The First Reported Outbreak of an Undetermined Species of Human Infection with Spotted Fever Group Rickettsia in Lu'an, China.

Objective: On May 2, 2017, an outbreak of unexplained fever with rashes was reported in Lu'an, China. In this study, we aimed to identify the possible pathogens, epidemiological characteristics, and risk factors of this outbreak. Methods: We conducted descriptive field epidemiological studies. Blood samples were tested using an indirect immunofluorescence assay for Rickettsia rickettsii antibody, and nested polymerase chain reaction and gene sequencing assays were performed. Results: We recruited 39 cases who had symptomatic onset from April 20 to June 8, 2017. Among these, 9 were suspected cases, 18 were probable cases, and 12 were confirmed cases. No one died. The main clinical manifestations were fever (100%), rash (100%), fatigue (97.3%), myalgia (83.8%), and anorexia (83.8%). None of the patients died. Thirty-seven patients who were treated with antibiotics during hospitalization showed significant improvement. The cases were distributed across 14 townships in 2 counties. The median age was 59 (43.0-81.0) years, of which 93.3% had a history of tea picking (28/30), and 77.3% (17/22) had a history of tick bites. The mean incubation period was 5.0 days (2.0-13.0 days). Serum IgG titers were higher in convalescent patients than in the general population (p = 0.016). Phylogenetic analysis revealed that the ompA sequences of Rickettsia sp. Lu'an-2018 had an 86.8%-99.0% sequence identity with the 23 strains of Rickettsia found worldwide. Conclusions: This was the first reported outbreak of an undetermined species of a human infection with the spotted fever group of Rickettsia in China, which might be caused by ticks biting local residents when picking tea.