The Topic of This Month Vol.20 No.9(No.235)

Japanese spotted fever

Japanese spotted fever, being a tick-borne rickettsial disease, was reported for the first time in Tokushima Prefecture in 1984. The antibody survey consequently carried out has revealed that there were cases of this disease among those suffering from scrub typhus-like illness occurring in Tokushima and Kochi Prefectures in 1983. The etiological agent, Rickettsia japonica (Uchida, T. et al., Int. J. Syst. Bacteriol., 42: 303-305, 1992), belongs to the spotted fever-group rickettsiae as do those of Rocky Mountain spotted fever and tick typhus and is transmitted by Ixodidae. As a vector tick of Japanese spotted fever, genus Haemaphysalis is being suspected (see p. 214-216 of this issue).

The clinical symptoms of Japanese spotted fever resemble those of scrub typhus. Primary rashes appear more often on the limbs rather than on the trunk and the tick bites are small in size as compared with those of scrub typhus, disappearing in a few days in some cases. For therapy, tetracycline is very effective as for scrub typhus (see p. 213 of this issue). The confirmatory diagnosis depends upon the detection of antibodies; no cross-reaction with scrub typhus occurs. Detection of rickettsia genome has recently been utilized (see p. 217 of this issue).

In Japan, scrub typhus has been regarded as important among rickettsial diseases and hundreds of patients are even nowadays reported annually (see IASR, Vol. 18, No. 9, 1997). Since the demonstration of spotted fever in Japan in 1984, the National Institute of Infectious Diseases (NIID) has been monitoring the incidents of spotted fever-like diseases via telephone with cooperation from prefectural and municipal public health institutes (PHIs) and clinical physicians and surveillance was carried out in 1995 through questionnaires by the Working Group for Tsutsugamushi Disease Surveillance in Japan, the Association of Public Health Laboratories for Microbiological Technology, which is composed of 73 PHIs and NIID.

Cases of Japanese spotted fever were reported from Chiba, Shimane, and Miyazaki Prefectures after Tokushima and Kochi Prefectures and totaled over 200 cases by 1998. Annual cases somewhat decreased after 1989, but have tended to increase again since 1995 (Fig. 1). Cases have been reported from 10 prefectures, all of which but Shimane Prefecture are located on the Pacific coast; cases tend to be more prevalent in Kagoshima, Tokushima, Kochi and Chiba Prefectures (Fig. 2). Only a case was reported in Mie Prefecture in 1988 and two cases in Kanagawa Prefecture in 1992; no further cases have been reported since then.

During the four years till 1998, 77 incidents were confirmed, 70 of which were reported through questionnaires including age, sex, estimated place of infection, type of work when acquiring infection, tick bites observed, and clinical symptoms. The following is a summary of the analysis of the reports.

Cases of spotted fever-like illness reported numbered 16 in 1995, 14 in 1996, 24 in 1997, and 16 in 1998. All were diagnosed as Japanese spotted fever from the significant elevation of the antibody titer determined by indirect immunofluorescence assay with R. japonica as the antigen. Monthly incidence of Japanese spotted fever in comparison with that of scrub typhus shows a large number of cases in July-September, when cases of scrub typhus are few (Fig. 3). In May-June, both of Japanese spotted fever and scrub typhus occur, but scrub typhus is restricted to the Tohoku and Hokuriku districts where Japanese spotted fever was never reported (Fig. 2). No sex differences are seen in cases of Japanese spotted fever, and all age groups are affected, but about 2/3 of them are over 50 years of age (Table 1) as is the case in scrub typhus cases (see IASR, Vol. 18, No. 9, 1997). The estimated places of infection with Japanese spotted fever rickettsia are mostly mountains. The type of work when acquiring infection is most frequently farming, followed by recreational, forestry activities, and collecting edible wild plants (Table 2). This tendency is similar to that seen in scrub typhus, but the proportion of infection when engaged in forestry work is higher in scrub typhus. It is generally accepted that places in which Japanese spotted fever and scrub typhus occur are geographically separate, but both diseases occur in the scrub typhus-prevalent prefectures. Since Japanese spotted fever has occurred in more and more prefectures, it has become difficult to differentiate the two diseases by relying on the place of contraction (see IASR, Vol. 18, No. 9, 1997 and p. 216 of this issue).

By the Law concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the New Infectious Diseases Control Law) enforced in April 1999, Japanese spotted fever as well as scrub typhus is grouped into the category IV notifiable infectious diseases requiring mandatory notification from the physicians (see IASR, Vol. 20, No. 4, 1999). The mode of transmission of Japanese spotted fever is still not fully understood in that species of the vector tick is only an estimate. However, the occurrence of patients is concentrated to summer when scrub typhus is rather rare. If cases suspicious of scrub typhus occur in summer and scrub typhus is ruled out, tests for Japanese spotted fever will be warranted. The criteria for reporting Japanese spotted fever, being the same as those for scrub typhus, are such cases that are suspected of Japanese spotted fever by the physician from the clinical symptoms and observations and from which the etiological agent or its genome was isolated or detected or which are showing a significant elevation of antibody titer.

Cases of infection with spotted fever-group rickettsiae occur in many places around the world and cases of overseas infection have occurred in Japan, one case in 1988 in Kanagawa Prefecture and two in 1991 and other two in 1996-97 in Tokyo (see p. 218 of this issue).

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