Background: Listeriosis is a foodborne infection with a low incidence but a high case fatality rate. Unlike common\r\nfoodborne diseases, the incubation period can be long. The first incubation periods were documented during a\r\nlarge listeriosis outbreak published in 1987 by Linnan and al. in the New England Journal of Medicine (range: 3 days\r\nto 70 days). Data on the incubation period of listeriosis are scarce. Our study aim was to estimate precisely the\r\nincubation period of listeriosis using available data since 1987.\r\nMethods: We estimated the incubation period of listeriosis using available published data and data from outbreak\r\ninvestigations carried out by the French National Institute for Public Health Surveillance. We selected cases with an\r\nincubation period calculated when a patient had a single exposure to a confirmed food source contaminated by\r\nListeria monocytogenes.\r\nResults: We identified 37 cases of invasive listeriosis (10 cases with central nervous system involvement (CNS cases),\r\n15 bacteraemia cases and 12 pregnancy-associated cases) and 9 outbreaks with gastroenteritis. The overall median\r\nincubation period of invasive listeriosis was 8 days (range: 1ââ?¬â??67 days) and differed significantly by clinical form of\r\nthe disease (p<0.0001). A longer incubation period was observed for pregnancy-associated cases (median: 27.5\r\ndays; range: 17ââ?¬â??67 days) than for CNS cases (median: 9 days; range: 1ââ?¬â??14 days) and for bacteraemia cases (median:\r\n2 days; range: 1ââ?¬â??12 days). For gastroenteritis cases, the median incubation period was 24 hours with variation from\r\n6 to 240 hours.\r\nConclusions: This information has implications for the investigation of food borne listeriosis outbreaks as the\r\nincubation period is used to determine the time period for which a food history is collected. We believe that, for\r\nlisteriosis outbreaks, adapting the exposure window for documenting patientsââ?¬â?¢ food histories in accordance with\r\nthe clinical form of infection will facilitate the identification of food products as the source of contamination. We\r\ntherefore propose to take an exposure window of 14 days before the diagnosis for CNS and bacteraemia cases, and\r\nof 6 weeks before the diagnosis, for pregnancy-associated cases.
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