The Topic of This Month Vol.25 No.2(No.288)

Imported dengue fever and dengue hemorrhagic fever in Japan, April 1999-December 2003

(IASR 2004; 25 : 26-27)

Dengue virus is transmitted to humans by bites of Aedes aegypti (photo 1) or Aedes albopictus (photo 2) (see p. 34 of this issue). Dengue virus infection causes two distinct syndromes, dengue fever and dengue hemorrhagic fever/shock syndrome. The three main symptoms of dengue fever (DF) are fever, exanthema and pain (arthralgia), and its case-fatality rate is low. Dengue hemorrhagic fever (DHF), first found in the Philippines in 1953, is characterized by fever, hemorrhagic manifestations, and circulatory disturbances and is likely to cause deadly shock if no adequate treatment is given. In Japan, DF epidemics, occurring in western Japan (in Nagasaki, Sasebo, Hiroshima, Kure, Kobe, and Osaka Cities) during 1942-1945, were reported. Dengue virus was introduced by DF cases returning by military boats from Southeast Asia and epidemics were evoked by Ae. albopictus inhabiting in this country. At present, dengue virus does not exist in Aedes mosquitoes in Japan, therefore no domestic infection occurs. Every year, however, there occur a considerable number of imported cases that are infected during staying in tropical or subtropical endemic areas for dengue virus and develop the disease after returning home (see IASR, Vol. 21, No. 6). Some visitors from endemic areas may also develop the disease in Japan.

The National Epidemiological Surveillance of Infectious Diseases (NESID): In the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) enacted in April, 1999, DF is listed in the Category IV notifiable infectious diseases and in the amendment made in November 2003, it was listed again in the new Category IV. DF cases reported after enactment of the Infectious Diseases Control Law have numbered 159, all of which were imported ones. Such cases numbered as few as nine in 1999 (during April and December) and 18 in 2000, but increased to 50 in 2001 and 51 in 2002. There were 31 cases in 2003 (Table 1). The increase during 2001-2002 may reflect not only the worldwide dengue epidemics but also the increased diagnoses of DF due to increased attention of physicians to DF after being categorized into the Category IV notifiable infectious diseases.

Monthly incidence may be influenced by two factors, the epidemic period of the travel destinations and the fluctuations of the numbers of travelers from Japan. Cases tended to increase during 2001-2002, particularly during summer and spring. In 2003, restriction on travel to prevalent areas of severe acute respiratory syndrome (SARS) affected the general overseas travel and no such seasonality was seen (Fig 1).

The destinations of overseas travel of cases included 25 different countries/areas (Table 2). Asian countries, mainly those of Southeast Asia, accounted for more than 90%. Cases infected during traveling to Thailand, Indonesia, and the Philippines during 2001-2002 were remarkable. Some cases among returnees from Oceania and Latin American countries also developed symptoms.

Cases reported from Tokyo were the largest in number among all prefectures (p. 30 of this issue), followed by Kanagawa and Osaka. Even in areas where no case has been reported, DF must always be paid attention for differential diagnosis among those who have traveled to endemic areas.

The male/female sex ratio of cases has been 3/2; there have been slightly more male cases (Table 1). The ages of cases are centered on 20s (44%); these and cases at 30s (25%) and 40s (18%) together accounted for 87% (Fig. 2).

Severe cases: As imported cases of dengue virus infection increase, several cases of DHF, which used to be extremely rare, have been reported after 2001 (Table 1). As the criteria for reporting DHF, the following four must be fulfilled in addition to the diagnosis of DF: 1) fever, 2) plasma leakage signs due to increased vascular permeability, 3) thrombocytopenia, and 4) hemorrhagic manifestations. A case of severe neurological disorder after infection with dengue virus has also been reported (see p. 32 of this issue). From now on, attention must be paid to cases of hemorrhagic fever and atypical cases not reported previously.

Laboratory diagnosis: After enactment of the Infectious Diseases Control Law, microbiological diagnosis can be made at the prefectural and municipal public health institutes (PHIs), but only limited number of specimens can be handled. The Department of Virology I, the National Institute of Infectious Diseases, receive a number of specimens for laboratory diagnosis, but there is a discrepancy between the number of virus- or antibody-positive cases and that of reported cases (Table 3). DF had to be reported within 7 days after diagnosis, but after the amendment in November 2003, any new Category IV infectious disease must be reported immediately after diagnosis by all physicians, and it is necessary to ask for more cooperation of physicians to report.

Tests for diagnosing DF at quarantine stations: In the Quarantine Law amended simultaneously with the Infectious Diseases Control Law, DF was included in the quarantine infectious diseases. At quarantine stations, those who are entering into Japan from endemic areas and suspected of DF can be subjected to medical examinations and laboratory tests (see p. 28-30 of this issue).

World situation and countermeasures in Japan: For the past few years, DF has been prevalent in summer in south Taiwan and also in large scale in Brazil (see p. 33 of this issue). In 2001, three DF cases infected during a group travel to the Philippines were admitted to the same hospital. It has been reported that the member participants of the group tour were not aware of the epidemics at the area (see p. 31 of this issue). It is necessary that the travel agencies must provide the epidemic information on the travel destination to the participants, and that the tourists themselves must pay attention to the epidemic information of the area and avoid mosquito bite during staying. Ae. aegypti , a vector mosquito of dengue virus, inhabits in urban areas and Ae. albopictus inhabits not only in suburban areas but also in urban areas (see p. 34 of this issue), therefore many infections occur in urban areas of endemic areas and care must be taken by visitors of not only tourists but also those who are staying on business.

The epidemic of DF occurring in Hawaii from September 2001 after 60 years started from an imported case and transmitted by Ae. albopictus inhabiting on the islands (see p. 33 of this issue). At present, Ae. albopictus , the vector mosquito, inhabits in Japan, and its distribution is going up north in Tohoku district (see p. 35 of this issue). There were epidemics of DF with Ae. albopictus as the vector mosquito in Japan in 1940s: the epidemic in Hawaii may serve as the alarm for the preparedness of DF in Japan. Physicians must be alert to the information on dengue epidemics, diagnose rapidly imported DF cases, and report immediately.

It is possible that Ae. aegypti carrying the virus enters into Japan by an airplane or a boat. Complying with the amendment of the Infectious Diseases Control Law in November 2003, when the new Category IV DF introduced, measures against animals and goods such as control of mosquitoes can be taken to prevent transmission in Japan (see IASR Vol. 25, No.1). It is known that severe viremia occurs in a feverish period of DF/DHF cases. When taking blood samples from cases in a viremia stage, basic precautions must be taken to avoid accidental needle stick injuries.

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