Genomic findings from this study revealed that the viruses circulating in Madagascar in the late 1970s did not give rise to the 1991 or 2008 Malagasy RVFV outbreaks, eliminating the role of this first isolate in enzootic maintenance and the successive outbreaks. Three distinct importation events, from the East African mainland to Madagascar, appear likely: one in the early 1970s, another in the mid- to late 1980s, and the most recent between 2003 and 2008. In all three instances, considerable RVFV activity was detected on the African mainland prior to the detection of these viruses in Madagascar. This pattern is particularly evident with the large sweeping outbreak in Egypt (1978 to 1979) prior to the Madagascar 1979 virus isolation and the extensive East African outbreak (2006 to 2007) that preceded the 2008–2009 activity in Madagascar.
The timing of the events associated with the recent outbreak in Madagascar also supports RVFV importation from East Africa, possibly even during the 2006–2007 outbreak. Although most cases were reported from January through April 2008 (1
), epidemiologic evidence has linked the 2008–2009 Malagasy outbreak to that occurring on the mainland a few years earlier. Specifically, livestock abortion was observed in one Malagasy district (Toliara II) beginning in early 2007, and a retrospective investigation revealed that RVFV had been circulating in the livestock population at least since December 2007 (1
). The same retrospective investigation demonstrated widespread exposure to RVFV among Malagasy slaughterhouse workers since at least 2007, as evidenced by the presence of antibodies in serum samples (1
). The combination of the genetic and serological data would be consistent with the view that in Madagascar, large outbreaks of RVF are associated with viruses introduced from the mainland, but the virus activity likely ramps up over the preceding year or two and likely persists at decreasing levels for a year or two following the large outbreak. Unfortunately, disease surveillance is insufficiently sensitive to detect these pre- and postoutbreak periods of virus activity. Increased surveillance and genetic characterization of interepizootic virus isolates would shed light on this issue. RVFV activity during an interepizootic/interepidemic period is not surprising, especially in the years immediately preceding (3
) or following (31
) an outbreak. Even when the virus fails to become established in the long term, it is likely that there is a temporary enzootic state with short-term circulation for a couple of years surrounding a RVFV outbreak.
Similar patterns of disease spread have been observed in other areas. For instance, in Mayotte, RVFV cases in humans and cattle in 2007 and 2008 also appeared to be an expansion of the East African outbreak (27
). In addition, outbreaks of other pathogens, including the 2003–2004 East Coast fever outbreak in Comoros and the 2005–2006 Chikungunya virus outbreak in the islands of Mayotte, Réunion, Seychelles, Mauritius, and Madagascar are thought to have originated on the East African mainland, specifically in Kenya and Tanzania (8
). These cases provide additional illustrations of the potential for infectious agents to be imported from coastal countries in Africa to islands located off the African continent and highlight the risk of subsequent establishment and circulation in a new environment. This is especially important given that replication-competent vectors of several viruses can already be found in these areas (12
While infected mosquitoes have certainly contributed to the widespread dispersal of RVFV, the role of mosquitoes in the importation of RVFV into Madagascar is unknown. Given the 400 km or more of open water between Madagascar and the African mainland, it is highly improbable, though not impossible, that wind-borne mosquitoes could be the origin of the imported virus. More likely, virus-infected mosquitoes could have been unwittingly transported in air or sea shipments to Madagascar. However, the most likely route remains importation of virus-infected cattle or other livestock. In nearby Comoros, the outbreak of East Coast fever in 2003 and 2004 was attributed to the importation of cattle from Tanzania (8
). Only limited information is available regarding the cattle trade to and from Madagascar, with some recent records documenting exportation of ruminants to the Comoros archipelago (1
) and importation of live cattle from New Zealand (United Nations Commodity Trade Statistics Database, 2010 [http://comtrade.un.org/db/
]). No official records are available to substantiate the importation of cattle from the African mainland to Madagascar; however, the legal and illegal importation of livestock and goods is a common practice throughout the region and has already been implicated as the possible cause of a case of RVFV in Comoros in 2007 (27
). Once infected livestock are introduced into an area, the disease spreads rapidly, particularly in countries, such as Madagascar, where the large-scale movement of cattle is common.
Detailed examination of the phylogenetic trees indicates considerable movement of virus-infected livestock or mosquitoes on the African mainland. It is obvious that during 2006 to 2008, the Kenya-I lineage (; see also Fig. S1 in the supplemental material) was distributed across the large distances separating Kenya, Tanzania, Madagascar, and South Africa. In addition, some of the Central African viruses are dispersed in different RVFV lineages, as are Zimbabwe lineages and South African viruses. Clearly, distant movements of RVFVs are not infrequent events.
In conclusion, the generation of whole genomic sequences and advances in Bayesian coalescent methods enabled us to conclude that the reported RVFV events in Madagascar can be attributed to importation or expansion from mainland Africa. These findings underscore the importance of maintaining accurate and complete import/export records and remaining vigilant for the signs and symptoms of RVFV infection, including unexpected deaths and/or abortions, in livestock and humans.