What is the incubation period for listeriosis?

Abstract

Background: Listeriosis is a foodborne infection with a low incidence but a high case fatality rate. Unlike common foodborne diseases, the incubation period can be long. The first incubation periods were documented during a large listeriosis outbreak published in 1987 by Linnan and al. in the New England Journal of Medicine (range: 3 days to 70 days). Data on the incubation period of listeriosis are scarce. Our study aim was to estimate precisely the incubation period of listeriosis using available data since 1987.

Methods: We estimated the incubation period of listeriosis using available published data and data from outbreak investigations carried out by the French National Institute for Public Health Surveillance. We selected cases with an incubation period calculated when a patient had a single exposure to a confirmed food source contaminated by Listeria monocytogenes.

Results: We identified 37 cases of invasive listeriosis (10 cases with central nervous system involvement (CNS cases), 15 bacteraemia cases and 12 pregnancy-associated cases) and 9 outbreaks with gastroenteritis. The overall median incubation period of invasive listeriosis was 8 days (range: 1-67 days) and differed significantly by clinical form of the disease (p<0.0001). A longer incubation period was observed for pregnancy-associated cases (median: 27.5 days; range: 17-67 days) than for CNS cases (median: 9 days; range: 1-14 days) and for bacteraemia cases (median: 2 days; range: 1-12 days). For gastroenteritis cases, the median incubation period was 24 hours with variation from 6 to 240 hours.

Conclusions: This information has implications for the investigation of food borne listeriosis outbreaks as the incubation period is used to determine the time period for which a food history is collected. We believe that, for listeriosis outbreaks, adapting the exposure window for documenting patients' food histories in accordance with the clinical form of infection will facilitate the identification of food products as the source of contamination. We therefore propose to take an exposure window of 14 days before the diagnosis for CNS and bacteraemia cases, and of 6 weeks before the diagnosis, for pregnancy-associated cases.

Figure 1

Distribution of the incubation period (in days) for 37 invasive cases of listeriosis.

Figure 2

Distribution of the incubation period (in days) of 37 invasive cases of listeriosis by clinical form of disease.

Figure 3

Distribution of incubation period for each clinical form of 37 invasive cases of listeriosis (box-plot). Line in the middle of boxes represents median of data. Boxes extend from the 25th percentile (X_[25]) to 75th percentile (X_[75]), representing interquartile range (IQR). Lines emerging from boxes extend to upper and lower adjacent values. The upper adjacent value is defined as the largest data point ≤X_[75] + 1.5 x IQR. The lower adjacent value is defined as the smallest data point ≥X_[25] – 1.5 x IQR. Dots are outliers (every point more than 1.5 x IQR from the end of a box).