A Cluster of Japanese Spotted Fever Cases Associated with Cemetery Visits in Wakayama City, Japan

Japanese spotted fever (JSF) is a tick-borne rickettsiosis caused by Rickettsia japonica. Although the number of JSF cases has been increasing, exceeding 300 per year since 2017, clusters of cases are rare. Here, we report a cluster of seven JSF cases, the first nonfamilial cluster of the disease documented in the Japanese literature, and describe the management of the outbreak through prompt investigation and control-and-prevention measures performed collaboratively by members from the clinical, laboratory, and public health fields. All seven cases in the cluster had visited a cemetery in September or October of 2019. R. japonica was detected in whole-blood and/or skin samples from six patients and in the larvae of Haemaphysalis hystricis collected in a field survey. The evidence suggested that this cluster of cases was caused by the conjunction of two circumstances within a short period of time: an increase in the number of visitors to a cemetery during a Buddhist event and an increase in the number of infectious tick larvae in the cemetery through hatching (vertical transmission from infected females). Delays in the treatment of JSF can lead to severe manifestations. Early interventions through collaborative efforts among members from the clinical, laboratory, and public health fields are important for controlling outbreaks, raising the awareness of the public, and diagnosing and treating patients.