Abstract: Objective To investigate an outbreak in a factory, determine the pathogen, source of infection and risk factors of transmission, and provide scientific basis for epidemic prevention and control. Methods From November 12 to 20, 2019, Bao'an District Songgang Station for Health Prevention of Shenzhen city carried out case investigation, occupational health and environmental health investigation on a factory suspected of Legionella disease outbreak according to the on-site epidemiological investigation steps, and collected samples for laboratory testing. Results from November 12 to 20, 2019, there were 45 cases, including 4 suspected cases .36 clinical cases and 5 confirmed cases, with an attack rate of 10.98% (45/410). The main symptoms were headache (86.67%), fatigue (77.78%), chest tightness (73.33%), dizziness (71.11%), etc. The white blood cell count was (9.81~30.29)×10⁹/L, neutrophil count (7.35~28.15)×10⁹/L. The onset time of the first case was 13:30 on November 15, and that of the last case was 21:10 on November 15, which was consistent with the characteristics of point source exposure. All of them are day workers in the assembly workshop on the second floor of the third factory. Among them, the morbidity rate of workers in workshop A and B was higher than that in workshop C (χ²= 37, P<0.05), and there was significant difference between male and female（χ²=34.90, P<0.001）. The minimum age of the patients was 18 years old, the maximum age was 48 years old, and the average age was 28 years old. Lactic acid bacteria, Bacillus cereus, Bacteroides fragilis, Enterococcus and Spirillum were detected in all environment and blood samples, and non-specific Legionella gene fragments were detected in some blood samples. The air-conditioning outlet on the ceiling of workshop B had the same homology with the patient's blood samples. Conclusion This is a outbreak of Legionella Pontiaceae fever caused by central air conditioning and humidifier. The source of the outbreak is a pipe air conditioning system.

Key words: Legionella, Pontiac fever, Outbreak, Factory