Spatial Aspects of a Smallpox Epidemic in a Small Brazilian City

Abstract

AN EPIDEMIC is usually recognized as only a phenomenon in time, but a disease also occurs in space. Moreover, when a contagious disease is being considered, the spatial distribution of cases clearly is fundamental to characterize epidemicity and to study the mechanism of the progression (epidemic flow, stream, or path).' For a demonstration of a time pattern, there may be an optimal geographic area that would be less satisfactory, for one set of reasons, if it were smaller and, for another set of reasons, if it were larger. Conversely, there may be an optimal period of time for the demonstration of geographic concentrations of cases. Studies of contagious disease frequently omit an adequate description of the spatial distribution of cases.2 Analytical studies of the spatial component of the concept of epidemicity are few. P. Stocks and M. Karns analyzed the spread of measles during 1926 in the St. Pancras Borough of London. The borough was divided into halfmile squares in order to identify the ratio of actual to expected cases in each square with respect to the epidemic center (modal square).3 Amplitude of waves and their time to traverse a square were used to obtain a rough measure of the rate of propagation of waves. Recently, A. Gilg used an essentially similar procedure in combination with other analytical methods to study an of fowl-pest.4 John M. Hunter and Johnathan C. Young conducted cartographic and autocorrelation analyses of the influence of socioeconomic and demographic characteristics on the observed spatial patterns of Asian influenza incidence in England.5 Gerald Pyle used a comparable approach to study incidence of measles in Akron, Ohio.6 With diffusion theory as his basis, Peter Haggett proposed various models for the analysis of measles epidemics.7 Juan Angulo and Haggett with their associates employed trend-