Surge in Japanese spotted fever analyzed by Okayama University: 10‐fold increase over 20 years, with a noticeable increase in the elderly and expansion into eastern Japan

Japanese spotted fever is a tick‐borne infection first reported in Japan in 1984. For some time afterwards, reports of outbreaks were limited to the Pacific coast and western Honshu, where it was recognized as an extremely rare disease. However, the number of cases in Japan has been increasing year by year since its designation as a category 4 infectious disease with the amendment to the Infectious Diseases Act in 1999 and the start of nationwide surveillance. Although antibiotics are often an effective treatment, the disease can be fatal in some patients, leading to disseminated intravascular coagulation and multi‐organ failure, so an accurate understanding of epidemic trends and epidemiology is needed, but there was insufficient knowledge of the regions and patient groups that are on the increase.

A research group led by Special‐Appointment Assistant Professor Yuki Otsuka, Associate Professor Hideharu Hagiya and Associate Professor Toshihiro Koyama of Okayama University used publicly available data on the number of Japanese spotted fever cases to estimate its incidence over 20 years for each age group and prefecture, and examined the data using trend analysis methods to quantify the rate of change for the first time. The annual incidence rate for Japan as a whole was 0.03 per 100 000 population in 2001, but in 20 years it had increased almost 10‐fold to 0.33, with an average annual rate of change of 12.3%. By age group, the annual rate of change was found to be particularly high in the 65 and over group. Comparisons by prefecture showed that while the average incidence was higher in warmer regions such as the Pacific coast and western Japan, as previously reported, the average annual rate of change was not necessarily higher in the traditional high incidence areas and tended to be higher in eastern Japan, which is considered to be relatively colder.

The impact of global warming and human activities has been suggested as the cause of the increase, and in addition to Japanese spotted fever, tick‐borne infectious diseases such as scrub typhus, Lyme disease and ehrlichiosis are endemic to Japan, and this research is important to help create public health measures to combat these diseases. The researchers' results were published in the American Journal of Tropical Medicine and Hygiene.

"The authorities and various facilities had increasingly reported that Japanese spotted fever was spreading rapidly in Japan, but as a general practitioner actually treating the disease, I wondered how much, where and among what part of the population was the increase happening," recalled Otsuka. "Our trend analysis in this case allowed the rate of increase to be quantified as well as the prefectures with the highest rates of increase to be identified. We hope it will help to infer the causes of the global tick‐borne disease epidemic and help to inform public health measures."

Hagiya added, "The epidemiology of infectious diseases changes gradually, so it is important to conduct trend analyses such as this study on a regular basis. We would like to continue to work on the epidemiological analysis of various infectious diseases."

Journal Information
Publication: American Journal of Tropical Medicine and Hygiene
Title: Trends in the Incidence of Japanese Spotted Fever in Japan: A Nationwide, Two‐Decade Observational Study from 2001‐2020
DOI: 10.4269/ajtmh.22‐0487