27

Distribution and Ecology of Potential Vector Mosquitoes ofWest Nile Fever in Japan
Mutsuo KOBAYASHI*, Shinji KASAI, Kyoko SAWABE and Yoshio TSUDA
Department of Medical Entomology, National Institute of Infectious Diseases,
1-23-1 Toyama, Shinjuku-ku, Tokyo 162-8640, Japan*e-mail: mutsuo@nih.go.jp

Abstract
West Nile virus (WNV) became established in New York in 1999, in the first outbreak in the Western
Hemisphere. WNV is propagated in many species of birds and has been detected in more than 300 bird species in the United States, although there are great differences in the viremia level among the various bird
species. Culex mosquitoes, including Cx. restuans, the Cx. pipiens complex and Cx. tarsalis are important
vectors of WNV in the USA. Laboratory transmission of WNV by Cx. pipiens form molestus showed thatthey were highly competent vectors of WNV, with infection and dissemination rates of 96% and 81%,
respectively. Three species, including two of the Culex pipiens group (Cx. pipiens pallens (as pallens in thisarticle) and Cx. pipiens form molestus (as molestus in this article)) and Aedes albopictus, account for morethan 99% of collected mosquitoes surrounding Tokyo. The Cx. pipiens complex (pallens, molestus andCx. quinquefasciatus) prefer both human and avian hosts, and there is no clear differences in avian
blood-feeding patterns between pallens and molestus, but there are significant differences among mammalian species, with molestus preferring human blood compared with pallens. Ducks (mallards & spotbill
ducks) and tree sparrows are major blood sources for the Cx. pipiens group in Japan, which are moderate orhigh amplifiers of WNV. From a larval bioassay with several kinds of insecticides it was demonstrated that
etofenprox, a synthetic pyrethroid, exhibited relatively low effectiveness against several mosquito coloniesof the Cx. pipiens group in an urban environment of Japan. The most important larval habitats of theCx. pipiens group and Ae. albopictus are catch basins along public roads and this kind of structure should betargeted to control mosquito larvae.
Key words: blood-feeding habit, distribution, insecticide resistance, larval habitat, MIROC K1 model,
WNV-vector mosquito

1. Introduction
The term “arthropod-borne virus” (arbovirus) has no
taxonomic significance, but is rather an ecological term
used to define viruses that require a blood-sucking
arthropod for transmission between hosts. There are
currently more than 530 viruses registered in theInternational Catalogue of Arboviruses, and most of theviruses listed in this catalogue are zoonoses or viruses
infecting vertebrate animals. WNV was isolated in 1937
in a Ugandan woman who survived the infection(Smithburn et al., 1940; Hayes, 2001), it has recently
emerged as a major public concern in Europe with theoccurrence of an epidemic, with reports of meningoencephalitis and encephalitis cases in urban areas since1996 (Platonov, 2001; Campbell et al., 2001). WNV became established in New York in 1999, in the first outbreak in the Western Hemisphere (Anderson et al.,1999). WNV is maintained in nature by many bird
Global Environmental Research
12: 27-33 (2008)
printed in Japan

species, and the virus has been detected in more than
300 bird species in the United States, although there are
great differences in the viremia level among bird species.
WNV has also been detected in 62 mosquito species,
including genera of Aedes, Anopheles, Coquillettidai,
Culex, Culiseta, Denocerites, Mansonia, Orthopodomyia,
Psorophora and Uranotaenia in the USA. During thetransmission season, 63 avian species, representing
30 families and 14 orders, have tested positive for WNV
in New York State. The highest infection rate of deadbirds for WNV was 67% (n=907) observed in American
crows at the epicenter (Bernard et al., 2001). Detectionof WNV from 9,954 mosquito pools was conducted andthe minimum infection rate per 1,000 mosquitoes (MIR)
was found to be highest at 3.53 for Cx. pipiens group at
the epicenter for the entire season and 7.49 for the peakweek in August. On Staten Island an MIR of 11.42 wasrecorded in the Cx. pipiens group (Bernard et al., 2001).
Culex mosquitoes, including Cx. restuans, Cx. salinarius,©2008 AIRIES

