The age-composition of natural populations of Culex tritaeniorhynchus was investigated in a rural district of Osaka Prefecture during the period from 1968 to 1973,and it was found that the seasonal fluctuations in the age-composition of Cx. tritaeniorhynchus populations were well correlated to the variation of mosquito abundance. The average parous rate of the populations was slightly higher in August (18.1%) than in July (15.4%). The daily survival rate of the female mosquitoes were thus estimated as 0.525 in July and as 0.555 in August, respectively. There was little difference in the daily survival rates among different years.
Continual survey in Nagasaki prefecture were established at Kuromaru village since last year. In this season, 13 places in Nagasaki prefecture and the suburbs of Taniyama city, Kagoshima prefecture, were selected additionally for the collection of mosquitoes. The hibernated female of Culextritaeniorhynchus collected in Nagasaki destrict from 16th March to 30th April in this season have been made, however, no virus could be isolated. During the period from 30th May to 6th September, 47 of 56 strains were isolated from Culextritaeniorhynchus mosquitoes, 5 of them from Culex vishnui and 2 of them from Culexpipiens pollens respectively. In addition, 2 strains regarded as Japanese encepha- litis virus (hereinafter referred to as JE) were isolated from Aedes vexans. The first isolation of JE virus from Culex tritaeniorhynchus mosquitoes was made on 30th May in Nagasaki area, while in Kagoshima area it was made on 17th June. Comparing the patterns of JE virus isolations, it was demonstrated that there were remarkable difference of JE virus dissemination by area and by periods. The ecological investigation of JE viruses has been stated on the bases of the JE virus isolations from mosquitoes, the rising of haemagglutination inhibition antibodies in swine sera and the occurrences of human encephalitic cases.
The seasonalprevalenceof immaturestagesof Aedestogoiin coastalrock pools was studiedat Abumize,Fukue Island,Nagasakiby estimatingthe absolutenumberof lar- vae (pupaeinclusive)in each poolby the modifiedremovalmethod.The methodwe used wasjustifiedbythe factthat the seasonaltrend of larvaecoincided verywellwiththe trend of adults. The seasonalabundanceof larvaewas differentin years but generallylowerin summer.It wasshownthat the importantfactorsinfluencing the abundanceinclude(1)dry- ing up ofpoolsby direct sunshineand littlerain fall, (2)water qualitythat is changeable by rain fall, spraysof sea water bystrongwind,and the successionof microorganisms, and (3) naturalenemiessuch as dytiscids(Coleoptera) and notonectids(Hemiptera).
The gonotrophic cycle of Culex tritaeniorhynchus, the principal vector of Japanese encephalitis, was studied in the field by the method of release and recapture of marked mosquitoes. It was found that the time period was 2.6 days between the emergence and the first blood ingestion, 2.9 days between the first and the second blood ingestion, and 2.9 days between the second and the third blood ingestion, and the pre-oviposition period was 5.4 days.
During the spring of 1967-1975,overwintered mosquitoes were collected by using dry ice-traps at terraced rice field areas of 14 localities in Osaka Prefecture. More Culex tritaeniorhynchus were collected from izumi-Mibayasi area than from any other areas. The collection site at Izumi-Mibayasi area was on the terrace in the middle of the River Makio, and characterized by clusters of the eulalia Miscanthus sinensis along the banks between terraced rice fields. Eighty-four emergence-traps were placed over the side of grassy banks or stone walls between rice fields in the early spring of 1968-1970. None of the traps yielded any Cx. tritaeniorhynchus. One trap yielded a single female of Cx. pipiens pallens each at Simamoto area and Higasiosaka-Kamisizyo area. At Izumi-Mibayasi area one trap yielded two females of Cx. pipiens pallens plus five females of Anopheles sinensis, and another yielded two females of An. sinensis. A total of 99 pools of Cx. tritaeniorhynchus (5,075 females) were inoculated into suckling mice. No virus was isolated.
Overwintered females of Culex tritaeniorhynchus, which is the most important vector of the Japanese encephalitis (JE) virus, were collected by using dry ice-traps from March to June at Izumi-Mibayasi area in Osaka Prefecture, 1967-1975. Ovarian dissections revealed that almost all of the females just having emerged from hibernation were nulliparous. From the beginning of September to the first week of October the numbers of host-seeking females of tritaeniorhynchus collected with the CO_2 baited light trap continued to decrease to zero. However, autumnal build-up of its larval population was observed. These results indicated that in the fall few newly-emerged females were developing an appetite for blood. From these results above mentioned, it is not likely that JE virus overwinters in the bodies of tritaeniorhynchus females.
Statistical data on the area of paddy fields and the number of livestock breeding farms in Osaka Prefecture during the period 1965-1986 were analyzed in relation to the recent reduction in the number of human Japanese encephalitis cases. Large numbers of human cases occurred in Osaka City in the epidemic years, 1965-1967. The reasons for the outbreak were considered to be : 1) the density of swine farms was relatively high in Osaka City; 2) the districts in the outskirts of Osaka City had developed paddy fields and a relatively high density of swine farms; and 3) the urban heat island wind enhanced centripetal invasion of the mosquitoes. Since 1973,the number of human cases has been only 0 to 3 a year in Osaka Prefecture and no cases occurred in Osaka City. Urbanization of the rural outskirts of Osaka City which resulted in significant decrease in swine farms and paddy fields throughout Osaka Prefecture may be responsible for the recent reduction in epidemics.
Four strains of Japanese encephalitis virus and 3 strains of group A arbovirus were isolated from mosquitoesof Aedesvexansnipponiicaught in Nagasaki area during epidemic season from 1965to 1968. The latter strains isolated were closely related to the strain MM 2021,Getah virus, a group A arbovirus. One of the isolates, MP 1805strain, was found to be able to form plaques in primary chick embryo cell cultures with the plaque
