We reviewed all outbreak-associated invasive L. monocytogenes cases reported to the Centers for Disease Control and Prevention from 1985 to 2015 with exposure date information available. Incubation periods were calculated using consumption dates and illness onset. If a single consumption date was reported, an incubation period was designated as “single.” An incubation period was designated as “possible” if a food was consumed multiple times, there was illness onset date uncertainty, or purchase date was used as a consumption date proxy. All possible exposure dates were included in a multiple exposure date simulation model producing a uniform frequency distribution of each possible incubation period [9]. For example, if a patient’s exposure occurred 5–10 days before illness onset, the integers 5–10 were randomly selected for 10 000 model iterations. We excluded cases with only a minimum (eg, patient reported consuming the implicated food at least 2 weeks before illness onset) or maximum (eg, patient reported consuming the food sometime in the 2 weeks before illness onset) possible incubation period available.

Clinical manifestations and the specimen source were used to classify cases into 4 clinical manifestation categories: (1) CNS cases (L. monocytogenes isolated from cerebrospinal fluid or from blood with clinical findings of CNS involvement), (2) bacteremia cases (L. monocytogenes isolated from the blood without CNS involvement), (3) pregnancy-associated cases (L. monocytogenes isolated from a pregnant woman, newborn, or products of conception), and (4) other listeriosis cases. Mother-infant pairs, which typically reflect transplacental infection, were counted as single cases. We classified foods implicated in outbreaks as produce, dairy, or meat/seafood.

Analyses were performed for single reported incubation periods alone and for single and possible incubation periods using US data alone and with European data from Goulet et al [8]. We calculated the median incubation period and the first and third quartiles for each clinical manifestation. Quantile regression was used to estimate the median incubation period, adjusting for clinical manifestation and food category.

This examination of US incubation period data for listeriosis, the largest to date, found a median incubation period of 11 days among cases with single and multiple possible incubation periods. The inclusion of cases with multiple possible exposure dates allowed for a large sample size and inclusion of cases linked to foods consumed on more than one occasion.

The median incubation period in this study is similar to that reported by Goulet et al [8] (8 vs 11 days), but different from the typically reported 3 weeks. However, the incubation periods for clinical manifestation categories differed between this study’s combined analysis and that of Goulet et al for bacteremia cases (9 vs 2 days, respectively), CNS cases (13 vs 9 days), and pregnancy-associated cases (11 vs 27.5 days).

The incubation period differences observed between US and European analyses, however, must be interpreted with caution given the relatively few cases in each category, particularly among pregnant women, and other host and external factors that may influence incubation period. These factors include the implicated food, the L. monocytogenes dose ingested, host susceptibility and response, and the virulence of the L. monocytogenes strain. In a healthcare-associated listeriosis outbreak in Norway caused by highly contaminated Camembert cheese, the median incubation period was approximately 3–4 days (range, 1–24) [10]. The short median incubation period may reflect the high number of bacteria (estimated 1 × 10⁸ organisms) and the fact that most patients had an immunocompromising condition. Conversely, a French outbreak linked to Brie cheese included a large number of pregnancy-associated cases (55%) with prolonged incubation periods (median, 32 days; range, 17–67), suggesting that host response and susceptibility or strain virulence played a role [11].

Although there was no significant difference between produce and other food commodities at the 50th percentile (P = .06), cases linked to produce had a significantly longer incubation period (6 days) at the 75th percentile with adjustment for clinical manifestation. Produce seems to be an increasingly common source of listeriosis outbreaks in the United States [12], and more enumeration testing after outbreaks would help determine whether produce items are usually associated with lower or higher L. monocytogenes doses than meat or dairy products. Produce is often eaten raw, so there is less opportunity for pathogens to be killed and items could vary dramatically in level of contamination, making generalizations difficult.

After a food recall or listeriosis outbreak public notices, person are often concerned if they have eaten the food involved. Our finding that incubation periods were ≤2 weeks in >75% and ≤3 weeks in 85% of non-pregnancy-associated cases (≤5 weeks in >75% and ≤6 weeks in 90% of pregnancy-associated cases), may help influence treatment and monitoring decisions. These data may also improve the food histories collected during outbreak investigations. Given that the exposure window of highest concern may be 5–6 weeks before illness onset for patients with pregnancy-associated illness and 3 weeks for others, food histories may be taken in this time frame. However, other factors, such as comparability with historic interview data and ease of questionnaire administration, should also be considered.