28

M. KOBAYASHI et al.

Cx. tarsalis are also important vectors of WNV in thenorthwestern and northeastern areas of the USA (Bernard
et al, 2001). In the USA there are three members of theCx. pipiens complex, specifically Cx. pipiens pipiens,
Cx. quinquefasciatus and Cx. pipiens form molestus(molestus in the USA). Experimental transmission of
WNV showed that they were highly competent laboratory vectors of WNV, with infection and dissemination
rates of 96% and 81%, respectively (Turell et al., 2006).
In field surveys for WNV detection and/or isolation,correct morphological identification is difficult among
the Cx. pipiens complex. In Japan about 110 mosquito
species have been recorded and some species of the Cx.
pipiens complex are common, such as, pallens, alongwith molestus and Cx. quinquefasciatus (Tanaka et al.,1979). It is crucial to understand which mosquito species are potential vectors for WNV and are common in
urban environments of Japan, which are densely populated. In this review we describe the present status of themosquito fauna, their population densities in urban areas,
the host feeding patterns of mosquitoes which seem tobe potential WNV vectors and occurrence of insecticide
resistance in Japan.

2. Mosquito Fauna in Urban Areas
Surrounding Tokyo and Rural Areas
Surrounding Narita International Airport
in Chiba Prefecture
Mosquitoes were collected using dry-ice traps (CDC
type) in urban areas surrounding Tokyo, Japan in 2003
and 2004. The collection sites beside houses werelocated in areas with high population densities. A battery-operated trap with 1 kg of dry-ice was operated
weekly for 24 hrs. A total of six mosquito species werecollected from 18 collection sites, and three species,
including the Cx. pipiens group (pallens and molestus,
Fig. 1A) and Ae. albopictus (Fig. 1B), accounted formore than 99% of the collected mosquitoes (Table 1).
The average number of female mosquitoes /trap/day for
the Cx. pipiens group and Ae. albopictus was 7.4 and 1.8
in 2003 and 9.4 and 2.0 in 2004 (Tsuda et al., 2006a),
respectively. There were large variations in the numberof collected mosquitoes among the collection sites. Thisvariation may be related to the environmental conditions
at collection sites, particularly to the number of larval
habitats around collection sites and the presence of adequate resting places for adult mosquitoes around houses.
Adult mosquitoes of Ae. albopictus were collected from
May to October, while those of the Cx. pipiens groupwere found from April to December. There are two
members of the Cx. pipiens group (pallens andmolestus) in the urban areas surrounding Tokyo, but
identification of each form is complicated and timeconsuming. To distinguish the females of two forms
there is a morphological method involving comparisonof the number of ommatidia of some parts of thecompound eyes of the mosquitoes. These countings
clearly showed that the Cx. pipiens group mosquitoes

contained two forms consisting of pallens and molestus(Tsuda et al., 2006 a).
There have been frequent instances of insects of public health importance being introduced from one country
to another. In 1930 Anopheles gambiae, a major vectorof malaria in Africa, was probably introduced into
Brazil from Senegal by a French naval vessel (Gratz
et al., 2000). Several reports regarding insects carried byaircraft have been published in Japan (Ogata et al.,
1974; Takahashi, 1984; Haseyama et al., 2007). The
survey by Ogata et al. (1974) was conducted by checking inside international aircraft just after their arrival at
Tokyo International Airport (Haneda) from 1972 to 1973.
Of 42 aircraft surveyed, ten species of pest insects were
captured in 24 aircraft. House flies (Musca domestica)
and Cx. quinquefasciatus were collected most abundantly, 59 and 24 individuals, respectively. Engorgedfemale Ae. aegypti and An. subpictus were also caught.
The important mosquito species were carried mainly byflights via Bangkok, Hong Kong or Manila (Ogata et al.,1974). There are big differences in the structures of
airport terminals and passenger boarding bridges amongreconstructed international airports, but accidental introduction of mosquitoes, flies, cockroaches and othermedically important insects from international aircraft is
not very rare in international airports.
Adult mosquitoes were collected in adjacent andsurrounding areas of Narita International Airport, Chiba
Prefecture in 2003 and 2004 (Fig. 2) and the samples
were tested for WNV by RT-PCR to check for accidental
introduction of WNV in areas near the international air-A

