Lassa Virus Seroprevalence in Sibirilia Commune, Bougouni District, Southern Mali.

Lassa virus (LASV) (family Arenaviridae, genus Arenavirus) is the etiologic agent of Lassa fever (LF), a viral hemorrhagic fever first documented in 1969 during an outbreak on the Jos Plateau in Nigeria (1). In humans, LASV infection is characterized by a variety of clinical manifestations that can range from apparently asymptomatic or mild disease to severe disease consisting of multiorgan failure (2,3). As much as 80% of infected persons are believed to experience mild disease, whereas 20% exhibit noteworthy and often severe clinical indicators that require medical attention (4). An estimated 300,000 LASV infections occur in West Africa each year, resulting in ≈5,000 deaths (5). Infection during pregnancy, especially during the third trimester, is particularly severe; estimated maternal mortality rates are 20%, and fetal mortality rates are ≈100% (6–8). As are most arenaviruses, LASV is maintained in nature in rodent hosts, specifically, the multimammate rat (Mastomys natalensis) (9). Most commonly, contact with infectious rodents or ingestion/inhalation of virus-laden particles is the source of human infection. Person-to-person transmission is also well documented and can result in outbreaks, especially in nosocomial settings, leading to mortality rates in >50% (7). Historically, LASV has been considered endemic to 2 geographic areas of West Africa: 1) Sierra Leone, Guinea, and Liberia; and 2) Nigeria. However, in recent years, an increased region of LASV endemicity has been suggested, which includes adjoining countries and areas farther north than previously suggested (10,11). In 2000, a German citizen received a diagnosis of LF after traveling through Ghana, Cote d’Ivoire, and Burkina Faso (12). More recently, cases of LF have been identified in Ghana (13), and the presence of LASV-infected rodents has been documented in Cote d’Ivoire (14). In a similar situation, LF was unknown in Mali until February 2009, when a young British man was medically evacuated to London after a 10-day history of fever (15). The infection was initially diagnosed as Plasmodium falciparum malaria, even though the patient did not respond to treatment for malaria. He died on arrival in London, and a postmortem diagnosis of LASV infection was confirmed by molecular techniques. In response to this case, rodent surveys were conducted in the village of Soromba (rural commune of Sibirila, Bougouni district, Mali), where the man was living and working when he became ill. The initial surveys found that 25% of M. natalensis rats had molecular evidence of active LASV infection, which was confirmed by virus isolation and sequence analysis (16). Similar studies conducted across Mali suggest that LASV is restricted to the southern tip of the country, in several villages near the border of Cote d’Ivoire (17). On average, 20% of peridomestic Mastomys rodents collected in these villages had serologic or molecular evidence of LASV infection, with peak prevalence rates >50%. Given the infection rates observed in rodents living in close proximity to humans in many villages in southern Mali, it seems likely that humans are frequently exposed to LASV infection and that LF may develop. Nevertheless, despite increased recognition of LF in Mali, to date no outbreaks have occurred, and the 2009 exported case remains the only confirmed human LASV infection contracted in Mali. Reports of a second case of LF associated with the British citizen are unconfirmed. Verbal accounts indicate that shortly after he was evacuated, his housekeeper and cook also fell ill and died. Samples were not collected for testing, in part because malaria was suspected. To better understand the risk for human LASV infection in southern Mali, we conducted a serologic survey of inhabitants of 3 villages within the rural commune of Sibirila to determine the proportion of persons who had been exposed to LASV.