Food-borne illnesses during pregnancy

We wish to thank Dr Khatter1 and Ms Taylor and Dr Galanis2 for their interest in our Motherisk Update “Food-borne illnesses during pregnancy” published in the April 2010 issue of Canadian Family Physician.3

We believe that some of their recommendations are not evidence-based. We concur that it is important for pregnant women to be very careful with regard to consuming certain foods. However, despite their impressions, we did not make contradictory statements, as will be clearly shown here.

Despite the increased relative risk for pregnant women contracting Listeria, the absolute risk is extremely low and avoiding deli meats altogether does appear to be rather punitive. A risk assessment of Listeria monocytogenes in ready-to-eat foods conducted by the Food and Drug Administration Center for Food Safety and Applied Nutrition and the US Department of Agriculture Food Safety and Inspection Service estimated the risk of developing listeriosis to be 1.2 × 10⁻⁵ (95% confidence interval 3.2 × 10⁻⁶ to 1.4 × 10⁻⁵) and 2.0 × 10⁻⁷ (95% confidence interval 4.8 × 10⁻¹¹ to 5.3 × 10⁻⁶) per serving of deli meat and soft cheese, respectively, in the perinatal population. Taking the reciprocal of these values, the model estimates 1 case of listeriosis in 83 000 servings of deli meat or 5 million servings of soft cheese consumed by pregnant women, which is a minimal risk.4

To put this in perspective, for a woman who is at less than 20 weeks’ gestation, a conservative estimate of the risk of fetal loss following contact with an individual in the contagious stage of fifth disease (parvovirus B19 infection) is 1.8 x 10⁻³. This calculation uses the lower estimates of the proportion of the population who are not immune (35%), the chance of maternal infection (20% in a day-care or school setting), the rate of vertical transmission (17%), and the estimated rate of spontaneous loss of fetuses affected by parvovirus B19 before 20 weeks’ gestation (14.8%). For a woman who acquires the infection after 20 weeks’ gestation, the estimated risk of fetal loss is lower (2.7 × 10⁻⁴), but still 1 order of magnitude higher than the risk of developing listeriosis following consumption of a single serving of deli meat. However, most government agencies do not recommend that pregnant women be routinely excluded from a workplace where an outbreak of fifth disease is occurring.5–8

We strongly believe that each woman should be informed of the nature and magnitude of the risks associated with the consumption of deli meats and soft cheeses. The decision to consume or avoid these foods should be made by her, based on this information, allowing her to make an informed decision. Subsequently, if she chooses not to avoid them, she should be advised on how she can minimize her exposure (and thus her risk). Factors that determine exposure include amount and frequency of consumption, duration of refrigeration before consumption, and temperature at which the food is stored. We acknowledge that L monocytogenes—unlike most bacterial pathogens—are able to grow at refrigeration temperatures (ie, 4°C). However, growth at 4°C is slow, and limiting the duration of storage will limit bacterial growth.9

Therefore, pregnant women should be advised to do the following with respect to deli meats and soft cheeses: 1) limit the amount and frequency of consumption; 2) limit the duration of storage by choosing the freshest foods (ie, those dated as close to the manufacturing or packaging date as possible) and consuming them in a timely manner; and 3) ensure that these foods are kept at the correct temperature (ie, less than 4°C) at all times, including during transport from the retail outlet to the home.

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