Retrospective search for dengue vector mosquito Aedes albopictus in areas visited by a German traveler who contracted dengue in Japan

Color Preference for Host-Seeking Activity of Aedes albopictus and Culex pipiens (Diptera: Culicidae)

A cue for long-range vision allows mosquitoes to identify hosts and differentiate the ecological niches (e.g., habitats). However, the visual factors involved in attracting mosquitoes to a host are complex and have not been fully understood. Therefore, we assessed color preference to Aedes albopictus (Skuse) and Culex pipiens (Conquillett) as diurnal and nocturnal species, respectively, using seven fundamental colors including black, white, red, yellow, green, blue, and purple with each trap at 100 lux in a laboratory. We used a binary behavioral assay using the Mosquito Preference Index (MPI) as a preference ratio with a range of 0-1. Our analyses showed that Ae. albopictus had a greater response to black (MPIs, 0.7), followed closely by red, blue, and purple (MPIs, 0.6). We also found that red, blue, and purple were significantly higher (P < 0.05) than those of green (MPI, 0.5), white (MPI, 0.3), and yellow (MPI, 0.2). Similarly, the MPIs for Cx. pipiens were significantly higher at black and red (MPIs, 0.7; P < 0.05) compare to those of white and yellow (MPIs, 0.3; P < 0.05). The color preference of Ae. albopictus showed significant correlation to luminous intensities (L-value) (r = -0.640; P = 0.000) and blue intensities (b-value) (r = -0.372; P = 0.000) for all seven colors. In addition, Cx. pipiens negatively correlated (r = -0.703; P = 0.000) between color preference and L-value. Our analyses provide a greater understanding of how color plays a role in visual sensory stimuli, and how that could potentially affect mosquito host-seeking behavior.

Aedes albopictus Strain and Dengue Virus Serotype in the Dengue Fever Outbreaks in Japan: Implications of Wolbachia Infection

From August 27 to October 15, 2014, a dengue fever outbreak with 158 autochthonous cases occurred after nearly 70 years of no reports of autochthonous cases in Japan. The most competent mosquito vector for dengue virus (DENV) transmission in Japan is Aedes albopictus. Since A. albopictus is widely distributed throughout Japan, we examined the susceptibility of this species to infection by DENV and the relationship of the endosymbiont Wolbachia (wAlbA and wAlbB) with susceptibility to DENV. The A. albopictus YYG strain, collected from Yoyogi Park in 2014, the epicenter of the dengue fever outbreak, was found to have lower susceptibility to DENV 1 and 3 than that of indigenous Japanese strains A. albopictus EBN 201808 (F1 from the field) and A. albopictus ISG 201603. Further, the A. albopictus EBN 201808 strain showed a same susceptibility to DENV3 as A. albopictus ISG 201603tet strain (Wolbachia-free). Susceptibility to DENV3 was not related to Wolbachia strains wAlbA or wAlbB in the A. albopictus ISG 201603 strain.

The 2014 autochthonous dengue fever outbreak in Tokyo: A case series study and assessment of the causes and preventive measures

Objective

In 2014, an autochthonous dengue fever outbreak occurred around the Yoyogi Park in Japan for the first time in 70 years. Despite no local cases reported since then, the risk of another outbreak remains high. This study reviews the autochthonous dengue fever cases of the outbreak, investigates its causes, and delineates preventive measures against autochthonous dengue epidemics.

Methods

We conducted a case series study of 15 patients who visited our institution during the 2014 outbreak. We collected and evaluated data on the surveillance of vector mosquitoes, weather, pest control, travelers’ origins and destinations, and imported dengue fever cases using reports made by public institutions.

Results

All patients recovered with supportive treatments and none met the diagnostic criteria for severe dengue infection. Twelve patients with positive real-time polymerase chain reactions were confirmed as having dengue virus-1 infections. We found no obvious associations between the number of mosquitoes and the weather, or between the number of imported dengue fever cases and that of travelers. Insect growth regulator (IGR) against vector mosquitoes has been used since 2014 for pest control, but the number of larvae has not declined in the Yoyogi Park, although that of imagoes has been relatively suppressed.

