Scrub typhus is a miteborne infectious disease caused by the bacterium Orientia tsutsugamushi. In Japan, the disease exhibits a characteristic seasonal pattern: most human cases occur in spring and autumn, where transmission is primarily associated with Leptotrombidium pallidum and L. scutellare mites. Transmission events during summer are uncommon; when they do occur they have been infrequently attributed to Leptotrombidium akamushi mites. The report summarized here documents a 2024 human case and supporting field investigations that indicate reemergence of summer scrub typhus associated with infected L. akamushi carrying a genotype not previously common at the site.
In August 2024, a pregnant woman who had attended a fireworks event in Niigata Prefecture, Japan, developed scrub typhus. Clinical and laboratory evaluation identified infection with O. tsutsugamushi. The pregnancy resulted in fetal death. The temporal link between attendance at the outdoor event and onset of disease prompted entomologic and ecological field investigations at the site to determine the likely source and vector responsible for transmission.
Molecular testing of patient-derived material detected a Vietnam-related Karp-like O. tsutsugamushi genotype. The investigators noted that this genotype is distinct from the Kato type that has previously been associated with L. akamushi mites in some settings. The detection of a Karp-like genotype in the patient indicates that genotypic diversity of O. tsutsugamushi is present in this event and may not conform to historical associations between specific genotypes and particular mite species in the region.
A site investigation focused on the area where the fireworks event occurred. Investigators documented that L. akamushi mites were seasonally predominant at that location during the summer period. Specimens collected during the field work were tested for Orientia sequences to assess the presence of the same genotype found in the patient. The report indicates investigators detected sequences identical to the patient strain in multiple field-collected specimens, linking local vector ecology to the human case.
Genetic sequences matching the patient’s strain of O. tsutsugamushi were identified in three L. akamushi mite larvae collected at the site. In addition, an identical sequence was found in one captured field mouse (the large Japanese field mouse, Apodemus speciosus). These concordant findings in mites and a rodent reservoir at the event site provide molecular evidence that infected L. akamushi were the likely source of the human infection.
Historically in Japan, scrub typhus transmission peaks in spring and autumn linked to L. pallidum and L. scutellare. Summer cases have been rare and when documented have been associated with L. akamushi. This investigation describes a summer transmission event in which L. akamushi mites carried a Karp-like genotype of O. tsutsugamushi that is described as Vietnam-related and distinct from genotypes more commonly associated with this mite species at other times or locations. The observed seasonal predominance of L. akamushi at the fireworks site during summer supports the plausibility of summer transmission in the local ecological context.
The combined clinical, molecular, and field data presented indicate reemergence of summer scrub typhus at the evaluated site in Niigata Prefecture. Detection of an identical, Karp-like O. tsutsugamushi sequence in the patient, multiple L. akamushi larvae, and a field mouse argues that infected L. akamushi were the source of human infection. The genotype identified differs from the Kato type more typically associated with L. akamushi, suggesting local genotypic variation or introduction of a novel strain in the area.
These findings highlight the need for continued surveillance of vectors and rodent reservoirs, seasonal awareness among clinicians, and consideration of genotypic diversity in understanding transmission dynamics. The report is a case-based investigation and provides molecular and ecological linkage at a single event; broader geographic or temporal conclusions were not reported in the abstract. Further details, methods, and expanded data were available in the full text linked via the cited publication for readers seeking comprehensive methodological descriptions and additional context.