Spatial and temporal dynamics of Dengue Hemorrhagic Fever Epidemics (Nakhon Pathom province, Thailand, 1997-2001) 2.2 Study area Nakhon Pathom province, is located in the central part of Thailand encompassing the latitude of 130 38’ 45.6” to 140 10’ 37.2” and the longitude of 990 51’ 10.8” to 1000 17’ 6”. It covers 2164.35 km2 and has a population of 774,276 inhabitants. There are seven districts and 106 sub districts, where the population density ranges from 316 to 630 inhabitants / sq km. The province health department has reported 14,079 DHF cases during 1983 - 1992-2001, and during the last ten years, two DHF epidemics have broken out, in 1997-1998 and 2000-2001. 2.3. Method of analysis The study aimed to describe the spatio-temporal dynamics of event, or it is for a sub district reaching a significantly higher than expected (epidemic) level of transmission of DHF. An epidemic in a sub district is defined as a monthly period of time when the whole province has a number of cases significantly higher than expected and, a sub-district exhibits a significantly higher number of cases than the other ones. An epidemic month has been defined as a month with a significantly higher number of cases during which the incidence is greater than expected value to the average plus one standard deviation of the monthly incidence specific of a glen month (i.e. January, February,…) observed from 1983 to 2001 (Barbazan P. et al 2001). During an epidemic month, an epidemic sub-district is a sub-district where the incidence rate (for 100 000 inhabitants) is superior to the average incidence plus one standard deviation observed among every sub-districts during that month. In a contagious model applied to infectious diseases, the spatial entities closer to an infective one are more at risk. In the spread of the epidemic phenomena among epidemic sub-districts, the observed distance between sub-districts reached each month by the epidemic, meaning facing an epidemic level of transmission is likely be smaller than the average expected distance between the whole sub-districts. To calculate these distances we used the euclidian distance between the centroid of sub districts.
H1 = observed distance < expected distance The Z test is used to compare the average distances.  Figure 2 Observed and expected distances correlating between each epidemic sub districts during one and sub district epidemic during the next month The method is applied to the study of two phenomena, 1) the occurrence epidemic sub district of cluster during one month and, 2) the spread of the epidemic from one month to the next one. A Cluster is a geographically bounded group of occurrences of sufficient size and concentration to be unlike to have occurred by chance(Knox 1989 quoted in Alexander and J.Cuzick 1997). Clusters identification: comparison of observed distance between epidemic sub-districts and expected distance between any sub-districts, during one epidemic month. The spread of epidemic of the diffusion of: the observed distance between epidemic sub-districts during one epidemic month and epidemic sub-districts during the next epidemic month is compared to the expected distance between epidemic sub-districts during one epidemic month and every sub-districts during the next epidemic month. The distance, at which the epidemic can spread from one month to the next one, was estimated using a 5 km buffer created around each epidemic sub district, 1 = less than 5 km.; 2 = 5 - 10 km.; 3 = 10 - 15 km.; 4 = 15 - 20 km. and 5 = more than 20 km. For each monthly epidemic sub district, every epidemic sub district observed during the next month fall into the five classes according to its distance to the last epidemic sub districts. 3. Result 3.1 DHF Incidence The incidence rate of DHF ranged from 76 to 2 835 cases per 100 000 inhabitants. 3.2. Epidemic months Twenty epidemic month have been identified from January 1997 to August 2001 (Table 1); the number of epidemic months in one sub district ranged from 0 month (in 29 sub districts) to 11 months (Figure 3). Table 1 Epidemic months chorological distribution from January,1997 to August,2001 (Nakhon Pathom, Thailand)
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