Conclusion

The 2014 outbreak emerged without particularly favorable climate conditions for vector mosquitoes. We found no obvious associations between the number of travelers or the imported dengue fever cases and the outbreak, but the increasing number of travelers may contribute to another outbreak. Pest control, including IGR, remains essential for infection control.

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We studied 15 patients with autochthonous dengue fever during a local outbreak.

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This outbreak emerged without favorable climate conditions for mosquitoes.

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There were no increased number of travelers or imported dengue cases.

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Pest control using insect growth regulator can be effective for infection control.

The threat of climate change to non-dengue-endemic countries: increasing risk of dengue transmission potential using climate and non-climate datasets

Background

Dengue is a major public health problem in the tropics and sub-tropics, but the disease is less known to non-dengue-endemic countries including in Northeast Asia. However, an unexpected dengue outbreak occurred in 2014 in Japan. Given that autochthonous (domestic) dengue cases had not been reported for the past 70 years in Japan, this outbreak was highly unusual and suggests that several environmental factors might have changed in a way that favors vector mosquitoes in the Northeast Asian region.

Methods

A Climate Risk Factor (CRF) index, as validated in previous work, was constructed using climate and non-climate factors. This CRF index was compared to the number of reported dengue cases in Tokyo, Japan where the outbreak was observed in 2014. In order to identify high-risk areas, the CRF index was further estimated at the 5 km by 5 km resolution and mapped for Japan and South Korea.

Results

The high-risk areas determined by the CRF index corresponded well to the provinces where a high number of autochthonous cases were reported during the outbreak in Japan. At the provincial-level, high-risk areas for dengue fever were the Eastern part of Tokyo and Kanakawa, the South-Eastern part of Saitama, and the North-Western part of Chiba. While a relatively small number of high-risk areas were identified in South Korea compared with Japan, the high-risk areas in South Korea include popular tourist destinations where international visitors have been increasing.

Conclusion

The recent dengue outbreak in Japan may signal that the two adjacent non-dengue-endemic countries are also exposed to the risk of temporal and sporadic behavior of dengue fever. It is critical to understand potential high-risk areas for future outbreaks and to set up appropriate prevention activities at the governmental-level.

Intercepted Mosquitoes at New Zealand's Ports of Entry, 2001 to 2018: Current Status and Future Concerns

Mosquito vectors are extending their range via international travel and trade. Climate change makes New Zealand an increasingly suitable environment for less tropically adapted exotic mosquito vectors to become established. This shift will add a multiplier effect to existing risks of both the establishment of new species and of resident exotic species extending into new areas. We describe trends in the border interceptions of exotic mosquitoes and evaluate the role of imported goods as a pathway for these introductions. Ae. aegypti and Ae. albopictus, the two most commonly intercepted species, were only intercepted in Auckland. Used tyres and machinery were the main mode of entry for both species. The majority of Ae. albopictus were transported as larvae by sea, while most Ae. aegypti were transported as adults by air. Continuing introductions of these mosquitoes, mainly arriving via Japan or Australia, increase the risk of the local transmission of mosquito-borne diseases in New Zealand in general and in the Auckland region in particular. These findings reinforce the need for a high performing and adequately resourced national biosecurity system, particularly port surveillance and inspection. Recommended biosecurity improvements are described.

Assessing dengue control in Tokyo, 2014

Background

In summer 2014, an autochthonous outbreak of dengue occurred in Tokyo, Japan, in which Yoyogi Park acted as the focal area of transmission. Recognizing the outbreak, concerted efforts were made to control viral spread, which included mosquito control, public announcement of the outbreak, and a total ban on entering the park. We sought to assess the effectiveness of these control measures.