B
Fig. 1 Typical urban mosquito species in Japan.
A. Culex pipiens pallens B. Aedes albopictus

Table 1 Mosquito species collected in urban areas surrounding
Tokyo, Japan in 2003 and 2004. (from Tsuda et al. 2006a)
Species

Female

Male

2003

2004

2003

2004

Aedes albopictus
Armigeres subalbatus
Culex bitaeniorhynchus
Cx. orientalis
Cx. pipiens groupCx. sasai
Cx. tritaeniorhynchus
Cx. (Culiciomyia) sp.
Ochlerotatus japonicus

614
0
2
1
2,582
0
2
1
1

908
1
1
0
4,225
0
0
0
0

182
0
0
0
21
0
0
1
0

355
0
0
0
35
1
0
0
0

Total

3,203

5,135

204

391

Ecology and Physiology of Potential West Nile Vectors

port. Collection sites of mosquitoes adjacent to Narita
International Airport and 8 or 14 km from the airport,
were selected for the possibility of release of hiddenmosquitoes from the wheel bays of airplanes. Russell(1987) reported that Cx. quinquefasciatus had a survival
rate averaging 84% in the wheel bays of a Boeing
747B aircraft on several commercial airways including
Melbourne-Singapore and Bangkok-Singapore. The
minimum external temperature during the flights was−54ºC at 11,890 m above sea level, but the minimum
temperature within the wheel bays did not fall below+8ºC and remained as high as +25ºC. These results
indicated that disinsection of aircraft wheel bays should
be considered to control hidden mosquitoes and othermedically important insects (Russell, 1987). During thetwo-year survey of the area around Narita International
Airport a total of 5,882 females of 16 mosquito species
in seven genera were collected and all pools were negative for WNV (Tsuda et al, 2006b). There are significant
differences in species composition of mosquitoes
between rural areas of Chiba Prefecture and urban areas
surrounding Tokyo. Major species of mosquitoes collected by dry-ice traps at three survey sites in Chiba
Narita International Airport
Nogehira
Shibayama(8 km)

Matsuo(14 km)Fig. 2 Collection sites around Narita International Airport in
Chiba prefecture. (redrawn from Tsuda et al., 2006b)

Table 2 Results of mosquito collection by dry-ice trap conductedin three survey areas around Narita International Airport,
Chiba Prefecture, Japan, from July to September 2003.
(from Tsuda et al., 2006 b)
Species
Females Culex pipiens group*
Aedes albopictus
Ae. flavopictus
Uranotaenia novobscura
Armigeres subalbatus
Anopheles sp.
Cx. tritaeniorhynchus
Tripteroides bambusa
Ochlerotatus japonicus
Ae. nipponicus
Cx. bitaeniorhynchus
Orthopodomyia anopheloides
Ae. vexans nipponii
Female total
Males
Ae. albopictus
Ae. flavopictus
Tp. bambusa
Male total
Total

Study area
Nogehira Matsuo Shibayama
178
139
46
11
6
8
12
3
2
0
3
0
1
409
4
0
2
6
415

130
96
34
18
17
5
2
3
3
4
0
2
0
314
1
5
1
7
321

* Includes Cx. pipiens pallens and form molestus.

18
0
0
0
2
3
1
0
0
0
0
0
0
24
0
0
0
0
24

Total
326
235
80
29
25
16
15
6
5
4
3
2
1
747
5
5
3
13
760

29

Prefecture consisted of the Cx. pipiens complex, Ae.
albopictus, Ae. flavopictus, Cx. tritaeniorhynchus andUranotaenia novobscura (Table 2). Our data clearly
showed that there were significant differences in thenumbers of mosquitoes collected at different survey
sites. It is important to know that the species composition of mosquitos in urban environments was extremely
simple. There are big differences in the natural environment, climate and socio-economical situation between
cities in urban areas and villages or towns in rural areas.
If WNV is introduced into Japan, mosquito control, particularly larval control, will have to start quickly. Fundamental data related to species composition, population
density of larval and adult mosquitoes and breeding sites
will be essential to the controlling of mosquitoes in avariety of areas where the activity of WNV is confirmed.

