Dengue virus and chikungunya virus are propagated among humans by Aedes aegypti and Aedes albopictus forming a human→mosquito→human infection cycle.  The former lives in the urban area and the latter in the both urban and rural areas.

For both dengue fever (DF) and chikungunya fever (CHIKF), clinical symptoms appear 3-7 days after infection.  The symptoms consist of fever, exanthema and pain (mainly arthralgia).  As they are clinically very similar and share the same endemic regions in Asia and Africa, differential diagnosis is almost impossible without the laboratory diagnosis (see p. 161 of this issue).

CHIKF, together with DF, now belongs to the Category IV infectious disease under the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (Infectious Diseases Control Law) revised in February 1, 2011.  All the cases should be reported under the National Epidemiological Surveillance of Infectious Diseases (NESID), and the physicians are under obligation of notifying them promptly after diagnosis (notification criteria is found in http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou11/01-04-42.html).  In the classification of control of pathogens, dengue virus is a class IV pathogen (http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou17/03.html).

1. Dengue fever
NESID: Cases of DF (including dengue hemorrhagic fever, DHF) notified during 2007-2010 were 531, all being imported cases.  In 2001-2006, 32-74 cases were reported yearly.  In 2007-2010, however, number of reports became higher and increased steadily, i.e., 89 cases in 2007, 105 cases in 2008, 92 cases in 2009 and as high as 245 cases in 2010 (Table 1) (IASR 25: 26-27, 2004 & 28: 213-214, 2007 for 1999-2006).  In the above figures, those infected and cured during stay abroad are not counted.

Seasonality: Monthly cases may be affected by two factors, number of travelers going abroad and prevalence of the disease in their destinations (see p. 162 & 163 of this issue).  The reported number of cases is high in August-September when many Japanese go abroad; the tendency was particularly conspicuous in 2010 (Fig. 1).

Suspected regions of acquiring infection: In 2007-2010, 42 countries/areas were counted as the suspected place of infection (Table 2).  Ninety percent were Asian, particularly Southeast Asian, countries, such as, Indonesia (51 cases among 79 visited Bali Island) (see p. 163 of this issue), India, the Philippines, and Thailand (see p. 162 of this issue).  Some DF cases were found among those who traveled to Central and South America, Oceania or Africa (see p. 164 of this issue).

Gender and age: While there is no difference in DF incidence by gender in endemic area, more male cases are reported than female cases among the imported cases in Japan (342 males vs. 189 females) (Fig. 2).  Most cases were in their 20s (41%), followed by 30s (21%) and 40s (16%), totaling 78% for these age groups (Fig. 2).

Severe cases: As increase of DF cases, DHF cases increased accordingly.  Three to seven cases were reported annually in 2006-2010 while in 2001-2005 only one to three cases were reported annually (Table 1).  The criteria for reporting of DHF are 1) fever, 2) plasma leakage, a sign of increased vascular permeability, 3) thrombocytopenia, and 4) hemorrhagic manifestations (http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou11/01-04-19.html).  None of the reported cases were fatal when they were notified.

2. Chikungunya fever
Two imported cases, Japanese who returned from Sri Lanka, were reported in November-December in 2006.  Since then till January 2011, total 19 imported cases, mainly from Indonesia and other endemic countries, were primarily diagnosed by Department of Virology I, National Institute of Infectious Diseases (NIID) (IASR 29: 345-346, 2008 & 30: 108-109, 2009).

NESID: Since Infectious Diseases Control Law came into force there have been five CHIKF cases reported, two males and three females in their 20s and 30s.  They were reported from February 1 to June 10 2011.  The suspected place of infection was Indonesia for four cases and Thailand for the remaining one case.

3. Laboratory diagnosis: Prefectural and municipal public health institutes (PHIs) and NIID conduct laboratory diagnosis of dengue and chikungunya virus infection, such as virus isolation, genome detection by PCR, or IgM antibody detection (Table 1).  DF and CHIKF were included in the quarantine infectious diseases by amendment in November 2003 and that in February 2011 of the Quarantine Law respectively.  Accordingly, quarantine stations are conducting medical examinations to those suspected of DF or CHIKF, who came back from endemic areas and genome detection by PCR and virus isolation.

4. Countermeasures to be taken in Japan: Dengue virus and chikungunya virus are not endemic in Japan, and all the cases reported in recent years are imported ones.  However, Ae. albopictus , vector of the both viruses, inhabits Japan, and its habitat extends to the northern Tohoku District (see p. 167 of this issue).  Another vector, Ae. aegypti , while nonexistent in Japan now, can enter by an airplane or a boat in future.

It should be recalled that during 1942-1945 Japan experienced an epidemic of DF mediated by Ae. albopictus in western part of the country.  Once the viruses are introduced in the manner that may start infection cycle, they may cause an epidemic.  Actually, DF transmission occurred in 2010 in France and Croatia, which had been DF/CHIKF free (see p. 165 of this issue) and a domestic infection of CHIKF was reported from France also in 2010 (see p. 161 of this issue).

Physicians should be aware of epidemiological situations of DF and CHIKF in the world, and should ask the travel history to persons having suspicious symptoms so as to avoid late diagnosis.  Febrile patients in viremic phase should be protected from mosquito bite so as to prevent the virus spread.  In the hospital, precaution should be taken to avoid cross infection through transfusion or needle-stick accidents.

In summer seasons, the Ministry of Health, Labour and Welfare gives warning to travelers going abroad on dengue and chikungunya virus infections using posters in airports and other places.  NIID is providing the updated information through web site (http://www.nih.go.jp/vir1/NVL/NVL.html).  As of June 10, 40 DF patients have been reported in 2011.  Those traveling abroad should seek information on the epidemic situation in their travel destinations and take necessary precaution such as protection from the mosquito bites.

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