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Spatial and temporal dynamics of Dengue Hemorrhagic Fever Epidemics (Nakhon Pathom province, Thailand, 1997-2001)
4.3 Validity of the epidemic pattern as a representation of the spread of the disease
The method to identify epidemic sub districts allows to identify an “epidemic” pattern, in any sub district, whatever is its density of population as the distribution of epidemic sub district was not correlated to the density of population (Pearson’s correlation = -0.24, P = 0.71). Moreover, we used the incidence rate for 100 000 inhabitants to reduce the bias from the size of the population in every the sub districts.
Duration of the epidemic in a sub district : the definition of an epidemic month in provinces is a minima definition : i.e. a province where incidence is statistically higher than expected, is supposed to face and abnormal (epidemic) phenomena. During that period of time the sub districts of this province where the incidence is statistically higher than in the other ones are supposed to be responsible of this abnormal phenomena.
4.4 Clusters
The study describes the emergence of clusters of cases in sub districts during specific periods of time (epidemic month in the province) and their monthly spread over the province.
Clusters of epidemic sub districts maybe due to a geographic heterogeneity (density of urbanization, road network). Meanwhile, considering the whole studied period, the relative average distance between every sub districts having been epidemic at least during one month (more than 67% of the sub districts: average distance of 77 sub district are 28.36 km.) was not significantly smaller than the global average distance between every sub districts (average distance of 106 sub districts are 21.83 km.); meaning that their global distribution is not related to geographical factors.
The spread of epidemic sub districts follows the Hagerstrand’s model that has been used to describe many types of phenomena, such as the waves of innovation. Lost their energy with distance from the source of the innovation (Gould P.G. 1969) or the spread of new ideas (Hagerstrand 1952). In public health research it has been applied to the infectious influenza (Cliff et al 1986). Applied to the DHF epidemic in Nakhon Pathom, it means that during the epidemic periods each epidemic sub districts is the origin of the emergence of an epidemic in other sub districts during the next month, the probability of this emergence significantly decreasing with the distance from formerly epidemic sub districts. This model is the contagious type and maybe opposed to the random or homogenous model. In these two models occurrence of epidemic is due to an overall phenomena (for example and increase in temperature) and could be observed in any sub districts. In this case, the distribution of epidemic sub districts would not show any significant trend.
In the figure 2 observed distances are smaller than expected ones, but exhibit similar monthly variations. This is mainly because of the border effect, the propagation in neighboring provinces being not taken in account. During the months where epidemic sub districts are located on the periphery of the province, the average distance to other sub districts is larger than during the months where epidemic sub districts are located near the center of the province as directly neighboring sub districts (epidemic of not) are comparatively less numerous.
4.5 Origin of epidemics
The initial date and place of the emergence of the epidemic in the province cannot be identified, as the incidence progressively increases from the endemic pattern to the epidemic one. Meanwhile, the contagious distribution and spread of epidemic sub districts strongly suggests that the epidemic due to the emergence of a new or rare serotype in the epidemic sub districts.
DHF is endemic in Thailand and the different serotypes are largely distributed, at least two or three are generally found at the same time. Moreover, the information on serotypes is rare and limited. Therefore, the emergence of a DHF epidemic cannot be measured by using only the occurrence of the dengue infection or of a specific serotype, and indirect methods are necessary, such as the identification of epidemic months used here.
Inside human community the spread of DHF viruses from one house to neighboring houses maybe due to displacement of vectors or of hosts. On another hand, because of the relatively short range of flight of vectors females the spread of viruses among communities separated by several km cannot be due to the active dispersal of mosquitoes. Moreover, despite the transport of mosquito by cars has been described (Kuno 1995) , infected hosts are more like to be at the origin of the spread of viruses among communities.
According to Hagerstrand’s model, the probability to be reached by a new virus is inversely correlated to the distance between communities and positively correlated to the intensity of the communication between people, density of traffic, and road network. The presence of sufficient densities of vectors in destination communities is also necessary to allow the transmission of the virus after it has been imported.
Areas not used by human to travel or limiting their displacements such as mountains, sea, boundaries act as barrier. This approach of the displacement of epidemics is likely to contribute to the delineation of areas at risk during epidemics, and to allow to public health to focus vector control activities in selected areas.
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