Ecology of Japanese Encephalitis Virus in Okinawa, Japan
II. Regional particularity of the virus dissemination in Okinawa island from 1966 to 1976
Muneteru URA
Department of Virology, Institute for Tropical Medicine, Nagasaki University
(Director: Prof. Kaoru HAYAS巾)
Epidemiological Laboratory , Okinawa Prefectural Institute of Public Health
(Director: Dr. Chokei YosHIDA)
Abstract: In the previous paper, it was indicated that the mosquito and pig infection in Okinawa island has usually been demonstrated from early April or May and vanished in late September or middle October since 1966. It was also pointed out that such a long period of JE virus dissemination in nature was never observed in Nagasaki area as well as in the main island of Japan. In this paper, the local peculiarity of JE virus dissemination by area in Okinawa island since 1966 will be described. The virus isolation from pig‑sera and vector mosquitoes and the detection of 2‑mercaptoethanol (2ME) sensitive antibody in pig‑sera in every epidemic season were usually made earlier in the northern area than in the middle and southern areas of the island. In 1976, JE virus was isolated from the overwintered female vector mosquitoes caught in February as indicated in the previous paper.
This noticeable evidence was demonstrated in the northern area of the island.
It was reported that the earliest isolation of JE virus from mosquitoes of Culex tritaeniorhynchus in Japan was made on 19th and 30th May in 1965 in Nagasaki area (Takahashl et al. 1968 ; Hayasni et al. 1965) and the latest one was demonstrated on 5th or 6th September in 1963 or 1965 in Kanto or Nagasaki area respectively (Ova et al. 1963 ; Shichijo et al. 1968). However, in the recent investigation on JE virus dissemination it was noted that the mosquito infection in Nagasaki area and also in the whole of Japan was demonstrated from the middle or the late part of July to the early or middle part of August, especially since 1973 (Fukumi et al. 1975 ; Wada et al. 1975 ; Hayashi et al. 1976). On the other hand, in Okinawa island, the isolation of JE virus from pig‑sera and vector mosquitoes was usually made from April or May to October or November (Ura, 1976). In this paper,
Contribution No. 772 from the Institute for Tropical Medicine, Nagasaki University.
Received for publication, December 7, 1976
166
such ユocal peculiarity of the virus dissemination in Okinawa island was observed for the
northern, middlle and southern parts of the island and the localized situation of the ecology of JE virus will be discussed.
MATERIALS AND METHODS
Ås shown in Fig. 1, Okinawa island was bounded into three districts for the geogra phical convenience ; namely, northern area north of Nago district containing a city, a town and 7 villages, middle area including from Ginowan and Nakagusuku to Onna and Ginoza districts containing 4 cities, a town and 9 villages, southern area south of Naha district contain!腸3 city, a town and 14 villages. Pig‑sera were collected at three slaughter houses located in the northern area (Nago〕, middle area (Gushikawa〕 and southern area 〔Madam‑
bashi on Ozato〕. Pig‑sera were also identified for their breeding farms as shown in Fig― 1,
Vector mosquitoes were caught by light traps at the pigsheds located in each area and also dry ice method was used in the field from January to March since 1973. The methods for the examination of hem昭glutination inhibition (HI〕 and 2‑mercaptoethanol (2ME) sensitive
Fig Map of Okinawa island and survey places.
淫･
Ihej
<?1‑
‑'‑^
牀Ijena
‑V'
North area
Middle area
South area 7.nmflrm
Tokashiki
Kadena^
chatanl Yomitan
3Gi議nowan, Uraso
Naha oMadambashi Kochi
Tok年中享ki1〔
7*Nakagusuk≡甘 Kitanakashiro
shihara一.､
ヽ｢
Tamagushiku いId主ユI
⊥tご‑･いeいt‑<s
<‑‑‑‑
Naki
!‑
ヽ■il! ヽ ｢■
y いy* ―
9Nago M,
―Lー― J
I
ヽ.
■:Ii=
ヽ ヽ
ヽ ヽ
ヽ ヽ
I‑ ｢■
立::コ ヽ
Scale :｢ ―――――→
10km
いI
㌔ J
♂ J
! ヽ.
ヽ ヽ
ヽ
＼
&
＼ 0
ヽ e
ヽ T1 月T｣>a
ト飛shih三三ba二
onlと1n Itornと1n
antibodies in pig―sera and for the virus isolation from pig‑sera and vector mosquitoes were
described in me previous paper.
