The Topic of This Month Vol.21 No.6(No.244)

Imported dengue fever in Japan

Dengue virus infection in man occurs by bite of infective mosquitoes, Aedes aegypti or Aedes albopictus, carrying dengue virus. Although some cases may recover spontaneously, showing only fever as the symptom, those showing characteristic symptoms may develop either of the two distinct illnesses, dengue fever or dengue hemorrhagic fever (see p. 114 of this issue).

Epidemics of dengue fever in western Japan were reported during the period of 1942 to 1945 (Hotta, S., Med. Entomol. Zool., Vol. 49, No. 4, p. 267-274, 1998). An epidemic arose in Nagasaki City in July 1942, followed by others in Sasebo, Hiroshima, Kure, Kobe and Osaka Cities. Since its inception in 1942, the epidemics in different areas probably arose independently. It apparently seems that dengue fever patients among returning warship crew members from Southeast Asia were the ones who introduced the virus to Japan and epidemics were roused in the arrived cities by indigenous mosquitoes Aedes albopictus prevailing. Besides, Aedes aegypti mosquitoes' presence was confirmed on the ships at that time. The continuation of epidemics for the following years may be explained by (1) importation of the virus on a yearly basis, or (2) transovarial transmission of the virus in Aedes albopictus and its hibernation in the eggs; neither possibility has yet been proven. The total number of patients was estimated at over 200,000 for that period (Hotta, S., J. Trop. Med. Hyg., Vol. 56, p. 83, 1953). Dengue virus type 1 responsible for these epidemics was isolated from blood samples of patients in Nagasaki, and a later survey detected the neutralizing antibody to dengue virus type 1 in yet other patients in Nagasaki.

At the present time, dengue fever derived from domestic infection does not occur since dengue virus does not inhabit Japan. Nevertheless, imported cases may occur by acquiring dengue virus infection during traveling in tropical and subtropical endemic areas maintaining dengue virus and developing symptoms after returning home. Hitherto, only few institutes in Japan have continuously carried out etiological diagnosis of dengue virus infection (see p. 114-116 of this issue), and only the records of imported dengue fever patients are available at the individual institutes. The numbers of yearly dengue patients diagnosed at the Department of Virology I, National Institute of Infectious Diseases (NIID) are displayed in Table 1. From 1985 to 1989, fewer than five cases were diagnosed annually, whereas more than 10 cases per year were diagnosed repeatedly after 1990. It is noteworthy that in 1998, more than 40 cases were diagnosed. The type of the dengue virus from 26 cases was identified by PCR during 1992-1999; nine (35%) were type 1, 11 (42%) type 2, four (15%) type 3, and two (8%) type 4. Cases diagnosed etiologically at the Osaka Prefectural Institute of Public Health and at the Institute of Tropical Medicine of Nagasaki University counted at fewer than 10 annually after 1994 and 1996, respectively. Those specimens were taken from suspected dengue cases and sent to these institutes by clinicians. These cases may represent only a fraction of the cases in the whole country.

The dengue patients diagnosed at NIID during 1985-1999 had visited 25 different countries (Table 2) before developing symptoms. Most of them (96%) visited Asian countries: 65 patients visited Thailand, 27 India, 26 the Philippines, 18 Indonesia, eight both Malaysia and Myanmar (Burma), seven Cambodia, and seven Singapore. In Table 2, the sum of each column exceeds the number of cases shown in Table 1, as some of them visited two or more countries.

Since 1990, in Southeast and South Asian countries, dengue fever and dengue hemorrhagic fever cases have been in the increase. More than 1,000 annual cases were reported also in Taiwan (Table 3). It is notable that cases acquiring infection during visiting Oceanic, South Pacific, Central American or African countries have recently been reported (Table 2). This may reflect diversification of visiting countries and the increase in dengue virus infection in both Central America and South Pacific countries (Pinheiro et al., Wld. Hlth. Statist. Quart., Vol. 50, p. 161, 1997). From the above data, dengue fever must be placed for differential febrile illness' diagnosis of returnees and visitors from not only Southeast and South Asian countries, but also such tropical and subtropical countries as Oceanic, South Pacific, Central and South American, and African countries.

During the four-year period from 1996 through 1999, the 74 cases of dengue virus infection diagnosed at NIID included 53 males and 21 females (Table 4). No sex difference has been found in the susceptibility to dengue virus. Nevertheless, Japanese males may have more chances of exposure to the virus in dengue-endemic areas than females. About 50% of the cases were ages 20s and those ages 10s, 30s and 40s accounted for 13, 21, and 13%, respectively (Fig. 1).

There is no domestic dengue virus infection, while there are vector mosquitoes, Aedes albopictus, in Japan. It is also possible that Aedes aegypti mosquitoes, not inhabitants of Japan, are temporarily brought in by air or ship into Japan and may survive during the summer. Since epidemics of dengue fever occurred in 1940s, attention must be paid to the possibility of provoking domestic infection by such mosquitoes.

In the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections enacted in April 1999, dengue fever is classified into the category IV notifiable infectious diseases. Nine and two cases were notified by clinicians in 1999 and as of May 28, 2000, respectively.

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