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United States Department of Agriculture

Agricultural Research Service


item Chretien, J.
item Bedno, S.
item Anyamba, A.
item Breiman, R.
item Njuguna, C.
item Sang, R.
item Sergon, K.
item Powers, A.
item Onyango, C.
item Small, J.
item Linthicum, Kenneth - Ken
item Tucker, C.

Submitted to: Emerging Infectious Diseases
Publication Type: Abstract only
Publication Acceptance Date: 3/19/2006
Publication Date: 4/28/2006
Citation: Chretien, J., Bedno, S., Anyamba, A., Breiman, R., Njuguna, C., Sang, R., Sergon, K., Powers, A., Onyango, C., Small, J., Linthicum, K., Tucker, C. 2006. Eco-climatic precursors to large chikungunya outbreaks in Kenya and Comoros, 2004-2005 [abstract]. International Conference on Emerging Infectious Diseases 2006, March 19-22, 2006, Atlanta, Georgia.

Interpretive Summary:

Technical Abstract: Background Chikungunya (CHIK) virus, transmitted by Aedes mosquitoes in Africa and Asia, causes dengue fever-like illness with severe arthralgias that can persist for months. Sylvatic cycles maintain low-level endemicity throughout much of Africa, but large epidemics, such as occurred in coastal Kenya and Comoros in 2004-5, are uncommon. Eco-climatic contributions to CHIK epidemics are not well-understood. Methods We conducted epidemiologic and entomologic investigations in Lamu, Mombasa, and Comoros in June 04-March 05 following reports of febrile-arthralgic disease outbreaks. We evaluated eco-climatic precursors using satellite data, including normalized difference vegetation index (NDVI; from MODIS), rainfall (from GPCP), outgoing longwave radiation (OLR; from Terra), and sea surface temperature (SST; from MODIS). Results Epidemiologic and laboratory studies confirmed extensive CHIK outbreaks in Lamu (beginning June 04; attack rate > 50%), Mombasa (beginning November 04), and the Comoros Islands (beginning January 05; attack rate > 50% on Grand Comoros). Entomologic investigations identified Aedes aegypti as the likely primary vector in Comoros. Time series NDVI analysis for Lamu showed drought from January-June 04, with the largest negative anomaly on record (1998-) in May (-15%). Rainfall anomalies in March-May were the most negative since droughts following the 1997-8 El Nino Southern Oscillation (ENSO; average, -50 mm/day). Seasonal composite OLR anomalies were positive (10-40 W/m2) for the Kenyan coast from March 04-February 05 (peak, March-April-May 04). SST anomalies near the Kenyan coast were neutral in May, negative in June-July (-1 to -1.5°C), and neutral until late 04. Conclusions A large CHIK epidemic in Lamu and subsequent outbreaks in Mombasa and Comoros followed persistent, unusually warm and dry conditions in coastal Kenya. Such conditions may favor CHIK transmission in Kenya through decreased extrinsic incubation period of virus in the mosquito, increased vector contact with people, or other factors. The lack of known CHIK outbreaks in Kenya following prolonged, heavy 1997-8 ENSO rains, which precipitated Rift Valley fever and other epidemics, is consistent with this hypothesis.

Last Modified: 8/24/2016
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