3. Host Feeding Patterns of Several Mosquito
Species Collected in Japan
The WNV virus has become firmly established in theWestern Hemisphere since it was discovered in the New
York City area in 1999 (Anderson et al., 1999). The
virus’s activity has spread at an unprecedented rate
throughout the continental United States and to Canada
and Mexico, where it is maintained in an enzootic cyclethat involves wild birds and ornithophilic mosquitoes(Komar et al., 2003). More than 60 mosquito species
have been found to be infected with WNV in North
America, and certain Culex mosquitoes appear to be theprimary vectors (White et al., 2001; Nasci et al., 2001).
To evaluate the role of Culex mosquitoes, the hosts ofblood-meals were identified by PCR amplification of a
portion of the cytochrome b gene of the mitochondrial
DNA and sequencing of the PCR products (Molaei et al.,2006). All Cx. restuans and 93% of Cx. pipiens acquired
blood from avian hosts; Cx. salinarius fed frequently on
both mammals (53%) and birds (36%). In this analysis
mixed-blood meals were also detected in 11% and 4%of Cx. salinarius and Cx. pipiens, respectively. Humanderived blood meals were identified from two
Cx. salinarius and one Cx. pipiens. From these results,
Cx. salinarius was found to be an important bridge vector to humans, while Cx. pipiens and Cx. restuans were
found to be more efficient enzootic vectors in thenortheastern United States (Molaei et al., 2006). Recent
analysis of host-feeding patterns of Culex mosquitoes
shows that a shift in host choice from birds to mammals,
including humans, as robins begin fall migratory movements, seems to be responsible for the seasonal rise in
human cases of WNV (Kilpatrick et al., 2006). Hostfeeding patterns of 21 mosquito species from New Yorkand New Jersey have been checked by DNA sequencingof the cytochrome b gene. Ae. vexans, Ochlerotatus
japonicus and Oc. trivitattus fed exclusively or almost
exclusively on mammals, but five species, two of
the Cx. pipiens complex (Culex pipiens pipiens andmolestus in the USA), Cx. restuans, Cx. salinarius and30

M. KOBAYASHI et al.

Culiseta melanura took a blood meal from both birds
and mammals. A total of 24 different avian host species
were detected among the avian-derived blood meals,
with blood meals from the American crow relatively
rare (Apperson et al., 2004). Although the activity of
WNV was not detected in field-collected mosquitoes inour survey during 2003-2006 in Japan, it is also
important to know which species of mosquitoes may
become bridge vectors in the urban environment ofJapan. Blood-feeding patterns of Cx. pipiens group(pallens and molestus) were separately analyzed by
DNA sequencing of the cytochrome b gene. Ae.
albopictus, the common Aedes mosquito in urban
environments of Japan was also analyzed. Cx. pipiens
group preferred both human and avian hosts, and noclear differences between the two members were foundin avian species composition, but there were significant
differences in mammalian species composition (Fig. 3).
Molestus preferred human blood compared with pallens.
Among avian species, ducks (mallards and spotbill
ducks) and tree sparrows were major blood sources for
the Cx. pipiens complex in Japan (Table 3). More than
60% of Ae. albopictus, a common daytime biter, took a
blood meal from humans and 11% from wild birds(Sawabe & Kobayashi, 2004; Sawabe et al., 2008)(Fig. 3). From these results it is concluded that two
members of the Cx. pipiens group and Ae. albopictus
n=119

n=38

(%)
100
80

n=114
3.5%
6.1%
5.3%

0.9%

Birds

44.7%

51.3%

Others & Mammals

60
40
20
0

Birds & Mammals

13.2%

10.9%

Mammals

84.2%

Others(Amphibians & Reptiles)