RESULTS
Virus dissemination by local area from 1966 to 1968
The examination of pig‑sera for HI and 2ME sensitive antibodies was fixed from= 1966 to 1968, because there were difficulties to isolate the virus from pig‑sera and vector mosquitoes due to the insufficient equipment during these 3 years･ So far as the dectection
◆
of HI and 2ME sensitive antibodies was concerned, it was indicated that JE virus might have been disseminated from the middle or the late part of April in these 3 years to the
late part of September in ]〓966 (Fig. 2, 3a, 3b〕 2MB sensitive antibodies were detected in
pig‑sera collected from January to March and after October in these 3 years especially at the northern area. These events suggested the pig infection with the virus in the interepidemic season. However, the identification of 2ME sensitive antibody in pig‑sera mu･st be made carefully by using the methods of 2ME‑iodoacetoamide treatment and analytical centrifu酢tion. In addition, it will also be necessary to isolate the virus from pig‑sera and vector mosquitoes in the interepidemic season for the improvement of the overwintering problems of JE virus in the island.
Virus dissemination by local area from 1969 to 1972
Even though the date of the virus isolation from pig‑sera and vector mosquitoes was different in these 4 years (April 16, 1969 ; June 2, 1970 ; May ll, 1972〕, it was found that the isolation of JE virus was usually made earlier in the northern area than in the middle
and southern areas一 of the island. It was also recognized that the detection of HI
antibody particularly 2ME sensitive antibody in slaughtered pig‑sera was made closely in
parallel with the mosquito infection and with the events of the virus isolation from pig‑sera･
●
So far as the detection of 2ME sensitive antibodies in pig‑sera was concerned, it was
indicated that the possibility o壬vector mosquito and pig infection in the interepidemic season in these 4 years would be presumed in the southern area especially in the northern area
(Table 1, Fig. 4a, 4b;5a, 5b;6a, 6b;7a, 7b〕.
Virus dissemination by local area from 1973 to 1976
In 1973, in the northern and middle areas, the virus isolation from vector mosquitoes
and pig―sera was made on 7th and 19th July respectively. In contrast, the 2ME sensitive
antibody in pig‑sera was found in the late part of May in the northern area. However, the 2MB sensitive antibody in the sera of pigs bred and slaughtered in the southern area was detected in January and February in 1973. It might be suggested that JE virus was disseminated in the southern localized area in this year (Table 2, Fig. 8a, 8b). In 1974 and 1975, the mosquito and pig infection was observed from the middle part of May to the late part of October and the early part of November in the northern area. In the middle
and southern areas, the virus iso一ation from pig‑sera and vector mosquitoes and also the
=
168
Fig. 2. HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in the main island of Okinawa in 1966.
Remarks : The dotted line means the rate of HI positive in swine sera and white stick means the rate of 2ME sensitive antibody possess in them. The
black sticks mean the number of human Japanese encephalitis cases.
Fig. 3a. HI antibody and 2ME sensitive antibody in s】aughtered swine sera collected in Okinawa island from 196･e.
Fig. 3b―
Remarks: The dotted line means the rate of HI positive in swine sera and w―bite stick means
the rate of 2ME sensitive antibody possess in them. The black sticks mean the number of
human Japanese encephalitis cases.
―
≡≒三±4…什.〓
‑｡ ‑ ‑‑̲〓一
1ロ〔
8 9C
―L上
記 B(
* 7(
r(一一 G〔
P u〕
･―.' J
13fr. 5〔
ヒ 1 :―ヨ Ll〔
(o
娼…:
丑Lj l〔
Weeks in month Number of reported 【 CaSeS
±=二三‑士≡去
Main ユsland
‑′＼^ A
イ｡
〓淫報彦皇日〓夘X養皇x i
立立立 〓=‑:〓‑‑‑‑‑‑―〓一一‑
･ooh ー･hいNorthare;M
ヽ ヽ いal
いvいいiい
∫
哺〓i康彦/態R彦喜〓/ i
〓張ddle area ( Gushikawa slaughter house)
.. .A
壷主鎧
i
i Vいいい s
い
＼′.甘′Lい―
實"音型E/音iE皇亜喜彦叩室. 二==二±
Souヒh a工｣ea
･ ∴｣∴..̲二O‑‑
IV
detection of 2ME sensitive antibody in pig‑sera were made from June to September 〔Table 2, Fig. 9a, 9b ; lOa, lOb〕 It was indicated that the virus dissemination in the northern area °night have been earlier and lon酢r than that in the middle and southern areas in these
2 years. In 1976, 2 strains of JE virus could be isolated from overwinteri咽female
mosquitoes of Culex tritaeniorhynchus caught in the northern area on 13th/15th February as described in the previous paper. It was suggested that JE virus might have overwintered
persisting週 the hibernated female vector mosquitoes and there might have been preserved
議
some particular situation in the northern area of the island. It was also noted that the virus isolations from vector mosquitoes in the middle and southern areas were made on the
=
late part of May and July in this year (Fig. 1la, lib〕･
Table 1 Detection of 2ME sensitive antibody and virus isolation from pig‑sera by month in the main island of Okinawa from 1969 to 1972
1970 』 1971
Month
January February March April May
June
July August September October N ovember December
毒pμ,4る/ ･･實巾w4違違
117 105 103 117
n―d.
