STUDIES ON ANNUAL CHANGES IN MICROFILARIAL PREVALENCE OF DIROFILARIA IMMITIS AMONG HOUSE DOGS FOR 27 YEARS IN NAGASAKI CITY, JAPAN

The positive rate of dogs with Dirofilaria immitis microfilariae among house dogs was examined for 27 years from 1968 to 1994 in the southern and northern parts of Nagasaki City. Roles of the number of the main vector, Culex pipiens pallens and of RPSS (rate of population utilizing a sewage system) were analyzed statistically in relation to the annual changes in microfilarial prevalence. In the southern and northern parts, there was a clear correlation among three parameters of the positive rate in the dogs, the number of Cx. p. pallens and RPSS. The present study proved that the decrease in the positive rate of dogs in both the southern and northern parts is mainly attributable to a decrease in the number of Cx. p. pallens, which resulted from the decrease of the breeding places of this mosquito following the spread of the public sewage system and the improvements in the use of roads and open roadside ditches.