Jpn. J. Infect. Dis., 53 (3), 126-127, 2000

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Laboratory and Epidemiology Communications

Annual Incidence of Tsutsugamushi Disease Caused by Different Serotypes of Orientia tsutsugamushi in Miyazaki Prefecture in 1991-1999

Seigo Yamamoto*, Kazuhiro Kizoe, Syuji Yoshino and Toshikatsu Hagiwara1

Miyazaki Prefectural Institute for Public Health and Environment, Gakuenkibanadai-Nishi 2-3-2, Miyazaki 889-2155 and 1National Institute of Infectious Diseases, Shinjuku-ku Tokyo 162-8640

Communicated by Motohiro Kato

(Accepted June 22, 2000)

There was a rapid increase of Orientia tsutsugamushi infection in various parts of Japan in the latter half of the 1970's (1). Also in Miyazaki Prefecture, the incidence of infection started to increase around 1980, and, in 1985, 189 cases were reported. We report here the annual incidence of O. tsutsugamushi in 1991-1999 in Miyazaki Prefecture.

IgM and IgG antibodies were assayed by indirect immunofluorescent antibody technique (2) by using the standard strains (Gilliam, Karp and Kato strains) and clinical isolates of Kawasaki and Kuroki strains as antigens. All strains were grown in L929 cells. Kawasaki and Kuroki strains were isolated in 1981 in Miyazaki Prefecture (3-4). An increase in antibody titers during disease progression or a single acute phase serum, whose IgM titer was higher than 1:40 and also higher than the IgG titer, were both used as criteria for confirmed cases. If a serum sample produced a titer to a particular antigen that was double that of other antigens, the serum was considered to be specific to the antigen.

Cases fulfilling the following criteria were considered to be clinical Tsutsugamushi disease, even if the serum was antibody-negative, i.e., if fever and rash and/or a lesion from a mite bite were observed, and if the onset of disease was from October to February, and if the serum was obtained within 10 days after the onset of disease.

A total of 679 cases were reported in the period between April of 1991 to March of 1999. Among them, 576 cases were confirmed by serodiagnosis (Fig. 1A). Three hundred seventy-one (64%) people were infected with Kawasaki type and 166 (29%) with Kuroki type; infections in the remaining 39 patients were difficult to classify because the sera reacted with multiple antigens (for the classification of this last group, genomic analysis of isolates was necessary) (Table). In 1985-1988, 74% of the patients were infected by Kawasaki type and 22% by Kuroki type (2). Therefore, the overall pattern of infection has remained unchanged in the past 15 years. The fluctuation of the total incidence was largely influenced by that of Kawasaki type. Kuroki type was less common in general, but was relatively frequent in 1993, 1998, and 1999 (Fig. 1B).

There are two regions, Kobayashi and Miyakonojo, endemic for Tsutsugamushi disease. However, the annual fluctuation was not identical in the two regions (Fig. 2). It is probable that the distribution and/or propagation of Leptotrombidium scutellare, a known vector of Kawasaki and Kuroki types of O. tsutsugamushi (5), differed in these two regions. The patients were diagnosed in a cluster in November and December (Table). Seventy-three percent of the patients were more than fifty years of age, reflecting the average age of the population in the endemic region.

REFERENCES

  1. Kawamura, A., Jr.(1995): Seasonal prevalence of the disease in Japan and neighboring countries. p. 10-17. In Kawamura, A., Jr., Tanaka, H.and Tamura, A. (eds.), Tsutsugamushi Disease. Tokyo University Press, Tokyo.
  2. Yamamoto, S., Kawabata, N., Ooura, K., Murata, M. and Minamishima, Y. (1989): Antigenic types of Rickettsia tsutsugamushi isolated from patients with Tsutsugamushi fever and their distribution in Miyazaki Prefecture. J. Jpn. Assoc. Infect. Dis., 63,109-117 (in Japanese).
  3. Yamamoto, S., Kawabata, N., Tamura, A., Urakami, H., Ohashi, N., Murata, M., Yoshida, Y. and Kawamura, A., Jr. (1986): Immunological properties of Rickettsia tsutsugamushi, Kawasaki strain, isolated from a patient in Kyushu. Microbiol. Immunol. 30, 611-620.
  4. Ohashi, N., Tamura, A., Sakurai, H. and Yamamoto, S. (1990): Characterization of a new antigenic type, Kuroki, of Rickettsia tsutsugamushi isolated from patients in Japan. J. Clin. Microbiol. 28, 2111-2113.
  5. Ohashi, N., Koyama, Y., Urakami, H., Fukuhara, M., Tamura, A., Kawamori, F., Yamamoto, S., Kasuya, S. and Yoshimura, K. (1996): Demonstration of antigenic variation in Orientia tsutsugamushi which were isolated in Japan, and their classification into type and subtype. Microbiol. Immunol. 40, 627-638.

* Corresponding author: Fax: +81-985-58-0930

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