37.8%

42.1%

pallens

molestus

Ae. albopictus

Fig. 3 Blood-feeding habits of three urban mosquitoes, Culex
pipiens pallens, Culex pipiens form molestus and Aedes
albopictus. (drawn from data in Sawabe et al., 2008)

Table 3 Identification of blood sources of three urban
mosquitoes in Japan. (from Sawabe et al., 2008)
Species ofBlood sources

pallens(n=119)molestus(n=38)

Birds
Ducks
Tree sparrows
Crows
OthersUnidentified

75
35 (29.4%)
30 (25.2%)2(1.7%)
6(5.0%)
2(1.7%)

22
12 (31.6%)
7 (18.4%)

1310(8.8%)

12(2.6%)(5.3%)

2
1(1.8%)(0.9%)

Mammals
Humans
Dogs
Cats
OthersUnidentified

59
24 (20.2%)
11 (9.2%)
10 (8.4%)

21
17 (44.7%)
1(2.6%)
1(2.6%)

10475(65.8%)

779
25(7.9%)(21.9%)

2

5(4.3%)

Others(Amphibians & Reptiles)(5.9%)(5.9%)

Ae. albopictus(n=114)(5.3%)

may be important bridge vectors for WNV in urban
areas of Japan.

4. Insecticide Resistance of Culex pipiens
pallens and Culex pipiens form molestus
in Japan
If WNV is introduced to a country, control of thevector mosquitoes will be one of the most important
countermeasures for preventing transmission of thevirus from birds to humans, and successful control ofmosquitoes at present heavily relies on the applicationof insecticides. As mentioned above, the Cx. pipiens
group is the most important of the potential vector
mosquitoes for WNV in Japan. Despite almost identicalmorphology, these two forms (pallens and molestus)
have some differences in physiological and ecologicalcharacteristics. Pallens inhabits areas above ground
while molestus inhabits mostly septic tanks below
ground in or around buildings. Therefore, molestus tends
to be the target of indoor control by insecticides. Since
the temperatures of these habitats are suitable in all seasons, molestus does not diapause in winter. Sources ofblood are probably sparse in such closed dark habitats;however, molestus can lay its first egg raft without ablood meal. It has been clarified that molestus willingly
goes up above ground after its first oviposition (Oda &Fujita, 1986). Using a PCR-based method to identify theCx. pipiens group (Kasai et al., 2008), we also confirmed that this mosquito species breeds in the stagnant
water of catch basins above ground in summer. It wasclarified that WNV inoculated into the thorax ofmolestus propagated to ca 106 PFU / mosquito 14 days
after inoculation (unpublished data). Therefore, at present we cannot exclude this species from the list of
potential vector mosquitoes of WN fever in Japan.
In order to confirm whether mosquitocides registeredin Japan are still effective against these mosquitoes, weinvestigated the insecticide susceptibility of the Cx.
pipiens group (Kasai et al., 2007). Overall 37 colonies
of pallens and 17 colonies of molestus were collected inurban environments in 2003 and 2004, and larval bioassays were carried out for five insecticides including apyrethroid-like insecticide (etofenprox), two organophosphates (fenitrothion and temefos), a chitin synthesis
inhibitor (diflubenzuron) and a juvenile hormone mimic(pyriproxyfen). As a result, almost all insecticides
demonstrated high effectiveness against both mosquito
species except etofenprox, which exhibited relatively
low effectiveness against several mosquito colonies(Fig. 4). Twenty-two colonies exhibited survival rates of>10% at an etofenprox concentration of 5.7 ppm which
was 100 times higher than the LC99 concentration,
concentration required to kill 99% of the test population
in a susceptible strain. Given that the total number ofcolonies was 54, overall 41% of the colonies contained
resistant larvae. It is noteworthy that three colonies ofmolestus exhibited a 100% survival rate at 5.7 ppm.
Low susceptibility of molestus to etofenprox may be dueEcology and Physiology of Potential West Nile Vectors

