Rift Valley fever during rainy seasons, Madagascar, 2008 and 2009

Abstract

During 2 successive rainy seasons, January 2008 through May 2008 and November 2008 through March 2009, Rift Valley fever virus (RVFV) caused outbreaks in Madagascar. Human and animal infections were confirmed on the northern and southern coasts and in the central highlands. Analysis of partial sequences from RVFV strains showed that all were similar to the strains circulating in Kenya during 2006-2007. A national cross-sectional serologic survey among slaughterhouse workers at high risk showed that RVFV circulation during the 2008 outbreaks included all of the Malagasy regions and that the virus has circulated in at least 92 of Madagascar's 111 districts. To better predict and respond to RVF outbreaks in Madagascar, further epidemiologic studies are needed, such as RVFV complete genome analysis, ruminant movement mapping, and surveillance implementation.

Figure 1

Distribution of Rift Valley fever in the 111 administrative districts in Madagascar, 2008 and 2009. 
Districts with laboratory-diagnosed confirmed or probable cases in humans and/or animals are indicated by yellow (2008), orange (2009), or red (both years). In districts without confirmed or probable cases, antibody data for Rift Valley fever virus immunoglobulin (Ig) levels in serum samples from at-risk professionals are indicated by green (IgM positive only), blue (IgG positive, IgM negative), or violet (IgG and IgM negative). No samples were received from districts shown in white.

Figure 2

Phylogenetic tree based on the entire small sequences from 33 Rift Valley fever virus strains described by Bird et al. (16), from 6 sequences representative of the Kenya 1, 1a, and 2 lineages described by Bird et al. (18), and from 1 Madagascar strain circulating in 1991 and 12 Madagascar strains circulating in 2008. Boostrap percentages (from 1,000 resamplings) are indicated at each node. ◆ indicates sequences from the 2006–2007 Kenya-1 lineage, ● indicates sequences from the 2006–2007 Kenya-1a lineage, ▲ indicates sequences from the 2006–2007 Kenya 2 lineage, and ◇ indicates the 2008 Madagascar sequences. Scale bar indicates nucleotide substitutions per site.