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ABSTRACT |
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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 aegypti and Aedes albopictus-human cycles are thought to maintain CHIKV between epidemics.1
During 2005–2006, large-scale chikungunya fever epidemics 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and South-African isolates,8 and followed outbreaks along the Kenyan Coast in 2004, in Lamu (Bedno SA and others, unpublished data) then Mombasa (Breiman RF and others, unpublished data; Sang R and others, unpublished data).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 epidemic 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the population) and 13,500 (75% of the population), respectively, based on serological surveys (Breiman RF and others, unpublished data).
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Normalized Difference Vegetation Index (NDVI), a measure of green vegetation density, was derived from measurements made by the VEGETATION optical instrument on board the SPOT-4 earth observation satellite at nominal spatial resolution of 1 km.11 The data were processed into monthly composites, and expressed as anomalies, or percent departure from reference monthly means over 1998–2005.12,13 Rainfall data is based on a satellite rainfall estimation algorithm using an area-averaged 10 x 10 km window centered on outbreak location.14 Daily rainfall estimates were summed to compute monthly totals. Data are expressed as monthly anomalies with respect to reference monthly means over 1995–2005. Outgoing longwave radiation (OLR) data were derived from measurements made by the NOAA polar-orbiting satellites series, and expressed as monthly anomalies with respect to reference means over 1979–2005. Negative OLR anomalies in the tropics represent regions of precipitating clouds, whereas positive OLR anomalies are associated with dry conditions.15
Cases of non-malarial febrile illness at Lamu District Hospital increased sharply in June 2004; of 88 patients who presented with symptoms consistent with chikungunya fever in August, 56 (63%) tested positive for acute CHIKV infection by PCR or IgM capture enzyme-linked immunosorbent assay (ELISA; Bedno SA and others, unpublished data). Longitudinal NDVI measurements for Lamu showed that the drought conditions preceding the chikungunya fever outbreak were the most severe since 1998 (Figure 1B
). Rainfall estimates were among the lowest for this time of year, usually the main rainy season (Figure 1B).
OLR measurements indicated unusually dry and warm conditions throughout East Africa, but especially along coastal Kenya, in May 2004 (Figure 2A–2C). NDVI measurements also indicated severe drought conditions along the northern Kenya coast during April–June 2004 (Figure 2D–2F). As in Lamu, longitudinal NDVI and rainfall measurements were well below expected levels before the chikungunya fever outbreak in Mombasa, which occurred in November 2004 (Figure 2G).
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Though preliminary, these results suggest that drought-affected populations may be at heightened risk for chikungunya fever, and underscore the need for safe water storage (especially ensuring tight lids on reservoirs to prevent mosquito breeding and egress) during drought relief operations. Vector precautions and control measures should be maintained after chikungunya fever outbreaks because as in Asia, large epidemics may recur as population immunity declines.1 Integrated satellite-based drought monitoring and epidemiologic surveillance could identify areas at risk for chikungunya fever epidemics, and allow countries to institute timely prevention or control programs.
Received November 2, 2006. Accepted for publication November 21, 2006.
Acknowledgments: The authors thank the Kenya Ministry of Health and the staff of Lamu District Hospital for collaborating on this study.
Financial support: This study was supported by the Department of Defense Global Emerging Infections Surveillance & Response System.
Disclaimer: The opinions expressed here belong to the authors and are not necessarily the official views of the U.S. Army, the U.S. Navy, or the U.S. Department of Defense.
* Address correspondence to Jean-Paul Chretien, Department of Defense Global Emerging Infections Surveillance & Response System, Division of Preventive Medicine, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Spring, MD 20910-7500. E-mail: Jean-Paul.Chretien{at}na.amedd.army.mil. 
Authors’ addresses: Jean-Paul Chretien, Department of Defense Global Emerging Infections Surveillance & Response System, Division of Preventive Medicine, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Spring, MD 20910-7500, Telephone: 301-319-9418, Fax: 301-319-9213, E-mail: Jean-Paul.Chretien{at}na.amedd.army.mil. Assaf Anyamba and Jennifer Small, Goddard Earth Sciences & Technology Center, Goddard Space Flight Center, Bldg. 33, Rm. G109 (MC 923.0), Greenbelt, MD 20771, Telephone: 301-614-6601, Fax: 301-614-6699, E-mails: Assaf{at}ltpmail.gsfc.nasa.gov, and jsmall{at}pop900.gsfc.nasa.gov. Sheryl A. Bedno, Unit 64109 Box 401, APO AE 09831-4109, Telephone: +254-20-272-9303, Fax: +254-20-272-4592, E-mail: sbedno{at}wrp-nbo.org. Robert F. Breiman, CDC-KEMRI, Unit 64112, APO AE 09831-4112, Nairobi, Kenya, Telephone: +254-20-271-3008 x166, Fax: +254-20-271-4745, E-mail: RBreiman{at}ke.cdc.gov. Rosemary Sang, KEMRI, P.O. Box 54840,00200, Nairobi, Kenya, Telephone: +254-20-722541, Fax: +254-20-720030, E-mail: Rsang{at}kemri.org. Kibet Sergon, CDC-KEMRI, Unit 64112, APO AE 09831-4112, Nairobi, Kenya, Telephone: +254-20-271-3008, Fax: +254-20-271-4745, E-mail: kibetsergon{at}yahoo.com. Ann M. Powers, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, Foothills Campus / Rampart Road, Telephone: 970-266-3535, Fax: 970-266-3599, E-mail: akp7{at}cdc.gov. Clayton O. Onyango, MRC-Fajara Laboratories, P.O Box 273 Banjul, The Gambia, West Africa, Telephone: +220-449-5442/6 Ext 345, Fax: +220-449-5919, E-mail: conyango{at}mrc.gm. Compton J. Tucker, Code 614.0, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, Telephone: 301-614-6644, Fax: 301-614-6695/9, E-mail: compton{at}ltpmailx.gsfc.nasa.gov. Kenneth J. Linthicum, USDA-Center for Medical, Agricultural & Veterinary Entomology, 1600/1700 S.W. 23rd Drive, Gainesville, Florida 32608, Telephone: 352-374-5700, Fax: 352-374-5850, E-mail: klinthicum{at}gainesville.usda.ufl.edu.
Reprint requests: Jean-Paul Chretien, Department of Defense Global Emerging Infections Surveillance & Response System, Division of Preventive Medicine, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Spring, MD 20910-7500, E-mail: Jean-Paul.Chretien{at}na.amedd.army.mil.
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