100

31

LC99×1 (0.057ppm)

Survival rate (%)

LC99×10 (0.57ppm)

80

LC99×100 (5.7ppm)

60
40

P03
P34
P36
P29
P13
P35
P31
P12
P14
P07
P30
P06
P09
P28
P08
P32
P25
P21
P15
P02
P01
P18
P19
P10
P20
P11
P26
P37
P22
P05
P16
P23
P24
P17
P04
P33
P27

0

M16
M03
M09
M07
M11
M01
M05
M15
M04
M14
M06
M08
M12
M02
M10
M17
M13

20

molestus

pallensFig. 4 Percentage of survival of Culex pipiens form molestus and Culex pipiens pallens collected fromJapan exposed to pyriproxyfen at three diagnostic concentrations that were calculated according
to their toxicities against a susceptible strain (LC99, LC99×10, LC99×100).
(redrawn from Kasai et al., 2007)

to the high likelihood of this species to have contact
with pyrethroids in underground habitats of buildings,
like commercial facilities. Pyrethroid insecticides are
popular as adulticides in Japan and it is suspected thatfrequent and long-term usage of these insecticides
results in the development of insecticide resistance of
this mosquito species to pyrethroids. Pyriproxyfen, apromising larvicide which is becoming popular in Japanas an IGR (insect growth regulator), exhibited enougheffectiveness against most of the colonies of Cx. pipiens
group at present; however, some colonies alreadyexhibited reduced susceptibilities compared to thestandard susceptible strain (Kasai et al., 2007). The long
history of insecticide usage teaches us that, due tocontinued use, no insecticide can escape the issue ofresistance. The number of registered mosquitocides islimited in Japan. Development of new mosquitocides ordiversion of agricultural insecticides to mosquitocides,
which are registered as insecticides for controlling
medically important insects, would be strongly desirable
in the case of an outbreak of WN fever in Japan.

5. Larval Habitats of the Culex pipiens groupand Aedes albopictus and Increasing Size of
the Areas of Ae. albopictus Distribution inJapan
Mosquito species exploit a wide variety of aquatic
habitats for larval development. Molestus and pallens,
which are common Culex mosquitoes in urban environments of Japan, can tolerate highly polluted aquatic
environments, such as septic tanks and catch basins containing high concentrations of organic matter. Recently,
water supply and drainage systems of urban areas inJapan have been widely re-constructed, polluted waterfrom residential houses does not drain into the gutters of
public roads. From our preliminary surveillance of larval mosquito habitats in urban environments, Cx.
pipiens group mosquitoes, were collected mainly fromcatch basins along public roads (unpublished data).
Larval habitats of Ae. albopictus in forested areas of thecountryside are fundamentally rotted holes in trees, butmany kinds of man-made containers such as cans, bottles, used tires, plastic containers, vases on graves andspecial types of basins for incense sticks at Japanese
temples are exploited. Because catch basins containmainly rainwater, it has been clearly shown that the larvae of Ae. albopictus breed in catch basins along publicroads and by public facilities and detached houses(unpublished data). From these results, it is clear thatcatch basins are the main larval habitat in urban
environments of Japan. It has also been shown that thetarget of larval control should be addressed to catchbasins in the case of an accidental outbreak of mosquito-borne diseases, such as West Nile, dengue andchikungunya fever, and even in times of ordinary conditions without an outbreak.
Since insects are cold-blooded animals, they are
influenced by environmental changes. Recently the
IPCC (Intergovernmental Panel on Climate Change)
reported that green house gas emissions have alreadychanged our climate. High temperatures may increase
the survival rates of vector mosquitoes during winterin temperate zones and change their behavior andecology. Recent increases in mean annual temperature
related to global warming in Japan have accelerated
the expansion of the areas of mosquito distribution, such as Ae. albopictus in the Tohoku District(Kobayashi et al., 2002). Using the MIROC K1
model (Model for Interdisciplinary Research On

32

M. KOBAYASHI et al.

Climate K-1), model developers clearly showed thatthe northern limit of Ae. albopictus will move north,particularly into the northern Honshu District ofJapan by 2035 and into southern and middle
Hokkaido Island by 2100 (Kobayashi et al., 2008).
The spreading of vector mosquitoes by world trade in
used tires has occurred in recent decades into Africa,
the Mideast, Europe and North and South America
causing an outbreak of chikungunya fever in northern
Italy in 2007 (Rezza et al., 2007). These results alert
us to the importance of continuous surveillance of
vector mosquitoes, particularly in the temperate zone
countries.