Methodology/Principal findings

We used a mathematical model to describe the transmission dynamics. Using dates of exposure and illness onset, we categorized cases into three groups according to the availability of these datasets. The infection process was parametrically modeled by generation, and convolution of the infection process and the incubation period was fitted to the data. By estimating the effective reproduction number, we determined that the effect of dengue risk communication together with mosquito control from 28 August 2014 was insufficiently large to lower the reproduction number to below 1. However, once Yoyogi Park was closed on 4 September, the value of the effective reproduction number began to fall below 1, and the associated relative reduction in the effective reproduction number was estimated to be 20%–60%. The mean incubation period was an estimated 5.8 days.

Conclusions/Significance

Regardless of the assumed number of generations of cases, the combined effect of mosquito control, risk communication, and park closure appeared to be successful in interrupting the chain of dengue transmission in Tokyo.

Evaluating the interventions implemented during an outbreak of mosquito-borne disease is of utmost importance, offering lessons for future control strategies. By retrospectively analyzing data of the first autochthonous dengue epidemic of the 21st century in Tokyo, Japan, we assessed the effectiveness of the interventions. Once a dengue outbreak was confirmed in late August 2014, the government of Japan took drastic mosquito control measures, targeting both adults and larvae. News of the outbreak was also widely disseminated via mass media along with experts’ recommendations as to how people could avoid the risks of dengue infection. As the outbreak was not immediately controlled, the focal area of transmission, Yoyogi Park, was closed on 4 September. Using a mathematical model, we assessed how well dengue virus transmission was intervened in relation to the start times of interventions. As we incorporated precise timing into the model, we directly modeled the time of infection and accounted for the time delay from infection to illness onset. Thus, we revealed that mosquito control and risk communication measures alone could not interrupt the chain of transmission; however, adding park closure to these interventions was substantially effective in reducing the number of transmissions.

Estimating Frequency of Probable Autochthonous Cases of Dengue, Japan

Imported dengue into naive areas is a recognized but unquantified threat. Differentiating imported and autochthonous cases remains problematic. A threshold approach applied to Japan identified several aberrant incidences of dengue. Despite these alerts, no epidemics occurred other than 1 in Yoyogi Park in Tokyo, which was probably an unusual event.

Study on the correlation between age and changes in mosquito bite response

Mosquito bite reactions consist of an immediate response characterized by an erythema and a wheal, which peaks at 20 min, and a delayed response characterized by a papule and/or an erythema, which peaks at 24 h. Mosquito bite reactions progress from stage I to stage V as an individual is repeatedly bitten as follows: stage I, neither immediate nor delayed reaction; stage II, delayed reaction; stage III, both immediate and delayed reaction; stage IV, immediate reaction; and stage V, neither immediate nor delayed reaction. In 1985, we conducted a cross-sectional study that demonstrated a positive association between age and stage in response to an Aedes albopictus bite among 162 healthy volunteers. However, it remains unknown how the mosquito bite reaction stage progresses in the same individual over decades. In the present study, the mosquito bite reaction of 10 subjects from the 1985 cross-sectional cohort was longitudinally evaluated over 30 years. We confirmed the time-dependent progression of stage in four subjects. One stage II subject and three stage III subjects had advanced to the next stage. We went on to find that, contrasting with the general trend found in the previous studies, the majority of the subjects (6/10) remained of stage III over the 30-year study period. Upon closer inspection, this apparently non-progressive group demonstrated a reduction in the size of the delayed reactions. The present study demonstrated a marked individual variability in course of the stage progression.

