Spatial and temporal dynamics of Dengue Hemorrhagic Fever Epidemics (Nakhon Pathom province, Thailand, 1997-2001)  Figure 3 DHF average incidence rate per 100, 000 inhabitants per sub district and number of epidemic-month from January 1997 to August 2001. 3.3 Epidemic sub districts During the first epidemic (1997-1998) a total of 49 epidemic sub districts have been identified, 61 during the second epidemic (2000-2001) and 31 during the two epidemics. The probability for one sub district epidemic during the first outbreak to be epidemic during the second epidemic is not significantly different of a random distribution. 3.4 Clusters In the comparison of observed and expected distance during one month the epidemic periods, the average observed distances were significant lower than the average expected distance (according to the H1 hypothesis), significant level 0.95. In 78.67% of the distances. 3.5 Spread The number of epidemic sub district during among the decrease with the distance to formerly epidemic sub districts distance during continuous epidemics months (Figure 4). Relative Frequency = Frequency for an epidemic sub district / frequency for all sub districts (The method used is strong enough or as looking at the map it appeared that during several more than one area (group of sub districts) was epidemic S+)  Figure 4 Comparison of DHF cases observed frequency of the distribution of classes of distance between every sub districts between epidemic during one month and every sub district epidemic during the next month VS the frequency of distance of class of distance between every sub districts. Relative Frequency (RF) = Frequency for an epidemic sub district / frequency for all sub districts 4. Discussion 4.1 Epidemic months The method used for the definition of epidemic month in province has been described (Barbazan P. et al, 2001). It allows to precisely frame epidemics, defined as periods during which the incidence is significantly over the observed average. 4.2 Choice of sub district as a spatial unit for this study But this method to identify epidemic months at the province scale could not be used directly at the sub districts because these data are available over a short period of time and because of the great variance of results, due to the very low values of incidence often recorded (during the study period a null monthly incidence was reported in 66% of the months / sub districts) followed by epidemic periods. The average surface of sub district is 21.83 km2 (standard deviation = 11.51). Sub districts comprise 3 to 24 villages, but villages cannot be used as spatial unit as many students and pupils do not live in their village and the address of patients is often only consistent at the sub districts scale. We considered in this study that displacements inside the sub district are sufficient to allow a mixity of the population and to consider the sub district as a homogenous unit towards DHF transmission. |