Spatial and temporal dynamics of Dengue Hemorrhagic Fever Epidemics (Nakhon Pathom province, Thailand, 1997-2001) Wutjanun Muttitanon1, 2,5*, Pongpan Kongthong1 , Chusak Kongkanon1, Sutee Yoksan2, 4, Jean Paul Gonzalez2, 3 and Philippe Babazan2, 3 1Department of Geography, Faculty of Education, Ramkhamhaeng University, Bangkok 10110, Thailand 2RCEVD - Center for Vaccine Development, Institute of Science and Technology for Research and Development, Mahidol University, Nakhon Pathom 73170, Thailand 3 Institute de Recherche pour le Developpement (IRD)Ur034, France. 4CVD, Mahidol University, Nakhon Pathom 73170, Thailand 5AIT – Asian Institute of Technologies, Pathumthani 12120, Thailand. Email: nungeog_ait@yahoo.com Acknowledgement This publication was supported by Institut de Recherche pour le Developpement (IRD)Ur034, France, and by fellowship Center for Vaccine Development, Institute of Science and Technology for Research and Development, Mahidol University, Thailand. We thank Dr. Sutee Yoksan for instituting the reporting system and helping with the analysis of the data. Abstract Several environmental factors modulate the distribution of dengue fever (DF) such as rainfall when the transmission peaks occur during the higher rainfall months, temperatures that reduce the mosquito extrinsic cycle duration, the human population density in urban areas, and the vector density. In order to take into account which variable. A Geographic Information Systems (GIS) has been build to create links between georeferenced data including medical records, socio-economic data, and environmental data. When applied to an epidemic-retrospective analytical study of DHF epidemics in Nakhon Pathom province (1997-2001), the GIS allowed a mapping of spatial variations of DHF incidence, the recognition of different temporal incidence patterns and the quantification of the spread of the diseases between defined spatial units. It showed that the diffusion process of these epidemics was of a contagious type as the distance between epidemics zones was significant lower than the average distance between one sub district to every sub-districts. This result indicates that these epidemics were likely to be due to the spread of a new or rare virus serotype in areas with a limited immune protection against the dengue virus for serotype differential level of specific herd immunity. 20-epidemic months was defined from 56 months from 1997-2001. There are negative correlation between the density of population and the DHF incidence rate. The average distance of epidemic sub district are not significantly smaller than the whole average distance, there are the global distribution. There are contagious diffusion , the probability of this emergence significantly decreasing with the distance from formerly epidemic sub districts. 1. Introduction Dengue hemorrhagic fever (DHF) is caused by the dengue virus, comprising 4 serotypes, belonging to the genus Flavivirus, family Flaviviridae. The principal vector of dengue viruses is the mosquito Aedes aegypti. Dengue hemorrhagic fever (DHF) is one of the most important public health problems for Thailand and many tropical countries around the world. No treatment or vaccine is available, and vector control is the only method to control the dengue. Dengue hemorrhagic fever (DHF) is a viral disease worldwide distributed among all tropical area. It is caused by the dengue (DEN) virus, (genus Flavivirus, family Flaviviridae) which present four antigenic forms or serotypes: Den-1, Den-2, Den-3 and Den-4. In Thailand DHF has been endemic since 1958, with a cumulated total of 1 369 542 DHF number of cases. Several epidemic manifestation have been observed at two to four years of intervals and remain of a main concern for the Public Health authorities (Ministry of Public Health). In most of the studied areas two or three serotypes have been found co-circulating. The main vector is the mosquito Aedes aegypti. As relevant to the virus transmission is anthropophilic, as females bite mainly human and also lay egg in man-made containers: (jars, cans, used tires...). Its short flight range, less than a kilometer., contributes also to a limited local spread of the disease. Vector control strategies are mainly based on mosquito population control by eliminating potential breeding sites. As a consequence of vector biological features and urban environmental structure, two type of virus transmission can be described: i) The contagious/continuous type, dealing with intra communities transmission, mainly dependent on the density of human population and houses, and the number of vector females likely to carry the virus from one house to another; ii) the contagious/discontinuous type, inter communities, mainly dependent on the traffic among villages and towns and the transportation of viremic hosts or infectious vectors. The knowledge of the mechanism of the DHF inter communities spread during epidemic periods is of primary interest in order to appreciate the areas at risk for high level of transmission and also to have precise view on the distance from the origin of an epidemic at which preventive control measures should be applied. Assuming the stability several years at the epidemic scale of geographical factors involved in dengue transmission (urbanization, cultural and social characteristics), the emergence of DHF epidemics in an endemic area is likely to be due either to favorable climatic conditions, or to the emergence of a new or rare virus serotype, or to a combination of the two phenomena. These variant origins of an epidemic can induce different patterns of diffusion of the disease. An epidemic due to favorable climatic factors may cover a large area leading to the possible emergence of epidemic transmission anywhere in the surveyed area (random distribution). The emergence of a new serotype is more likely to exhibit particular spatial characteristics. It may start at the place where this serotype first arrived and then propagates to places where the specific herd immunity (towards this serotype) is low enough and the density of mosquito high enough, to allow a high level of transmission. Assuming the second hypothesis the spread of a new serotype is likely to follow the main model of diffusion described for the spread of other types of moving phenomena (Hagerstrand 1952 quoted in Meade et al. 1988, pp 255). According to this model applied to the diffusion of an infectious disease, the probability for an area to be reached by the disease will depend on the distance to the formerly contaminated areas and, clusters of epidemic areas will secondary appear. To test the validity of this model, a study was developed, covering the 1997-1998 and 2000-2001 DHF epidemics in Nakhon Pathom province in order to monitor spread of significantly higher levels of incidence rate (epidemic) among sub districts. 2. Material and methods 2.1 Data Data on DHF were kindly provided by the Ministry of Public Health, Demographic data by the Administrative department, Ministry of Interior; and geographic map by the Royal Thai Survey Department.  Figure 1 Total DHF incidence in Nakhon Pathom Province; district level, recorded from January 1997 to August 2001
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