Predicting the Start of the Aedes albopictus (Diptera: Culicidae) Female Adult Biting Season Using the Spring Temperature in Japan

Aedes albopictus (Skuse) (Diptera: Culicidae) is distributed widely and is common in much of Japan. In Japan, female adults begin to bite in between April and June, except in the southern subtropics where the mosquito has no dormant period. It is difficult to estimate the first Ae. albopictus biting day because it varies annually depending on the location. Over several years, we surveyed the mosquitoes at different locations that covered a range of warmer to cooler areas of Japan. We found an association between the timing of first biting day by Ae. albopictus and spring temperature. In spring months, the strongest correlation was found with mean April temperatures, followed by March. Based on these data, it may, therefore, be possible to apply a simple formula to predict the timing of the first biting day at various geographical locations in Japan. Forecasting maps were created using a simple prediction formula. We found that the first biting day for Ae. albopictus changed depending on early spring temperatures for each year. There was an approximate 20-d difference in first biting day between years with warmer and cooler springs. This prediction model will provide useful insight for planning and practice of Ae. albopictus control programs, targeting larvae and adults, in temperate regions globally.

Aedes (Stegomyia) albopictus' dynamics influenced by spatiotemporal characteristics in a Brazilian dengue-endemic risk city

Brazil reported the majority of the dengue cases in Americas during the last two decades, where the occurrence of human dengue cases is exclusively attributed to the Aedes (Stegomyia) aegypti (Linnaeus). Nowadays, other recognized Dengue virus (DENV) vector in Asian countries, Aedes (Stegomyia) albopictus (Skuse), has been detected in more than half of the 5565 Brazilian municipalities. Therefore, the aim of the present study was to investigate the presence of, and determine the Ae. albopictus' dynamics influenced by spatiotemporal characteristics in a dengue-endemic risk city of Belo Horizonte, Minas Gerais State's capital. Aedes albopictus were collected across four consecutive DENV transmission seasons from 2010 to 2014. These mosquitoes were caught in three selected districts, which had been reported in the previous ten years as having high mosquito densities and an elevated concentration of human dengue cases during epidemic seasons. All field-caught Ae. albopictus was individually processed by real-time RT-PCR, to research the DENV presence. The third season (p<0.05) and the Pampulha district (p<0.05) had the highest proportions of field-caught Ae. albopictus, respectively. The second season had the highest proportion of DENV-infected field-caught females (p<0.05), but there was no difference among the proportions of DENV-infected Ae. albopictus when comparing the collection in the three districts (p=0.98). Minimum (p=0.004) and maximum (p<0.0001) temperature were correlated with the field-caught Ae. albopictus in four different periods and districts. In the generalized linear model of Poisson, the field-caught DENV-infected Ae. albopictus (p=0.005), East district (p=0.003), minimum temperature (p<0.0001) and relative humidity (p=0.001) remained associated with the total number of human dengue cases. Our study demonstrated that the number of field-caught DENV-infected Ae. albopictus was inversed correlated with the number of human dengue cases. Our study raises the possibility that the DENV circulating in mosquitoes Ae. albopictus is happening in non-epidemic periods, showing that this species may be keeping only the presence of the virus in nature. Further long-term studies are necessary to better understand the role of Ae. albopictus in DENV transmission and or its vectorial competence in Belo Horizonte and in other endemic cities in Brazil and in the New World countries.

Dengue Virus Isolation in Mosquito Aedes albopictus Captured During an Outbreak in Tokyo, 2014, by a Method Relying on Antibody-Dependent Enhancement Mechanism Using FcγR-Expressing BHK Cells

Dengue virus (DENV) isolation from mosquitoes is necessary for providing definitive evidence of virus circulation, and is critical for further virological characterization and determination of epidemiological characteristics. By using Aedes albopictus mosquitoes captured during an outbreak in Tokyo in 2014, we compared the DENV isolation rates of a conventional virus isolation method that uses C6/36 mosquito cells as assay cells with those of a virus isolation method that relies on an antibody-dependent enhancement (ADE) mechanism by using FcγR-expressing baby hamster kidney (BHK) cells and an antibody with ADE activity. The number of DENV genome copies and infectious virus titers in cell culture supernatant fluids of FcγR-expressing BHK cells were significantly higher than those of the C6/36 cells. In addition, DENV was isolated from a mosquito pool by using FcγR-expressing BHK cells only in the presence of infection-enhancing antibody. Infectious virus was detected in six mosquito pools only by using FcγR-expressing BHK cells. The results suggest that the method that relies on ADE mechanism by using the FcγR-expressing BHK cells and an antibody with ADE activity is useful for DENV isolation from mosquitoes caught in the field.

