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In high-elevation areas in western Kenya, the abundance of Anopheles arabiensis is either very low or absent. The western Kenya highlands (an area with an elevation > 1,500m above sea level) have also been experiencing extensive deforestation, and deforestation has been suggested as one of the important factors that facilitate malaria transmission in the highlands. This study investigated whether climate conditions in the western Kenya highlands (Kakamega, elevation 1,500 m above sea level) were permissive to the development and survival of An. arabiensis and whether deforestation promoted An. arabiensis survivorship of immature and adult stages, using life-table analysis. We found that in larval habitats located in forested areas, only 4–9% of first-instar larvae developed into adults and the development length exceeded 20 days. Mean water temperature of aquatic habitats in the deforested area was 4.8–6.1°C higher than that in the forested area, larval-to-adult survivorship was increased to 65–82%, and larval-to-adult development time was shortened by 8–9 days. The average indoor temperature in houses in the deforested area was 1.7–1.8°C higher than in the forested area, and the relative humidity was 22–25% lower. The median survival time of adult mosquitoes in the deforested area was 49–55% higher than those in the forested area. The net reproductive rate of female mosquitoes in the deforested area was 1.7- to 2.6-fold higher than that in the forested area. Compared with previously published data on An. gambiae, the net reproductive rate of An. arabiensis was only 0.8–1.3% of Anopheles gambiae in the forested area and 2.3–2.6% in the deforested area. Therefore, the current ambient climate condition is less permissive to An. arabiensis than to An. gambiae in western Kenya highlands. However, environmental changes such as deforestation and global warming may facilitate the establishment of An. arabiensis populations in the highlands.
Received September 26, 2006. Accepted for publication June 5, 2007.
Acknowledgments: This study is published with the permission of the Director, Kenya Medical Research Institute. The authors thank Robin Oriango, Paul Omondi, and Nicholas Juma for technical help, and Cynthia Spurgeon and two anonymous reviewers for valuable comments on the manuscript.
Financial support: This work was supported by grants from the National Institutes of Health (NIH): D43 TW01505 and R01 A150243.
* Address correspondence to Yaw A. Afrane, Climate and Human Health Research Unit, Centre for Vector Biology and Control Research, Kenya Medical Research Institute, P.O. Box 1578, Kisumu, 40100 Kenya. E-mail: yafrane{at}kisian.mimcom.net, yaw_afrane{at}yahoo.com
Authors’ addresses: Yaw A. Afrane and Andrew K. Githeko, Climate and Human Health Research Unit, Centre for Vector Biology and Control Research, Kenya Medical Research Institute, Kenya, Telephone: +254 720–805738, Fax: +254 57–2022981, E-mail: yafrane{at}kisian.mimcom.net, yaw_afrane{at}yahoo.com. Goufa Zhou and Guiyun Yan, Program in Public Health, College of Health Sciences, University of California at Irvine, Irvine, CA 92697–4050. Bernard W. Lawson, Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana.
Reprint requests: Dr. Guiyun Yan, Program in Public Health, College of Health Sciences, University of California at Irvine, Irvine, CA 92697–4050, E-mail: guiyuny{at}uci.edu.
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