6. Conclusion
There are significant differences in species composition of mosquitoes between rural areas of Chiba Prefecture and urban areas surrounding Tokyo. During thetwo-year survey surrounding Narita International Airport 16 mosquito species were collected, while three
species, the Culex pipiens group and Ae. albopictus, accounted for more than 99% of collected mosquitoes inareas surrounding Tokyo.
Some mosquito species in urban environments ofJapan are potential vectors of WNV, dengue and chikungunya fever. The present status of mosquito fauna,
population density in urban areas and blood-feeding
habits of female Culex mosquitoes which are competentvectors of WNV are essential data for construction of amosquito control strategy. Development of insecticide
resistance in the Cx. pipiens group was also recognizedin urban environments of Japan. The most importantlarval habitats of these urban mosquitoes were catchbasins along public roads so this kind of structure
should be modified to control mosquito larvae.

Acknowledgements
This work was partially supported by a grant from“the Global Environment Research Fund” by Ministryof the Environment, Japan.
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33
Mutsuo KOBAYASHI

Mutsuo KOBAYASHI is Director of the
Department of Medical Entomology,
National Institute of Infectious Diseases.
He is interested in a wide variety of fieldsof medical entomology including physiology, biochemistry and host-pathogen
interrelation. Recently, he has focused on
the expanding distribution of medicallyimportant insects, particularly the Asiantiger mosquito, Aedes albopictus in the northern parts of the Honshu
District and a venomous spider, Latrodectus hasseltii, in Osaka Prefecture.
He is a member of a special council organized by Ministry of theEnvironment to evaluate the effects of global warming on human healthand suggest adoptable countermeasures in Japan. He is President of theJapan Society of Medical Entomology and Zoology (2006-2008).

Shinji KASAI is a Senior Research Scientist in the Department ofMedical Entomology, National Institute of Infectious Diseases. He
received his Ph. D. in Agriculture from the University of Tsukuba in 1998.
He spent two years at Cornell University as a post-doctoral research
fellow in the Department of Entomology. Through his research he aims to
elucidate physiological and ecological characteristics of medicallyimportant insects, focusing especially on the molecular mechanisms ofinsecticide resistance, monitoring of resistance levels of field populations,searching for new targets for insect regulators and establishing molecular
identification methods for vector insects.

Kyoko SAWABE is Chief of the Laboratory of Physiology andBiochemistry in the Department of Medical Entomology, National
Institute of Infectious Diseases. Her research involves the relationshipbetween vector insects and their pathogens. To evaluate the vector
competence of Japanese mosquitoes for flaviviruses, including theJapanese encephalitis virus, the West Nile virus and the Culex flavivirus,
she performs virus isolation in mosquitoes and carries out blood-meal
analyses of these mosquitoes. Another area of her research is in clarifying
the overwintering and hibernation mechanisms of Culex mosquitoes using
a physiological approach.

Yoshio TSUDA is Chief of the Laboratory of Taxonomy and Ecology,
Department of Medical Entomology, National Institute of Infectious
Diseases. His major fields of study are the ecology of vector mosquitoes
and disease ecology. He has carried out ecological studies on malaria,
dengue fever and Japanese encephalitis vectors in Japan and Southeast
Asia. He is interested in the ecology of urban mosquitoes in Japan andstudies the role of urban mosquitoes in transmission of avian malaria
parasites among Japanese wild birds in Tokyo.

(Received 14 February 2008, Accepted 6 June 2008)