Mosquito-borne viral infections: Zika virus infection, Chikungunya fever and Dengue fever, 2011 to June 2016, Japan
(IASR 37: 119-120, July 2016)All mosquito-borne infectious diseases are classified as category IV infectious diseases under the Infectious Diseases Control Law (Table 1). This article focuses on Zika virus infection, Chikungunya fever and Dengue fever. As these are associated with acute febrile illness and clinically similar in manifesting syndromes such as fever, rash, and arthralgia, and as there are many asymptomatic cases, differential diagnosis solely based on clinical information is difficult (see notification criteria for each disease described below). In Japan, majority of the reported cases for these three diseases have been acquired abroad (i.e. imported cases), but an autochthonous dengue fever outbreak was confirmed in Japan in 2014 for the first time in nearly 70 years (IASR 36: 33-35, 2015).
Epidemiologic situation
Zika virus infection: Zika virus infection in humans was reported from Africa in the 1950s and from Asia in the 1970s. In 2007, Zika virus was confirmed for the first time in the Federated States of Micronesia, causing an outbreak on Yap Island; during 2013-2014, French Polynesia reported that approximately 30,000 people were infected (see p. 121 of this issue). In 2013-2014, three cases, all infected abroad, were also reported in Japan (IASR 35: 45-46; 243-244, 2014). In 2015, circulation of Zika virus in Brazil and other areas in Central and South America was confirmed, and has since been spreading. In Brazil alone, approximately 40,000 people have been infected with Zika virus so far, including 12 deaths (WHO, Zika situation report, 16 June 2016). Since 2015, it has spread to 38 countries or areas in Central and South America and the Caribbean, 12 countries or areas in Asia and the Western Pacific region, the Republic of Maldives in the Indian Ocean and the Republic of Cabo Verde in North Africa (WHO Zika situation report, 16 June 2016).
Following 2-12 days of the incubation period, about 20% of Zika virus-infected persons become symptomatic and develop symptoms such as maculopapular rash, fever (majority <38.5°C), arthralgia, myalgia, conjunctivitis, and malaise. In the 2013 epidemic in French Polynesia, association between Zika virus infection and Guillain-Barré syndrome was suspected. In the 2015 epidemic in Brazil, link between Zika virus infection and Guillain-Barre syndrome among adults and microcephaly among newborns was suspected. On 1 February 2016, WHO declared a “public health emergency of international concern (PHEIC)” regarding clusters of microcephaly and Guillain-Barré syndrome associated with Zika virus infection. On 15 February 2016, the Japanese government classified Zika virus infection (includes Zika virus disease and congenital Zika virus infection) as a category IV infectious disease that requires all cases to be notified by clinicians (notification criteria: http://www.niid.go.jp/niid/images/iasr/37/437/de4371.pdf). Since then till week 23 of 2016, a total of 7 cases infected in Central & South America and Oceania were reported (as of 15 June 2016) (Table 2).
Chikungunya fever: Since 1 February 2011, chikungunya fever has been a category IV infectious disease (notification criteria: http://www.niid.go.jp/niid/images/iasr/37/437/de4372.pdf). Since then till week 22 of 2016, a total of 69 cases (average 13 cases per year) were reported (as of 8 June 2016); all were infected abroad (Table 3 in p. 121 of this issue). Forty (58%) cases were male and 29 (42%) were female; 23 (33%), 19 (28%), and 14 (20%) cases were in their 20’s, 40’s and 30’s, respectively (median: 34 years; range 11-71 years). No clear seasonality was found (Fig. 1). Suspected place of infection are shown in Table 3 in p.121 of this issue and have been reported in IASR 36: 47-48, 2015.
Dengue fever and dengue hemorrhagic fever: From 2011 to week 22 of 2016, a total of 1,357 dengue cases (1,310 dengue fever, 46 dengue hemorrhagic fever and 1 asymptomatic) were reported (notification criteria: http://www.niid.go.jp/niid/images/iasr/36/421/de4211.pdf). Among them, 162 were autochthonous cases from the 2014 outbreak, and the remaining 1,195 were infected abroad. Among the cases, 833 (61%) were male and 524 (39%) were female; 386 (28%), 328 (24%), and 239 (18%) cases were in their 20’s, 30’s and 40’s, respectively (median: 32 years; range 0-82 years). Notifications of infection abroad were highest during August-September (Fig. 2), and this reflects an increase in the number of persons who travel abroad and dengue activity levels in their destinations (see p. 131 of this issue). Suspected place of infection are shown in Table 4 in p.121 of this issue and have been reported in IASR (36: 33-35, 2015, Table 3).
Mosquito vectors in Japan (Table 1)
Aedes albopictus mosquito capable of transmitting the causative viruses of the three diseases inhabits Japan. They are active during the day in outdoor settings, and feed on persons who enter environments with vegetative cover, such as parks (see p. 126 of this issue).
Laboratory diagnosis
On account of the similar symptoms and geographical distributions of the three diseases, laboratory tests are indispensable for diagnosis (see pp. 124 & 132 of this issue). Prefectural and municipal public health institutes (PHIs) and quarantine stations are ready to provide the service of PCR-based tests. Some PHIs may also provide antibody tests. Currently, laboratory tests available in clinical setting are the dengue virus non-structural protein NS1 antigen detection ELISA and immuno-chromatography for detection of both anti-dengue virus IgM antibody and NS1 antigen.
Treatment and prevention
A physician who examines a patient suspected of any of the three infections should, as appropriate, obtain advice from specialized medical facilities or refer the patient to such facilities (Clinical Examination Guidelines on the Mosquito-Borne Infections, 3rd Ed.) (see p. 123 of this issue). As no specific therapy is available, patients are treated symptomatically, including fluid therapy.
To prevent infection in endemic areas, appropriate use of repellents (see p. 128 of this issue) and minimizing skin exposure during outdoor activities is important. In order to reduce the potential for outbreaks in Japan, it is important to reduce containers and other artifacts that accumulate water which may serve as habitats for mosquito larvae (see p. 126 of this issue) (“Guidance for the local government on the prevention/control of mosquito-borne infections such as dengue fever and chikungunya fever” revised on 12 February 2016). On account of the risk of mother-to-child (see p. 124 of this issue) and sexual transmission of Zika virus, pregnant women or women expecting pregnancy should refrain from travel to Zika virus endemic regions. Furthermore, men who return from such regions should use a condom when they have sex with a partner for at least 8 weeks following return (or for the entire gestation period if the partner is pregnant), or abstain from sex.
Concluding remarks
It should be reminded that mosquito-borne infections can be introduced into Japan as had occurred in 2014 with dengue fever. Rio de Janeiro in Brazil, where the Olympic and the Paralympic games are to be held this year, is in the subtropical zone and the average temperature in August to September exceeds 20°C (http://www.data.jma.go.jp/gmd/cpd/monitor/nrmlist/NrmMonth.php?stn=83743). Those who are going to the Olympic games are advised to take appropriate measures described above against these viruses by minimizing the risk of mosquito bites.
To prevent the spread of mosquito-borne infectious diseases such as dengue fever and chikungunya fever, the Ministry of Health, Labour, and Welfare issued the “Special Guidelines on Prevention of Mosquito-borne Infections” in April 2015, to which guidance on Zika virus infection was added in March 2016. The guidelines recommend routine implementation of mosquito control measures, early detection of mosquito-borne infections, and emergency response including provision of appropriate medical care (see p. 129 of this issue). Updated information on Zika virus risk assessment is available from the National Institute of Infectious Diseases home page.