Drought-associated chikungunya emergence along coastal east Africa

Epidemics of chikungunya fever, an Aedes spp.-borne viral disease, affected hundreds of thousands of people in western Indian Ocean islands and India during 2005-2006. The initial outbreaks occurred in coastal Kenya (Lamu, then Mombasa) in 2004. We investigated eco-climatic conditions associated with chikungunya fever emergence along coastal Kenya using epidemiologic investigations and satellite data. Unusually dry, warm conditions preceded the outbreaks, including the driest since 1998 for some of the coastal regions. Infrequent replenishment of domestic water stores and elevated temperatures may have facilitated Chikungunya virus transmission. These results suggest that drought-affected populations may be at heightened risk for chikungunya fever, and underscore the need for safe water storage during drought relief operations. Transmitted by Aedes spp. mosquitoes, Chikungunya virus (CHIKV) causes febrile illness with joint pain. Illness is rarely fatal but can be severe and prolonged, and lead to other complications. Sylvan transmission cycles involving forest- dwelling Aedes spp. and wild primates maintain endemicity throughout tropical Africa. In tropical Asia, urban Aedes ae- gypti and Aedes albopictus-human cycles are thought to main- tain CHIKV between epidemics. 1 During 2005-2006, large-scale chikungunya fever epidem- ics affected western Indian Ocean Islands and India (Figure 1A). 2-7 The outbreak strain from the western Indian Ocean Islands was related to previous East-, Central-, and South- African isolates, 8 and followed outbreaks along the Kenyan Coast in 2004, in Lamu (Bedno SA and others, unpublished data) then Mombasa (Breiman RF and others, unpublished data; Sang R and others, unpublished data). The total number of cases across the outbreaks during 2004-2006 is unknown, but they constitute, by far, the largest chikungunya fever epi- demic on record. In La Reunion alone, the World Health Organization estimated that more than 200,000 people had been infected. 2 Estimated infections in Comoros and Lamu were 215,000 (63% of the population) and 13,500 (75% of the population), respectively, based on serological surveys (Breiman RF and others, unpublished data). To explore eco-climatic contributions to epidemic chikun- gunya fever emergence in Kenya, we obtained satellite data through an early warning system developed for Rift Valley fever. 9 This is part of the US Department of Defense Global Emerging Infections Surveillance and Response System (DoD-GEIS), 10 which participated in the Lamu field investi- gations with the Kenya Ministry of Health and the US Cen- ters for Disease Control and Prevention. Normalized Difference Vegetation Index (NDVI), a mea- sure of green vegetation density, was derived from measure-