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Emerging Infectious Diseases

Transmission of Lassa virus (family Arenaviridae) from its natural rodent reservoir to humans can cause hemorrhagic fever, a clinical syndrome associated with high death rates. Lassa fever is endemic in West Africa and has been reported from Sierra Leone, Guinea, Liberia, and Nigeria (1-4). The geographically restricted occurrence of the disease is not well understood as its rodent host (Mastomys species) is prevalent in much larger areas of sub-Saharan Africa. The importation of Lassa virus into other regions, for example by travelers, is rare, with only a few cases documented (5-9). Although imported disease often raises public concern because of the possibility of human-to-human transmission; the highly pathogenic nature of the virus; and the lack of an effective, safe therapy, the actual risk for infection from an imported case appears to be low (5,7), and adequate guidelines have been published for disease management in patients and contacts (5,10). Arenaviruses can be divided phylogenetically, serologically, and geographically into two major complexes: the Old World complex (e.g., Lassa virus, lymphocytic choriomeningitis virus [LCMV]) and the New World complex (e.g., Tacaribe virus, Junin virus, Machupo virus) (11). Isolates of Lassa virus also differ in their genetic, serologic, and pathogenic characteristics (11-13). This variability is evidenced by the poor crosscomplement fixation and cross-neutralization among Lassa virus isolates of different geographic origins (3,14). Serologic differences were demonstrated by testing a panel of Lassa virusspecific monoclonal antibodies against many Lassa virus isolates (13). The single-stranded arenavirus genome consists of a small (S) and a large (L) RNA fragment, sizes 3.4 kb and 7 kb, respectively. The S RNA encodes the viral glycoprotein precursor protein (GPC) and the nucleoprotein (NP). The L RNA encodes the viral polymerase and a small, zinc-binding (Z) protein. Sequencing of the complete S RNA of two Lassa virus strains, originating from Sierra Leone (strain Josiah) (15) and Nigeria (strain Nigeria) (16), as well as sequencing of short S RNA fragments of additional isolates (e.g., strain LP from Nigeria) (11,17,18) showed considerable genetic Imported Lassa Fever in Germany: Molecular Characterization of a New Lassa Virus Strain