in fact, super extra nasty if you're pregnant because it crosses the placental barrier and infects the fetus. This frequently results in miscarriage or fetal death.

[As a side note, most listeria infections in pregnancy occur in the 3rd trimester, when suppression of Th1-mediated immunity is at its maximum.]

The 20-fold figure is based on a single CDC study in 1986. They tracked all reported cases of listeria for one year in six health departments across the country, and came up with a rate of about 5 per million population overall. 27% were in pregnant women (pw); the authors record 67 perinatal cases, which at the time was 12.7/100,000 live births. The CDC reports an annual birth rate of 13.9 per 1000 population, so 1.1% of population is pregnant women (let's estimate, based on being pregnant 40/52 weeks). So if you're not pregnant, your incidence of listeria is 5 per million, and if you are, it's 127 per million pw. This a 25-fold increase in risk.

Data from France, where the overall incidence is about 3.5 per million, reported 24% of all cases occurred in pw. The birth rate is 11.99 per 1000- 0.92% of the population consists of pw; this works out to 2.6 cases per million nonpregnant population and 86 cases per million pw, or a 33-fold enhancement in risk.

A more recent seven-year CDC FoodNet survey ("Pregnancy-associated listeriosis in Hispanic women in FoodNet Sites", also here) found a rate of 3 per million population, and 16% were in pregnancy (2.5 per million non-pw, 43 per million pw)4. Of all cases, 7% of patients were Hispanic, but 28% of pregnancy-associated cases were in Hispanic women (eating queso fresco seems to be the cause of this increase). So that makes an 17-fold risk enhancement.

To further muddy the waters, of 75 cases reported in Finland over 9 years, only 4 (5%) were in pregnant women. On the other hand, in several reported many-case outbreaks, up to 85% of infections were in pregnant women. Except when they weren't: one in Boston had
14% infection in pregnant women. And then another review says maybe overall it's more like 35%? They think? And another study of isolated infections found 11% in pregnant women.

In case you need to feel alarmed about eating anything, Listeria infections or contamination have been found not only with cold-smoked fish (lox, etc.), feta, brie, other soft cheeses, and hot dogs, there have also been cases and outbreaks from coleslaw, pasteurized-but-somehow-contaminated chocolate milk, undercooked chicken, deli meat, hummus, melons, lettuce, packaged sandwiches, pasta salad, egg salad, mushrooms, dips, and pretty much anything else you could imagine.

So overall, the moral is, the most risk is from preserved fish, cheese from unpasteurized milk, and deli meats. Possibly also avoid vegetables, milk, hummus, Finland, and eating out if you're feeling particularly paranoid. The risk of listeriosis is increased substantially in pregnancy, and what do you know, it might really be 20-fold.

But don't be too alarmed. Remember even with the highest numbers, the total incidence of Listeria in pregnancy is only 0.01%.