Geographical structure of dengue transmission and its determinants in Thailand

Abstract

SUMMARYExpansion of dengue has been attributed to urbanization. To test this concept, we examined dengue transmission intensities in Thailand. We used the inverse of mean age of dengue haemorrhagic fever (DHF) cases as a surrogate of dengue transmission intensity (or force of infection). The transmission intensity in Bangkok decreased rapidly since the mid-1990s, to levels that are currently lower than in other regions. Regression analysis revealed that transmission intensity is highest in the Northeastern rural region, mainly due to scarcity of private water wells. Private wells reduce the need for household water containers, the major breeding sites for vectors. Cumulatively, these results show that urbanization is not necessarily associated with intense dengue transmission in Thailand. Paradoxically, the DHF incidence in Bangkok has surpassed other regions despite declines in transmission intensity. This finding implies the existence of endemic stability (i.e. low incidence of a clinical illness in spite of high transmission intensity).

Socio-geographical factors in vulnerability to dengue in Thai villages: a spatial regression analysis

“…This study did not find an association between dengue occurrence and wells, as had been identified in a previous district-level study (Nagao et al, 2008). This is not surprising since public and private wells in Thailand are constructed with the purpose of providing water for daily life in rural areas and the small population in each village is barely sufficient to sustain transmission once an index case occurs.…”

“…In contrast to previous region-and district-level studies (Chareonsook et al, 1999;Nagao et al, 2008), the present investigation presents a village-level analysis. The association found between dengue incidence and proximity to the nearest urban areas implies that the villages surrounding cities have a higher dengue risk than those located far away from urban areas.…”

“…The role of urbanisation with regard to local vulnerability to dengue transmission in the rest of the region, however, was much less clear. Recent region-and district-level analyses (Chareonsook et al, 1999;Nagao et al, 2008) showed that the incidence of DHF and the intensity of transmission have been higher in rural regions than in Bangkok since the mid-1990s. The reasons for this apparent reversal in epidemiological pattern remain unknown.…”

Decreases in dengue transmission may act to increase the incidence of dengue hemorrhagic fever

“…Each of them models the four serotypes of dengue virus through modifications of a multistrain model first introduced by Andreasen et al (27) (see Methods). Each (58). Wavelet analyses were performed on logtransformed incidence data, by using the Morlet wavelet.…”

“…Similar to the The Breteau Index, defined as the number of positive water containers for mosquito larvae/pupae in 100 randomly sampled households (60), is known to be a relatively sensitive indicator of transmission (61). Mean age of DHF was obtained as described elsewhere (58). (b) DHF incidence rates per 100,000, averaged over 1994 -1996, plotted against the mean June Breteau Index for the 23 amphoes.…”