A Pontiac fever outbreak associated with exposure to a bath center in Shanghai city in 2020: a survey report

  Objective  To report a survey on an outbreak of Pontiac fever associated with history of exposure to a bath center in Shanghai in 2020 for providing a reference to investigation of relevant epidemics.

  Methods  A questionnaire survey was conducted among 139 cases of fever of unknown origin (all with the history of exposure to a bath center) reported by fever clinics during 18 – 21 December, 2020 in Fengxian district of Shanghai. A subsequent case- control study was carried out among 74 cases with explicit information on activities in the bath center and 44 healthy controls ever exposed to the bath center during the same period. Deep throat sputum samples were collected from 15 cases for nucleic acid detections of multiple pathogens with reverse transcription-polymerase chain reaction (RT-PCR) and Legionella culture with buffered charcoal yeast-extract (BCYE) plate. Residual chlorine detector was used in field study to measure residual chlorine in water of various pools in the bath center. Logistic regression analysis was adopted to explore factors correlated with the incidence of the fever of unknown origin.

  Results  The most common observed symptoms among the 139 fever cases were weakness (proportion: 51.08%, case number: 71), headache (41.73%, 58), and myalgia (41.73%, 58). The median incubation period for the symptom incidence was 36 hours, with an range of 12 – 68 hours. In the case-control study, univariate analysis revealed that bathing in pools of the bath center was a risk factor for fever incidence (odds ratio OR = 9.036, 95% confidence interval 95%CI: 3.521 – 23.189); multivariate logistic regression analysis also showed that the cases ever bathing in a special pool for body beatification was at a significantly increased risk of fever incidence (OR = 5.456, 95%CI: 3.521 – 23.189). Legionella nucleic acid was detected in 4 cases. Legionella strains were cultured in 4 environment swab samples collected in the bath center. The residual chlorine of water in two of bathing pools (0.2 mg/L and 0.09 mg/L) were lower than that of the national standard.

  Conclusion  Based on epidemiologic evidence, clinical symptom of the cases, and laboratory test results, the outbreak of fever of unknown origin was a Pontiac fever caused by Legionella associated with the exposure to a bath center.