･ク
ーク
･ケ
^
^
2(S 1(S l〔N
3E習 サ言
Total 〓 663 45
〕 0 ) 0
〕 0
KN)
)
〕 2(S)
KS)
調潜る/:皇国4違違
120 0 120 0 145 0
0 0 0
i―‑4一〓4‑4三享違違
90 0 120 1(N) 0
150 0 120 0 120 0 0
i2皇一…三享違違
86 5(S) 90 0 150 1(N) 118 1(N〕
0 0 0
0
119 2(S〕 119 3(S〕 5捌146 2(N) 0
15028((實)(
)32(
)(皇8,い103KS)0
86 10實8
150 35指8
119 2鞘8
90 19指8
57 0 59 0
2
4
(N〕
(S〕
(M) (S)
5E習8
･(N) TS)
149 17實8 8措8
60 0 岳 6014指8 o
1(S〕
1(N〕
≡:oo
｡0〓30〓cs (s;
萱9: :〓―Z.萱〔s〕 o:
4 い一10119 958 49 39 1,254 100
Remarks : (N)‑‑ North area (M〕‑‑‑Middle area (S>‑=‑South area
15
‑I:三O‑〓 士IO4一O
― ― ‥ ― ―〓 ―
1 .I,
1972
iWSK1
97 14言8
(N) 124 24(M) (S〕
ユ〓70
Fig― 4a. HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in
Okinawa island in 1969.
Fig.4b.
Remarks:ThedottedlinemeanstherateofHIpositiveinswineseraandwhitestickmeans therateof2MEsensitiveantibodypossessinthe‑Thearrowsmeanthevirusisolation fromslaughteredswineseraandmosquitoesofCule
meanthenumber｡fhumanJapaneseencephalitiscase;〓ritaeniorhynchus.Theblacksticks
Main island North area
Fig. 5a. HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in Okinawa island in 1970.
ユOJ 39(召B(
い*7〔
三ベ6〔
rJUい) /Kい5〔
r‑IH4J如 しH―√1 0其〓3C 2C l｡
Weeksin month Numberof reportedr cases
Main island
. 】.∫
〓血清g皇彦x 潜/淫彦家家/張 家屋ElB寛X調寛X潜IX罹
North area
i傾 言調M〓の家E屋l彦潜/溶x[彦EK重 x屋 』主立家≡L虹
ladle area Middle area
10〔
∴淫m藍X
̲し,岩8〔
n Q)70 u3‑‑6占 召..其50 空言40 リ
±±=二±÷o̲=一
｢い八八張1〓
South area
∫
EEg家//E潜旬EE皇i
‑‑̲‑.〓1. I‑〓 〓
/E家皇./国‑i/彦X彦皇潜x寛i
/rM
‑一三二二二
South area
F.p｢g. 5b.
Remarks: The dotted line means the rate of HI positive in swine sera and white stick means
the rate or 2ME sensitive antibody possess in them. The arrows mean the virus週solation
from slaughtered swine sera and mosquitoes of Culex tritaeniorhynchus. The black sticks mean the number of human Japanese encephalitis cases.
Main island North area
Fig. 6a. HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in Okinawa island in 1971.
Middle area South area
Fig. 6b.
Remarks: The dotted line means the rate of HI positive in swine sera and white stick means
the rate of 2ME sensitive antibody possess in them. The arrows mean the virus 週solation
from slaughtered swine sera and mosquitoes of Culex tritaeniorhynchus. The black sticks mean the number of human Japanese encephalitis cases.
