蚊の Age に関する研究 VI : フイラリア症流行地における媒介蚊集団の Age 構成と疫学的意義について

Abstract

Two field surveys were carried out in filariasis endemic areas in order to assess the relationship between ages of mosquitoes and their infection with various stages of developing filarial larvae. One survey was done in Okino Erabu Island, in the southwestern part of this country, where bancroftian filariasis has been endemic with a microfilaraemia rate of about 10% among the inhabitants. Here Culex pipiens fatigans is the only certain vector. Another survey was carried out in Hachijo-Koshima Island, a small islet some 250km south of Tokyo in the Pacific Ocean. This is the only site of malayan filariasis so far known in Japan with a microfilaraemia rate of 19% among inhabitants at the time of our examination, Aedes togoi is the only vector. In both islands female mosquitoes were collected inside houses from 20 to 22 hours nightly by using sucking tubes and were dissected on the following morning. First the ovary was isolated and the physiological age of the individual mosquito was determined by observing the number of relics at pedicel of the ovariole, according to Detinova's method (1962). The whole body was then dissected out and examined for filarial larvae. Among 349 dissected mosquitoes of C. pipiens fatigans, which were caught resting indoor at Okino Erabu Island in May, 1963, 50.7% were nulliparous, 39.70% uniparous, 8.65% biparous and 0.89% triparous. Of these 45.2% were found to have fed. Although 8.8% were infected, the infective rate was 0.29%. It is of interest to note that the infected rate increased with an increase of age of the mosquitoes : i. e., 7.06% were infected among nulliparous, 9.77% among uniparous, 10.34% among biparous and 66.7% among triparous. This naturally follows, since the older mosquitoes have had more opportunities to take infective bloodmeals. The nulliparous mosquitoes also were often found to be infected; however, all larvae were in the early phase of development. And most noteworthy is the fact that the mature infective larvae were found only in the biparous and older mosquitoes. This suggests that the age composition of the population, particularly the relative proportion of biparous and older groups is of prime importance as a factor to be considered in connection with the transmission of the infection to man. The results obtained with Aedes togoi in Hachijo-Koshima Island in August, 1963 showed the same trend as that described above. Among 108 mosquitoes dissected, 43.3% were nulliparous, 46.3% uniparous, 9.3% biparous and 0.93% triparous. Of these, 77.8% had fed, 6.48% were infected, and 1.85% were infective. The rate of infected mosquitoes was 2.13% among nulliparous, 6.0% among uniparous, and 33% among biparous. The infective larvae could be detected only in the biparous mosquitoes. In both areas the rate of infected mosquitoes was higher among those collected from houses where microfilarial carriers dwelt, than among those collected from houses of non-carriers. However, the infective rate was no different among mosquitoes in either carriers' houses or those of non-carrieres. Since the mosquitoes could have moved around for a considerable distance by the time ingested microfilariae reached the mature stage, it is reasonable to find infective mosquitoes in either kind of house. Hence it is recognized that any of the inhabitants of the area were exposed to the same risk of infection in their respective areas. In both endemic areas no significant trends to familial aggregation of infected cases were observed.