Since the first field and laboratory investigations conducted by IPM in the 1970s, 15 arboviruses have been isolated in Madagascar, 10 of which are known to be pathogenic for humans and 3 of which (dengue virus type 1, chikungunya virus, and RVFV) have been responsible for large outbreaks (4
). The RVF outbreak in 2008–2009 is the largest detected in the country, although during this outbreak the reporting and sampling for suspected cases in humans and animals were not optimal. RVF is a rural disease occurring mainly during the rainy season. Transfer of information from the central administration to the health centers and back was challenging during this period in this low-income country. This lack of recorded information prevented us from describing the outbreak in more detail and estimating the extent of human disease. However, the extent of the disease could be established through a retrospective study in the outbreak area, comparing the crude death rate observed during the outbreak period with an expected death rate computed from data for previous years. The effect on livestock is even more difficult to quantify because of the lack of animal population data and the limited number of specimens submitted for diagnosis.
The addition of the Madagascar RVFV 2008 strains to the Kenya-1 2006–2007 lineage raises the question of this lineage’s introduction from Kenya. We detected IgG against RVF in serum from 18 of 24 goats sampled by the end of June 2008 in Toliara II district (no. 602) on the southeast coast where abortions were reported in early 2007 (J.-M.R., unpub. data). RVFV was probably circulating in this area while it was circulating in Kenya, and perhaps it was introduced from Kenya at that time. Official records document exportation (but no importation) of ruminants from Madagascar to the Comoros archipelago; subsequent exportation to the African continent is possible (L.T.R., pers. comm.). However, illegal importation cannot be excluded. The question of introduction remains unanswered. More complete phylogenetic studies, and full sequences of Madagascar 2008 isolates, are needed to detect the circulation of >1 lineages during the 2008 outbreak and to get a better understanding of the movements and evolution of Madagascar and Kenya isolates.
The nationwide cross-sectional serosurvey supplemented the information obtained during the outbreak. The serologic observations suggest that the virus has probably circulated in the past in most districts and more recently, in 2008–2009, at least in all regions of the country. The sample collection from persons at risk started 3 months and ended 12 months after the detection of the last case of the 2008 outbreak. Despite not having data on duration of IgM against RVFV in humans, we suspect that IgM may have already disappeared in some of the serum samples tested and that the area of recent RVFV circulation is indeed larger than the one we studied. The lack of evidence of virus circulation in some adjoining districts from arid southern Madagascar may be also explained by our small sample size from some of them (Appendix Table
). However, the serologic investigation conducted among cattle sampled after the 2008 RVF outbreak indicated that the virus has circulated in the following districts: Midongy-Atsimo (no. 318), Vangaindrano (no. 320), Ampanihy (no. 605), Sakaraha (no. 620), Betioky (no. 612), and Toliara-I. (no. 601) (Elisabeth Jeanmaire, unpub. data) and reduced the area of contiguous districts where the virus circulation was not detected to the following 6 districts: Iakora (no. 311), Befotaka (no. 307), Amboasary-Atsimo (no. 603), Ambovombe-Androy (no. 604), Tsihombe (no. 621), and Bekily (no. 607).
Recent circulation of RVFV in the country was extensive. The detection of the same haplotype from serum sampled at the same period, from the 2 first reported outbreak places 500 km apart may be explained by the large-scale movement of cattle within the country. Observed movements of cattle from rural areas to provincial capitals and between provincial capitals and Antananarivo, could explain the rapid spread of the virus. However, we do not know where the outbreak started; thus, reemergence of RVFV from different places cannot be ruled out. We found the results of some unpublished studies reporting the movement of ruminants in some areas, but a comprehensive study of these movements is needed for a better understanding of the epidemiology of the disease and to organize its surveillance and control.
The geographic distribution of RVF encompasses all 4 ecozones of Madagascar (www.nationalgeographic.com/wildworld/terrestrial.html
). This finding suggests that mosquito transmission may occur in all of them. Extensive entomologic studies conducted out in the 1980s in Madagascar have shown that some species described as vectors on the African continent were present in some or all 4 ecozones (19
). This finding implies that cycles of transmission involving different species may occur in Madagascar. Until now, little information on RVFV vectors in Madagascar has been available (19
). Thousands of mosquitoes were collected in the highlands during the 2008–2009 outbreaks. The results of the virus detection are still pending and will contribute to the knowledge of the RVFV vectors in Madagascar.
The model used to predict at risk RVF situation has been efficient in the Horn of Africa (3
). When this model was applied to Madagascar, the area where probable and confirmed cases were reported was not predicted to be at risk (24
), suggesting that the model needs to be adjusted for Madagascar. This last point and the questions raised above underline the need for research studies and surveillance on RVF in Madagascar to better predict, declare, and respond to RVF outbreaks.