The Topic of This Month Vol.28 No.8(No.330)

Imported dengue and dengue hemorrhagic fever in Japan, as of July 2007

(IASR 28: 213-214; August, 2007)

Dengue virus is transmitted to humans by bites of Aedes aegypti or Aedes albopictus , forming an infection cycle of human → mosquito → human.  Dengue virus infection causes two distinct syndromes, dengue fever and dengue hemorrhagic fever/shock syndromes (see IASR 21: 114, 2000).  The three main symptoms of dengue fever (DF) are fever, exanthema and pain (arthralgia), and its case-fatality rate is low.  On the other hand, dengue hemorrhagic fever (DHF) is characterized by fever, hemorrhagic manifestations, and circulatory disturbances and is likely to cause deadly shock if no adequate treatment is given.  At present, dengue virus does not exist 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 of dengue virus and develop the disease after returning home.  Some visitors from endemic areas may also develop the disease in Japan (see p. 217&218 of this issue).  

In the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law), DF is listed in the Category IV notifiable infectious diseases, and physicians are obliged to notify as soon as after diagnosis.  

Dengue virus is classified into the group 4 pathogen under the revised Infectious Diseases Control Law enacted in June 2007 (see IASR 28: 185-188, 2007).

The National Epidemiological Surveillance of Infectious Diseases (NESID): Cases of DF notified after enactment of the Infectious Diseases Control Law in April 1999 have been counted at 375, all of which were imported ones.  After 2004, 49 cases in 2004 increased to 74 in 2005, and 58 cases in 2006 and 33 cases in the first half of 2007 (up to July 17) totaling 214 cases were reported (Table 1) (for the situation during 1999-2003, refer to IASR 25: 26-27, 2004).  The increase and decrease in reported cases may reflect the worldwide epidemics, particularly those in Asia.  The incubation period of DF is as short as 3-7 days, therefore such cases who acquired infection and developed symptoms during staying in the destination and were treated and recovered there are not the subjects of notification, thus causing vague situation.

Seasonality: Monthly cases may be affected by two factors, prevalence of the disease in the destination and the period during which many visitors are coming from Japan.  Cases increase during August-September every year when there are many travelers, such being conspicuous particularly in 2004 and 2005 (Fig. 1).

Suspected regions of acquiring infection: The destinations of cases diagnosed during 2004-2007 were 26 different countries (Table 2).  Asian countries mainly in Southeast Asia were overwhelming, accounting for 90%, particularly, those traveling to Indonesia, the Philippines, or India, acquiring infections there in 2005-2007 were conspicuous.  Those who were estimated to have acquired infection in Oceania, Central and South America or Africa were also reported.  Of 9 cases acquiring infection in Micronesia in 2004, 6 were of the same traveling group.

Gender and age: Of all cases, 136 were males and 78 females (Fig. 2).  Most cases were of the age of 20s (37%), and 30s (25%) and 40s (16%), totaling 78% (Fig. 2).  Ae. aegypti , a vector mosquito of DF, inhabits in urban areas and Ae. albopictus inhabits not only in suburban but also in urban areas, 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.

Severe cases: As imported cases of dengue virus infection increase, one to 4 cases of DHF, which used to be extremely rare, have been reported every year after 2001 (Table 1).  As the criteria for reporting DHF, the following four must be fulfilled: 1) fever, 2) plasma leakage signs due to increased vascular permeability, 3) thrombocytopenia, and 4) hemorrhagic manifestations (

These reports include one DHF case having died after returning from Sri Lanka in September 2005 (see IASR 27: 14-15, 2006).  Beside, another Japanese case that acquired infection and died in Vietnam in December 2006 was reported.

Laboratory diagnosis: After enactment of the Infectious Diseases Control Law, etiological diagnosis of dengue virus infection can be made at the prefectural and municipal public health institutes (PHIs), the National Institute of Infectious Diseases (NIID), and quarantine stations (Table 3).   The Department of Virology I, NIID receives every year a number of specimens for differential diagnosis from other flavivirus infection.  In the Quarantine Law amended in November 2003, DF was included in the quarantine infectious diseases.  At quarantine stations, those who are entering Japan from endemic areas and suspected of DF can be subjected to medical examinations and laboratory tests (see p. 215 of this issue).

World situation: In Singapore, a large epidemic occurred during 2004-2005 due to type 1 virus and in 2007, another epidemic due to type 2 virus has begun.  There occurred a large epidemic in Indonesia in 2007, and on April 9, declaration of emergency was issued in Jakarta.  In Vietnam, cases and deaths as of June 16, 2007 tended to be 25 and 40%, respectively, more of those of the same period of the preceding year.  In Thailand, Cambodia and Myanmar, also cases and deaths have been increasing.  Although vector mosquito control has been conducted in Taiwan after the big epidemic of 2002, cases and deaths increased again in 2006 (see p. 215 of this issue).  Since 2005, chikungunya fever, which is an important differential disease, has been prevalent in the Indian Ocean Islands, India and Sri Lanka.  Two imported Japanese cases were reported in December 2006.

Countermeasures in Japan: Ae. albopictus , a vector mosquito of dengue virus and chikungunya virus, inhabits Japan, and the northern limit of its distribution is approaching toward Aomori Prefecture, and further proceeding northwards (see p. 219 of this issue).  It is possible that Ae. aegypti carrying the virus enters Japan by an airplane or a boat.  Like DF epidemics were evoked by Ae. albopictus in western Japan during 1942-1945, environmental conditions supporting DF prevalence are well prepared once dengue virus invades Japan even at present.

Physicians are required to question for the voyage history, paying attention to the information about DF epidemics in the world, conduct early diagnosis, prevent febrile patients in viremia from mosquito bite, and take fundamental care to prevent nosocomial infection by transfusion or accidental needle-stick of patients or patientsf blood possibly possessing the virus.

The Ministry of Health, Labour and Welfare calls for attention of travelers by putting up posters in summer every year.  The website of NIID provides the latest information on demand (  As of July 17, 2007, more cases than those in January-July every year have been reported and travelers have to be more careful about oversea information on epidemics and not to be bitten by mosquitoes.

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