The impact of rainfall and temperature on the spatial progression of cases during the chikungunya re-emergence in Thailand in 2008-2009

Background: In 2008, chikungunya virus (CHIKV) re-emerged in Thailand aftermore than a decade of absence. Cases first appeared in the extreme southern region of the country and advanced northward approx. 300 kmover the next 18 months. The spatial advance of CHIKV cases appeared to occur at two rates, initially progressing slowly and then increasing in speed. We hypothesize that climatic variation affected the transmission of CHIKV in the country. Methods: To determine the effect of climate on CHIKV transmission, we evaluatedmodelswhere climate affects the transmission rate from mosquitoes to humans; extrinsic incubation period; fertility rate ofmosquitoes; and the mortality rate of mosquito larvae. We compared these models to models that did not include climate effects. Results: The inclusion of climate data greatly improvedmodel fit withmodels assuming climate affected the fertility rate of mosquitoes providing the best fit to data. Conclusion: These results suggest that climatic variation contributed to the slower rate of incidence observed inMarch 2009. Overall, a gradient in transmission probability and mortality and fertility rates of mosquito is observed over the entire area with the most southern districts experiencing the most efficient transmission.