The identification of vector species and their relative population densities is important for a better understanding of the transmission of zoonotic and human pathogens and the epidemiology of emerging and reemerging diseases. In the Republic of Korea, lymphatic filariasis was prevalent in the inland areas of Gyeongsangbuk-do (do=province) and coastal and southern areas of Jeollanam-do and Jeju-do. However, the prevalence of filariasis has decreased sharply due to medical treatments of microfilaria-positive patients and increased use of mosquito nets [1-5]. The islands of Heuksan, Sinan-gun (county), Jeollanam-do, were identified as endemic foci of fila-riasis in the mid-1980s (mean microfilaria positive rate: 12.5%). However, the positive rate in endemic regions dropped to 0%, and it was finally reported that filariasis had been eradicated from Korea [6].

Ochlerotatus togoi (formerly known as Aedes togoi) is the primary vector of lymphatic filariasis along the coastal areas of Jeju-do and other southern islands, whereas Anopheles sinensis sensu stricto, associated with rice paddies, is found in inland areas [3,7]. In these regions, however, vector mosquitoes and their larval densities have not been fully studied. There is also little information on their distribution and relative population densities of filariasis vectors and other mosquitoes in previous endemic areas. In the present study, mosquito surveillance was conducted to identify relative distributions and population den-sities of filariasis vectors in previous endemic areas in Korea.

Oc. togoi and Cx. pipiens were the most frequently collected mosquitoes captured by black-light traps in southern islands of Korea during this survey. An. (Hyrcanus) group was collected at high densities only in the Baekdo-ri village, suggesting that it is a potential important vector of filariasis in this area. The mosquito population on Geomun-ri was the highest during the early rainy season and peaked a month earlier than the population on Maejuk-ri, due presumably to differences in climate (e.g., temperature and rainfall) between the 2 areas. However, both areas had a lower population after July because of frequent typhoons during the summer season (data not shown). The larval densities were the highest in June, when the average rainfall and temperature in these areas were 195-210 mm (data not shown) and 20-27℃ (Tables 7, 8), respectively. The appearance of Oc. togoi larvae coincided with decreases in the salinity of rock pools due to frequent rains (Table 10). Thus, lower salinity during this period may be a reason for increased population densities as observed in previous reports [10,11]. Larval Oc. togoi densities decreased during the drier summer due to strong direct sunlight and high water temperature [12]. In a previous study, increase in salinity played a role in reducing the susceptibility of Oc. togoi larvae [4,13]. The presence of <0.5% salt concentrations during the summer and the rainfall amount influenced the number of larval survival [7,14].

The number of Oc. togoi collected in our study was greater than a previous survey collection by Lee et al. [15] at the same location on Heuksan Island. This difference may have been due to differences in collection methods between the CDC miniature type (American Biophysics Co., East Greenwich, Rhode Island, USA) used in the Lee et al. study [15] and black-light traps used in our study. However, the proportion of Oc. togoi in the collected mosquitoes in this study was also lower than that reported by Kim et al. [1] and Lee [16]. These previous studies reported that collection of Oc. togoi and Cx. pipiens ranged 70-90% and 5-26%, respectively, in Jeju-do.

As both vector species are present in Korea, the most likely explanation for the eradication of filariasis is suggested to be an aggressive treatment program executed during the 1970s and 1990s [2,3,17,18], with the consequent benefit that there were fewer vector mosquitoes that have contracted the disease from human carriers. Furthermore, transmission of mosquito-borne diseases has been greatly reduced by the ommercialization of mosquito repellents and insecticides, as well as the use of bed nets and screened windows, all of which decrease human-vector contact [19]. However, a steadily high prevalence of the vector mosquitoes may constitute a potential risk for reemergence of brugian filariasis in these areas.