Dissecting Japan's Dengue Outbreak in 2014

Despite Japan's temperate climate, a dengue outbreak occurred in Tokyo for the first time in over 70 years in 2014. We dissected this dengue outbreak based on phylogenetic analysis, travel interconnectivity, and environmental drivers for dengue epidemics. Comparing the available dengue virus 1 (DENV1) E gene sequence from this outbreak with 3,282 unique DENV1 sequences in National Center for Biotechnology Information suggested that the DENV might have been imported from China, Indonesia, Singapore, or Vietnam. With travelers arriving into Japan, Guangzhou (China) may have been the source of DENV introduction, given that Guangzhou also reported a large-scale dengue outbreak in 2014. Coinciding with the 2014 outbreak, Tokyo's climate conditions permitted the amplification of Aedes vectors and the annual peak of vectorial capacity. Given suitable vectors and climate conditions in addition to increasing interconnectivity with endemic areas of Asia, Tokyo's 2014 outbreak did not come as a surprise and may foretell more to come.

Autochthonous dengue fever, Tokyo, Japan, 2014

After 70 years with no confirmed autochthonous cases of dengue fever in Japan, 19 cases were reported during August-September 2014. Dengue virus serotype 1 was detected in 18 patients. Phylogenetic analysis of the envelope protein genome sequence from 3 patients revealed 100% identity with the strain from the first patient (2014) in Japan.

Autochthonous dengue fever, Tokyo, Japan, 2014

After 70 years with no confirmed autochthonous cases of dengue fever in Japan, 19 cases were reported during August–September 2014. Dengue virus serotype 1 was detected in 18 patients. Phylogenetic analysis of the envelope protein genome sequence from 3 patients revealed 100% identity with the strain from the first patient (2014) in Japan.

Susceptibility of Indigenous and Transplanted Mosquito Spp. to Dengue Virus in Japan

Dengue fever, an acute, mosquito-borne, febrile illness caused by Flavivirus spp., is a problem in Africa, South and Southeast Asia, Latin America, and the Caribbean. A dengue outbreak occurred after nearly 70 years of absence or no detection, and then 158 autochthonous cases occurred in Japan from August to October 15, 2014. The most competent mosquito vectors for dengue virus transmission were Aedes aegypti and A. albopictus. Since A. albopictus is widely distributed across Japan and A. aegypti recently invaded Japan by airplane, we examined the susceptibility of these species to infection by dengue virus.

Biting Density and Distribution of Aedes albopictus during the September 2014 Outbreak of Dengue Fever in Yoyogi Park and the Vicinity of Tokyo Metropolis, Japan

A total of 160 autochthonous dengue cases transmitted by Aedes albopictus were reported between August and October of 2014 in Tokyo Metropolis, Japan. Ae. albopictus is a medically important vector of dengue virus, which has expanded its geographic distribution in temperate regions. Understanding the distribution and biting density of Ae. albopictus during the 2014 dengue outbreak in Tokyo is important to evaluate the epidemic risks of dengue fever in other highly populated cities in Europe and Asia. Of the 160 patients, 134 visited the same park (Yoyogi Park) located in central Tokyo. Mosquitoes infected with dengue virus were collected from this park, suggesting that it was the exclusive location for the transmission of dengue. This study aimed to collect referential data to estimate the transmission threshold of dengue virus in terms of biting density of Ae. albopictus and determined high transmission risk areas of dengue virus in Yoyogi Park and its vicinity. The overall mean density of biting Ae. albopictus (7.13/man/8 min) was sufficiently high for successful transmission of dengue virus, and areas with biting densities higher than the overall mean density were classified as high risk areas for the transmission of dengue virus in Yoyogi Park.