The Topic of This Month Vol.18 No.9(No.211)
Scrub typhus or tsutsugamushi disease is an infectious disease due to Rickettsia (Orientia) tsutsugamushi transmitted by the bite of trombiculid mites and has long been regarded as an endemic disease occurring in summer at river terraces along the coast of the Sea of Japan in Niigata, Yamagata and Akita Prefectures (classical type of tsutsugamushi disease). In the fall of 1948 after the World War II, American soldiers stationing in Japan were affected by tsutsugamushi disease at the foothills of Mt. Fuji. Similar diseases were found later on Izu Seven Islands (Shichito fever) and in Chiba [Hatsuka (twenty-day) fever] and Kagawa (Umayado fever) Prefectures. The main vector mite of classical tsutsugamushi disease is Leptotrombidium akamushi, but a new type of tsutsugamushi disease found after the war is transmitted mainly by L.scutellare and L.pallidum. These mites transmit tsutsugamushi disease during late fall to winter west of the Kanto district and in spring and during fall to winter mainly in the Tohoku and Hokuriku districts.
The system of reporting tsutsugamushi disease conforming to the Communicable Disease Prevention Law was started in 1950. At that time, the classical type of tsutsugamushi disease was prevailing. The patients numbered about 100 per year, but decreased rapidly to about 10 per year during 1965-1975. From around 1980, patients of the new type of tsutsugamushi disease increased suddenly, reaching 957 in 1984. Although decreased slightly after that, the patients increased again, reaching 941 in 1990. Since 1992, they have again been decreasing, but hundreds of patients and a few deaths are still being reported even today (Fig. 1).
According to the vital statistics from the Ministry of Health and Welfare (MHW) during 1991-1995, patients of tsutsugamushi disease were reported from all over Japan except for Hokkaido, Shiga, Nara, Kagawa, and Okinawa Prefectures (Table 1). The patients have been decreasing in number year by year, but the area of occurrence of patients seems to be rather expanding. Sixteen deaths (10 males and six females) have been reported from 11 prefectures including Akita and Kagoshima. Their ages were 50-59 years in two males and two females, 60-69 years in three males and two females, 70-79 years in two males and one female, and over 80 years in three males and one female. The average morbidity of tsutsugamushi disease (per 100,000 population) in five years during 1991-1995 by prefecture was the highest in Kagoshima and Miyazaki followed by Akita, Niigata, and Oita (Fig. 2). The incidence of tsutsugamushi disease by month is the highest in May and November every year (Fig. 3). The peak incidence in May reflects the patients prevailing in the Tohoku and Hokuriku districts and that in November those west of the Kanto district. In contrast to the much higher incidence in November than in May during 1991-1994, the May incidence increased while the November incidence decreased in 1995.
The Working Group for Tsutsugamushi Disease Surveillance (the working group) set up under the Association of Public Health Laboratories for Microbiological Technology has been collecting information on tsutsugamushi disease cases since 1989 with cooperation of prefectural and municipal public health institutes (PHIs) of the whole country. A total of 2,512 cases reported by PHIs during 1991-1995 have been summarized with special reference to the age, sex (no information in 1991), estimated place of infection, and the type of work at the time of infection (see IASR, Vol. 13, No. 11; Vol. 14, No. 12; Vol. 15, No. 12; Vol. 16, No. 12; and Vol. 18, No. 1). In the reports on tsutsugamushi disease cases, male and female cases are almost equal in number. By the age group, among male patients, 60-69 years were the largest in number, followed by 50-59 years; among females, 70-79 years numbered the largest next to 60-69 years, indicating an aging tendency of female patients. With respect to the estimated place of acquiring infection and the type of work at the time of infection, about 25% of the total cases furnished no information. In the cases with information furnished, the estimated places of acquiring infection were mostly mountainous districts followed by farmlands. The type of work at the time of infection was farming followed by forestry work (Table 2).
The confirmative diagnosis of tsutsugamushi disease depends on antibody determination by the complement fixation test (CF) with the antigens of three strains of R.tsutsugamushi (Gilliam, Kato, and Karp) recommended by the 1993 MHW notification "On the implementation of countermeasures for increase of tsutsugamushi disease." CF, however, will not be able to detect the antibody of a low titer if there is an anticomplement activity, nor the IgM antibody, which may serve as an indicator of the acute phase of infection; therefore, the Committee for reference reagents of rickettsial disease diagnosis under the Research Group of Reference on Microbiological Examination of Public Health recommends the immunofluorescence (IF) and immunoperoxidase (IP) methods (see IASR, Vol. 8, No. 11, 1987). In the reports furnished by PHIs during 1991-1995, 82% of the tsutsugamushi disease cases were serodiagnosed confirmatively by IF, 16% by IP and 1% by CF. Besides, it was revealed that diagnoses of about 20% of all the cases tested have been suspended because of unavailability of the paired serum samples resulting in the failure of confirming the significant rise in the antibody titer. The working group recommends the detection by PCR of the rickettsial gene DNA in the acute phase blood samples (blood clot) (see IASR, Vol. 18, No. 1, 1997).