臥, 帆
Middle ar
―
紅E屋//〓E/清〓/ i憎岩屈L 血判硝. U胞J. I濾割切
〓
3o=―
L‑‑〓〓te―b r王a, A!I
"=='= ‑ い〓〓
凹實彦喜勤型･
十
u‑n‑!＼∧
追』ヨ
紅[報紹皇E言n 箸+盟
Jい大
I eb いhT ･ Apr
〓
m賂
出aま―n island
Fig, 7a. HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in Okinawa island in 1972.
Middle area
Fig. 7b.
Remarks: The dotted line means the rate of HI positive in swine sera and white stick means the rate of 2ME sensitive antibody possess in them. The arrows mean the virus isolation from slaughtered swine sere and mosquitoes of Culex tritaeniorhynchus. The black sticks mean the number of human Japanese encephalitis cases.
Fig. 8a. HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in Okinawa island in 1973,
100 臣gOrl 召8D Ul精呂 ヨ5n叫0 彦30 2｡
ユロ Weeksin month Numberof 喜≡冒≡…ted5
二二‑±
―
/P/―[家//家E潜E h例North area
〓
彦言E /皇国/iE生//5屋/ i
二 〓二一±二二二二二二
.一 ‑〓I‑‑
/主E皇病夘E/旬E s.?
South area
〓 ̲:〓二‑〓〓一.
朋 =三三±二
North area
I,l一IU 与l WM
̲〓=―l
μain island
･. I:‑.‑〓〓〓L一t〓〓
Middle area
Fig. 8b.
Remarks: The dotted line means the rate of HI positive in swine sera and white stick means the rate of 2ME sensitive antibody possess in them. The arrows mean the virus isolation from slaughtered swine sera and mosquitoes of Culex tritaeniorhynchus. The black sticks mean the number of human Japanese encephalitis cases.
Table 2 D己tection of 2MB sensitive antibody and virus isolation from pig―sera
by month in the main island of Okinawa from 1973 to 1976 Year 】 1973
Month
w
毒軍縮
凹 u〕
i養〕皇=三4 ol皇/i
1976
〔c uo
毒養養:μ4 o一皇/i
〔JJ CO
i=享 享̲41違
January February March April May
June
July
August
September
()ctober N ovember December
116 3(S) 0 90 3(S〕 0 90 0 90 0
90 1(N) 0 (N) 149 〔M) (S)
120 16指o)
54エ8
(N) 4(M) (S)
120 12指8 4指8
(N〕
119 〔M) 2(N) (S) 120 1(N) 0
90 0 60 0
90 0 120 0 90 0 120 0 150 5(N)
120 1噸8
120 15指8 120 20指8
90 3(S〕
･20 指8
90 0 60 0
0 0 0 0
90 0 120 0 120 0 115 0
6:N一15 12 12≡1｡慧
0 0 0 0
2CN) 3(N)
≡(s〕F ll≡.0 1;.E≡三三(s)
〓‑ 〓o〓〓4
Tota1 1,254 42 12 1,290 45
0 0 0 30
90
90
60
0 0
2CN〕
0 0
2(N)
堀 6(N〕
10,(N) cM)5(N)
5〔N〕 4〔N〕
0
so KM〕 2指)) 60 3摺)) o
30 1(M) O n.d. .d. n.d
21 Remarks : See Table 1.
實EE蠕皇宮i/g〓E/藍室 . "n紅型温.〓主張〓d*
//皇/蠕E/清室iE〓屋寛室
South area
‥∴二‑‥‥ ･･I
4軸
0 0
1(N二
0 0
1,41040 12 i 39024
174
Fig. 9a, HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in Okinawa island in 1974,
Middle area
Fig―. 9b.
Remarks: The dotted line means the rate of HI positive in swine sera and white stick means the rate of 2ME sensitive antibody possess in them｣ The arrows mean the virus isolation from slaughtered swine sera and mosquitoes of Culex tritaeniorhynchus.
Mai―n island North area
Fig｣ lOa. HI antibody and 2ME sensitive antibody in slaughtered swine sera cellected in Okinawa island in 1975.
屋紅屋/家屋/家言清彦E
Main island
‑･‑ ∴ 一
紅張群E屋// 家喜R
[orth area
〓〓〓.〓〓〓
張一家〓彦lp岳―i リ〓 ^ 三一=こ‑I
[E/日当iE/旬養皇喜d
South area
〓‑〓;一.〓
夘/w/屋/ 張〓―〓型屋/旬精〓 hi
―九rA/fV
[m家型F屋/ gi潜 x傾實x h馴
､L巾u什
Middle area South area
Fig. lob.
