Amplification of the Sylvatic Cycle of Dengue Virus Type 2, Senegal, 1999-2000: Entomologic Findings and Epidemiologic Considerations

Mawlouth Diallo, Yamar Ba, Amadou A. Sall, Ousmane M. Diop, Jacques A. Ndione, Mireille Mondo, Lang Girault, Christian Mathiot


Emerging Infectious Diseases. 2003;9(3) 

In This Article

Abstract and Introduction

After 8 years of silence, dengue virus serotype 2 (DENV-2) reemerged in southeastern Senegal in 1999. Sixty-four DENV-2 strains were isolated in 1999 and 9 strains in 2000 from mosquitoes captured in the forest gallery and surrounding villages. Isolates were obtained from previously described vectors, Aedes furcifer, Ae. taylori, Ae. luteocephalus, and-for the first time in Senegal-from Ae. aegypti and Ae. vittatus. A retrospective analysis of sylvatic DENV-2 outbreaks in Senegal during the last 28 years of entomologic investigations shows that amplifications are periodic, with intervening, silent intervals of 5-8 years. No correlation was found between sylvatic DENV-2 emergence and rainfall amount. For sylvatic DENV-2 vectors, rainfall seems to particularly affect virus amplification that occurs at the end of the rainy season, from October to November. Data obtained from investigation of preimaginal (i.e., nonadult) mosquitoes suggest a secondary transmission cycle involving mosquitoes other than those identified previously as vectors.

Dengue is a viral disease transmitted by mosquitoes and caused by four viral serotypes (dengue virus serotypes 1-4 [DENV 1-4]) belonging to the genus Flavivirus of the Flaviviridae family. In terms of illness and death, dengue is the most important viral disease transmitted to humans by mosquitoes.[1] This virus is distributed nearly worldwide and represents a serious public health problem in Southeast Asia, the Caribbean, the Pacific Islands, and Latin America. While in Asia and the Americas human-to-human transmission by mosquitoes is the current form of virus circulation, in West Africa sylvatic circulation is predominant. In Africa, the existence of dengue dates back to 1956,when a retrospective serosurvey confirmed that a dengue epidemic occurred in Durban, South Africa, in 1926-1927.[2] Additional evidence was obtained in the 1960s, when DENV-1 and DENV-2 were isolated for the first time from human samples in Nigeria.[3]

In Senegal, evidence of dengue virus circulation was obtained when DENV-2 was isolated for the first time in 1970 from human blood.[4] After this isolation, an entomologic surveillance program was undertaken in Senegal. By that time, in light of the yellow fever virus transmission cycle established by Haddow,[5] much interest was being shown in sylvatic cycles of arboviruses in general. The main objective of this program was to identify the vectors and describe the sylvatic cycle of dengue virus transmission. Results obtained from these studies identified several DENV-2 epizootics through periodic amplification of the sylvatic cycle in Kedougou, Senegal.[6,7,8] Although DENV 1-4 were isolated incidentally in Senegal from humans, only DENV-2 was shown to be circulating regularly in mosquito, human, and monkey populations with a sylvatic focus in Kedougou.[9,10]

After 8 years of silence, DENV-2 reemerged in 1999. In this paper we analyze data obtained during this 1999-2000 epizootic in light of climatic changes and vector ecology. We also discuss new insights in terms of the maintenance and emergence mechanisms of DENV-2.