Dengue epidemics on the Pacific Coast of Mexico.
Dengue epidemics in three Mexican cities were investigated with retrospective surveys and virus isolations from acute cases. These epidemics were part of the continuing extension of dengue in Mexico since 1978 after 15 years without transmission. Serotype 1 dengue infection predominated in all epidemics, but in one city, type 2 strains were also isolated. The following findings were consistent in all three cities: 1) illness history provided evidence of presence of infection months before the epidemic became evident, 2) there was a very sharp and progressive increase of dengue illness attack rates by age, 3) there were no interpretable relationships between illness and the presence of different types of breeding sights in the home, 4) socioeconomic status was strongly negatively related to illness, and 5) geographical factors not related to the other measured variables had a strong effect on illness rates reflecting the focality of transmission. In addition, there was evidence of a strong protective effect against illness by the use of mosquito netting or the presence of screens on houses, but these relationships were not universal to all three cities.