Remarks: The dotted line means the rate of HI positive in swine sera and white stick means the rate of 2ME sensitive antibody possess in them. The arrows mean the virus isolation from slaughtered swine sera and mosquitoes of Culex tritaeniorhynchus. The black sticks mean the number or human Japanese encephalitis cases,
Fig. 1la. HI antibody and 2ME sensitive antibody in slaughtered swine sera collected in Okinawa island in 1976.
Fig. lib.
Remarks: The dotted line means the rate of HI positive in swine sera and white stick means the rate of 2ME sensitive antibody possess in them･ The arrows mean the virus isolation from slaughtered swine sera and mosquitoes of Culex tritaeniorhynchus.
痛彦/報張芳田, 十 畑=刑一世｢〓西
V一11L国
ヨ]
‑皇国X 喜擱軌型
―
↓出御いj?a//皇皇張‑彦長屋
･ ==‑〓‑･･一〓
Main island
p^
≡｡Ⅱ盛続出ふ〓4m清皇rePO
〓彦E家淫j家pg旬h醐=
O一‑4二 〓立
〓〓〓 〓
〓E//喜皇h.
}止ddle且rea
―.― Fu.の｡――癖.芸― ――べ.一y血一―l.｣A長:r;丑1岨]
―
adg 590I80 i70サfi.60 sJ50slw
"￨30 ァ120p,‑1Q.
Keeksin‑‑‑‑
monthJal Numberof reported cases5
South area
̲I̲―̲〓 ･̲三±±=
〓〓
176
DISCUSSION
The variation in time and period of virus isolation from vector mosquitoes by place in the epidemic season was investigated in Nagasaki area in 1965 (Hayashi et al. 1965).
It was also indicated that the pig infection informed by HI antibody rising in the pig‑sera● has usually extended from the southern part to the northern area of Japan (Hayashi, et al.
1975). However, there has been little knowledge about continuous and detailed investiga‑
tions on the local particularity of JE virus dissemination in the limited places. It was demonstrated by the author in the previous paper that the time and period of JE virus diss‑
emination in Okinaw且island located in subtropical area was eariler and lon酢r by about two
or甘hree months than in Nag―asaki area and in the whole of Japan 〔Ura, 1976). In this
paper, it is reported that the local peculiarity of JE virus dissemination in the northern, middle and southern areas of Okinawa island was investigated.
When the potential motion of JE virus in the interepidemic season was focused on the detection of the 2ME sensitive antibody in pig‑sera, it might be presumed that the pigs happen to be infected with the virus at that time. On the standpoint of such a conside‑
ration, It was noticed that the 2ME sensitive antibody was detected in the pig‑sera collected in the northern area and occassionally in the southern area of the island from January to
March since 1966 〔Fig. 2a, 3a, Table 1, 2). Following these events in the interepidemic season, the virus isolation from pig‑sera and mosquitoes was always made earlier in the northern area than in the middle and southern areas in every epidemic season. Iha (1971) has reported that in the main island of Okinawa, the gonoactivity of Culex tritaeniorhynchus female mosquitoes seems little affected with shortening of day length in cold winter months
and to continue feedi腸activity in warmer temperature during winter season whereas, the■
female mosquitoes of Culex tritaeniorhynchus in Nagasaki area definately affected with shortening of day length in spite of the environmental temperature (Kawai 1969〕.
Furthermore, It was a noticeable finding that in the northern area, the virus isolation from overwintered female mosquitoes of Culex tritaeniorhynchus caught in the field in the northern area was made as presented in the previous paper (Ura, 1976〕 These events might have been related to the industrial development of the island. Accordingly, the rice‑field area remarkably decreased in extent especially in the middle and southern areas of the island, and it may be considered that the source for the breeding of the vector mosquitoes diminished since 1973. In contrast, it was considered that certain favourable circumstances particularly for the overwintering of the virus in interepidemic season must have remained■ in the northern area of the island.
ÅcKNOWLEDGEMENT
Author has been greatly indebted to the Virological Laboratory of the Institute for Environmental Pollution and Public Health, Nagasaki Prefecture for the kind cooperation of
comparative mvestig―ation on JE virus ecology. Author also express sincere thanks to Dr.
Tetsu Shimabukuro, chief of Diagnostic Section of Animal Disease, Animal Health Center, okinawa Prefecture for the kind supply of suckling mice, and to Mr. Kunio Nakachi, Mr.
Takao Kishimoto, Mrs. Yoshiko Riga, Mrs. Kazuko Shimojana, staffs of our Institute, for the kind cooperation of mosquito collection and classification. Author would like to thank prof. Kaoru Hayashi for his continuous encouragement throughout this study.
