Abstract
The positive rate of Dirofilaria immitis infection was studied in the eastern, western, southern and northern parts of Nagasaki City 3 times between 1968 and 1983. Chronological changes in the positive rate of dogs for microfilariae in these 4 parts and the roles of epidemic factors in the changes of the positive rate were evaluated. The positive rate decreased in the northern part, where houses and buildings increased annually, whereas the number of dog owners and the density of dogs decreased. Improvements in the drainage such as side ditches, which are primary breeding sites of vector mosquitoes (Culex p. pallens), are assumed to have led to a decrease in the mosquito density, resulting in a decrease in the positive rate of dogs. However, the positive rate increased in the eastern, western, and southern parts. The density of dogs decreased also in these parts, but it remained higher than in the northern part as the number of houses was smaller, and more people kept dogs. Also, as the sewage system did not seem to be so well developed as in the northern part, more mosquitoes might be present. The high positive rates in these parts may be ascribed to these conditions.
