Comparison of population of vector mosquitoes of Dirofilaria immitis and their natural infection rates in southern and northern parts of Nagasaki City, Japan.

Abstract

Female mosquitoes including Culex pipiens pallens, the main vector of Dirofilaria immitis, and some secondary vectors, were collected at 4 to 10 sites in Nagasaki City from 1983 to 1989 using light traps, and the number of the mosquitoes and the natural infection rate with D. immitis were compared between the southern and the northern parts of the city. In 1983, Cx. p. pallens was extremely prevalent in the southern part but not in the northern part. After 1986, the prevalence of Cx. p. pallens became very low in both the parts. The rapid decrease of prevalence of this species may be attributable to the decrease in breeding sites by improvement of roads and open roadside ditches in parallel with spread of sewage systems. On the basis of the number of infected mosquitoes in Nagasaki City, it was suggested that, Cx. tritaeniorhynchus and Ae. albopictus are important vectors in addition to Cx. p. pallens, but Ae. togoi does not play a significant role in the transmission of D. immitis. INTRODUCTION We previously reported that the percentage of house dogs having the larvae of Dirofilaria immitis increased from 1968 to 1983 in the eastern, the western and the southern parts of Nagasaki City, but this percentage decreased during the same period in the northern part of the city (Oda et al., 1993) . We also assumed that the decrease of the infection rate of house dogs in the northern part be due to the spread of public sewage systems contributing to the reduction of breeding sites of the main vector mosquito, Culex pipiens pallens. To test the validity of this assumption, we compared the number and the natural infection rate of Cx. p. pallens, caught by using light traps, between the southern and the northern parts of this city during the period from 1983 to 1989, and analyzed the data in relation to the development of public sewage systems. Furthermore, we attempted to clarify the role of some secondary vector mosquitoes in the transmission by comparing the population densities and the natural infection rates. PLACES AND METHODS Between 1983 and 1989, we collected mosquitoes including Cx. p. pallens, which is the main vector mos quito, and three secondary vectors ( Cx. tritaeniorhynchus. Ae. albopictus and Ae. togoi) , using light traps (20 watt black light) , in the southern part of Nagasaki City where the microfilaria positive rate was high and in the northern part where the positive rate was low. In 1983, we placed a trap in each of two southern districts *Department of General Education, the School of Allied Medical Sciences, Nagasaki University, 1-7-1 Sakamoto, Nagasaki 852, Japan 2Reference Center, Institute of Tropical Medicine, Nagasaki University, 1-12-4 Sakamoto, Nagasaki 852, Japan 3Sasebo City Health Center, 1-10 Yahata-machi, Sasebo 857, Japan 4Department of Bacteriology, Nagasaki University School of Medicine, 1-12-4 Sakamoto, Nagasaki 852, Japan 5Department of Medical Zoology, Faculty of Medicine, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113, Japan 6Nagasaki City Health Center, 2-34 Morimachi, Nagasaki 852, Japan 7Scientific Data Center of Atomic Bomb Disaster, Nagasaki University School of Medicine, 1-12-4 Sakamoto, Nagasaki 852, Japan