REFERENCES
l) Fukumi, H., Hayashi, K., Mifune, K., Shichijo,A., Matsuo,S., Omori, N., Wada,Y., Oda, T.,
い Mogi,M. & Mori, A. (1975) : Ecology of Japanese encephalitis virus in Japan. I. Mosquito and
pig infection with the virus in relation to human incidences･ Trop. Med. , 17, 97‑no.
2) Hayashi, K., Mifune, K., Shichijo,A., Suzuki, H. & Matsuo,S. (1976) : Ecology of Japanese encephalitis virus in Nagasaki area, Okinawa and Amami islands. Trop･ Med. , 17,159‑176.
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studies on Japanese encephalitis virus･ Isolation of JE virus from mosquitoes co一lected Nagasaki and
Kagoshima districts, Japan, 1965. End. Dis, Bull. Nagasaki Univ., 8, 61=73･
4〕 Iha,S. (1971) : Feeding reference and seasonal distribution of mosquitoes in relation to the epidemiology of Japanese encephalitis in Okinawa main island. Trop. Med. 12, 143‑168.
5) Kaw―ai. S. 〔1989〕: Studies on the follicular development and feeding activity of the females of
cule;一c tritaeniorhynchus with special reference to those in autumn. Trop. Med. , ll, 145‑169.
6) Ova, A., Takahashi, M. , Ogata, T., Kataoka, M. & Okuno, T. (1963) : Studies on arbovirus. Results
of the field work in 1963. Ann― Rep. of Natl― Inst. Health., 17, 75‑76･
7) Shichijo, A., Mifune, K., Hayashi, K., Wada,Y.,Ito, S., Kawai, S., Miyagi, I. & Oda, T. (1968):
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8) Takahasi, K.,Matsuo, R., Kuma, M. & Noguchi, H. (1968〕 Aino virus, a new member of simbu group of arb｡virus from mosquitoes in Japan. Jap. J. Med. Sci. Biol., 21,95‑101.
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甜 (4〕, 151‑164.
10) Wada, Y., Oda,T.,Mogi,M., Mori, A., Omori, N., Fukumi, H., Hayashi, K., Mifune,K., Shichijo, A. & Matsuo, S. (1975) : Ecology of Japanese encephalitis virus in Japan. II. The population of
亀 vector mosquitoes and the epidemic of Japanese encephalitis. Trop. Med., 17, 111‑127.
――― 〓 ― ― ―‑ ―〓―― 〓―――〓‑｣ ‑――― 〓 〓 ―〓 ‑― ‑ ―――― l
沖繩における日本脳炎ウイルスの生態の研究
II.沖繩本土における日本脳炎ウイルスの撒布の地域特異性
宇良宗輝(長崎大学熱帯医学研究所ウイルス学部門,沖縄県公害衛生研究所)
前報で沖繩本島では豚血清及びコガタアカイエカから日脳ウイルスが分離される期間は例年4月上旬 か5月上旬から9月下旬あるいは10月中旬までであって,ウイルス撒布期間は著しく長期に亘ること を明かにした.このように流行期の始まりが早いことやウイルス汚染期間が長いことは長崎地方を含 めて日本本土では決して認められない現象であることを指摘した.本報では,沖繩本島におけるウイ ルス撒布の状況を地域別に詳細に観察した所見を述べた.即ち,沖繩本島を南部,中部,北部地域に 区分し,豚血清及び蚊から最初にウイルスが分離されるのは常に北部地域であって,中部及び南部地 域はこれよりややおくれることが判かった.豚血清中の2ME感受性抗体の検出状況やその保有率も ウイルス分離の所見と平行していた.1976年,北部地域では2月13日/15日に野外で捕集した越冬コ ガタアカイエカから日脳ウイルスが分離され,北部地域では4月以後年間を通じて蚊及び豚血清から ウイルスが分離されると共に豚血清中の2ME感受性抗体も平行して検出された。これに比べ中部及 び南部地域のウイルス汚染は北部地域よりおくれ5月下旬から始まった。このように北部地域は1976 年には年間,ウイルスの生存環が保持されたことを示していた。亜熱帯地帯に位置し,かつ比較的狭 小な島である沖縄本島において日脳ウイルスの撒布の様相は例年定常的といえるほど明瞭な地域差を 示すことを明かにした。
熱帯医学 第18巻 第4号165‑178瓦1976年12月
178
