Contour mapping of the temporal-spatial progression of a contagious disease

A methodology for contour-map study of contagious-disease epidemics is presented. Its application is exemplified in a smallpox epidemic occurring in a small Brazilian town. Computer-controlled contour-mapping of dates of introduction of variola minor into 169 households and the coordinates of the affected dwellings did not show a single contour pattern, but a group of subareal patterns of within-household outbreaks. Introduction by adults and pre-school children were distributed throughout the whole city area. However, introduction by school children formed two groups of contours and of affected dwellings. Each group was included in a discrete area corresponding to the zone of pupil recruitment of the two schools enrolling 91% of the school-child introductory-cases. The latter were responsible for introduction of the disease into 45% of the city's affected households. Altogether, both zones practically covered the whole city area. In either zone, several patterns surrounded the corresponding school. Even though no time value was entered for any school, contour maps clearly evidenced the influence of those two schools on spread of the epidemic. An estimated rate of linear spread of variola minor was 1.35 m per day in a city subarea.