Changes in the Transmission Dynamic of Chikungunya Virus in Southeastern Senegal.

In Senegal, chikungunya virus (CHIKV) is maintained in a sylvatic cycle and causes sporadic cases or small outbreaks in rural areas. However, little is known about the influence of the environment on its transmission. To address the question, 120 villages were randomly selected in the Kedougou region of southeastern Senegal. In each selected village, 10 persons by randomly selected household were sampled and tested for specific anti-CHIKV IgG antibodies by ELISA. We investigated the association of CHIKV seroprevalence with environmental variables using logistic regression analysis and the spatial correlation of village seroprevalence based on semivariogram analysis. Fifty-four percent (51%-57%) of individuals sampled during the survey tested positive for CHIKV-specific IgG. CHIKV seroprevalence was significantly higher in populations living close to forested areas (Normalized Difference Vegetation Index (NDVI), Odds Ratio (OR) = 1.90 (1.42-2.57)), and was negatively associated with population density (OR = 0.76 (0.69-0.84)). In contrast, in gold mining sites where population density was >400 people per km2, seroprevalence peaked significantly among adults (46% (27%-67%)) compared to all other individuals (20% (12%-31%)). However, traditional gold mining activities significantly modify the transmission dynamic of CHIKV, leading to a potential increase of the risk of human exposition in the region.

Chikungunya Outbreak in Kedougou, Southeastern Senegal in 2009-2010.

BACKGROUND: In Senegal, Chikungunya virus (CHIKV), which is an emerging mosquito-borne alphavirus, circulates in a sylvatic and urban/domestic cycle and has caused sporadic human cases and epidemics since 1960s. However, the real impact of the CHIKV sylvatic cycle in humans and mechanisms underlying its emergence still remains unknown. METHODOLOGY: One thousand four hundred nine suspect cases of CHIKV infection, recruited from 5 health facilities located in Kedougou region, south-eastern Senegal, between May 2009 to March 2010, together with 866 serum samples collected from schoolchildren from 4 elementary schools in May and November 2009 from Kedougou were screened for anti-CHIKV immunoglobulin (Ig)M antibodies and, when appropriate, for viral nucleic acid by real-time polymerase chain reaction (rPCR) and virus isolation. In addition, mosquitoes collected in the same area from May 2009 to January 2010 were tested for CHIKV by rPCR and by virus isolation, and 116 monkeys sera collected from March 2010 to May 2010 were tested for anti-CHIKV IgM and neutralizing antibodies. RESULTS: The main clinical manifestations of the CHIKV suspect cases were headache, myalgia, and arthralgia. Evidence for CHIKV infection was observed in 1.4% (20 of 1409) of patients among suspect cases. No significant difference was observed among age or sex groups. In addition, 25 (2.9%) students had evidence of CHIKV infection in November 2009. Chikungunya virus was detected in 42 pools of mosquitoes, mainly from Aedes furcifer, and 83% of monkeys sampled were seropositive. CONCLUSIONS: Our findings further documented that CHIKV is maintained in a sylvatic transmission cycle among monkeys and Aedes mosquitoes in Kedougou, and humans become infected by exposure to the virus in the forest.