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HIGH SEASONAL VARIATION IN ENTOMOLOGIC INOCULATION RATES IN ERITREA, A SEMI-ARID REGION OF UNSTABLE MALARIA IN AFRICA

Entomologic studies were conducted in eight villages to investigate the patterns of malaria transmission in different ecologic zones in Eritrea. Mosquito collections were conducted for 24 months between September 1999 and January 2002. The biting rates of Anopheles arabiensis were highly seasonal, with activity concentrated in the wet season between June and October in the highlands and western lowlands, and between December and March in the coastal region. The biting rates in the western lowlands were twice as high as in the western escarpment and 20 times higher than in the coastal region. Sporozoite rates were not significantly different among villages. The risk of infection ranged from zero on the coast to 70.6 infective bites per year in the western lowlands. The number of days it would take for an individual to receive an infective bite from an infected An. arabiensis was variable among villages (range = 2.8–203.1 days). The data revealed the presence of only one main malaria transmission period between July and October for the highlands and western lowlands. Peak inoculation rates were recorded in August and September (range = 0.29–43.6 infective bits/person/month) at all sites over the two-year period. The annual entomologic inoculation rates (EIRs) varied greatly depending on year. The EIR profiles indicated that the risk of exposure to infected mosquitoes is highly heterogeneous and seasonal, with high inoculation rates during the rainy season, and with little or no transmission during the dry season. This study demonstrates the need to generate spatial and temporal data on transmission intensity on smaller scales to guide targeted control of malaria operations in semi-arid regions. Furthermore, EIR estimates derived in the present study provide a means of quantifying levels of exposure to infected mosquitoes in different regions of the country and could be important for evaluating the efficacy of vector control measures, since Eritrea has made significant steps in reducing the burden of malaria based on the Roll Back Malaria initiative of the World Health Organization.

Received July 7, 2003. Accepted for publication July 30, 2003.

Acknowledgments: We are grateful to the zonal staff of the National Malaria Control Program for conducting the field surveys, and thank David Sintasath, Fesahaye Seulu, and G. E. Asmelash for their valuable technical support. The generous support of Saleh Meky (Minister of Health), Berhane G Tensae (DG Health Services), Dr. Andom Ogbamariam (Director of Communicable Diseases Control), and Linda Lou Kelley (Chief, Health Strengthening Office, United States Agency for International Development [USAID]/Asmara) is acknowledged. This work was performed by the Eritrea National Malaria Control Program to improve the knowledge base for making malaria vector control decisions.

Financial support: This study was supported by the USAID through a mechanism for cooperation with the Ministry of Health, Eritrea, and with technical support from the USAID Environmental Health Project.

Authors’ addresses: Josephat Shililu, National Malaria Control Program, Division of Communicable Diseases Control, Ministry of Health, PO Box 212, Asmara, Eritrea and International Centre of Insect Physiology and Ecology, PO Box 30772, Nairobi, Kenya. Tewolde Ghebremeskel, Solomon Mengistu, Helen Fekadu, and Mehari, Zerom, National Malaria Control Program, Division of Communicable Diseases Control, Ministry of Health, PO Box 212 Asmara, Eritrea, Telephone: 291-1-125529, Fax: 291-1-125835. Charles Mbogo and John Githure, International Centre of Insect Physiology and Ecology, PO Box 30772, Nairobi, Kenya, Telephone: 254-2-861680, Fax: 254-2-860110. Robert Novak, Illinois Natural History Survey, 607 E. Peabody, Champaign, IL 61820, Telephone: 217-333-1186, Fax: 217-333-2359. Eugene Brantly, Environmental Health Project, 1611 N. Kent Street, Suite 300, Arlington, VA 22209-2111, Telephone: 703-247-8730, Fax: 703-243-9004. John C. Beier, Department of Epidemiology and Public Health, University of Miami, School of Medicine, Highland Professional Building 1801 NW 9th Avenue, Suite 300 (D-93), Miami, FL 33136, Telephone: 305-243-2977, Fax: 305-243-3324.

Reprint requests: Josephat Shililu, National Malaria Control Program, Division of Communicable Diseases Control, Ministry of Health, PO Box 212, Asmara, Eritrea, Telephone: 291-1-125-529, Fax: 291-1-122-899 or 291-1-125835; E-mail: shililu{at}gemel.com.er.

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