Lassa Fever: a rodent-human interaction

Lassa fever is a viral hemorrhagic fever, caused by an arenavirus discovered in 1969 in Nigeria. Nosocomial outbreaks and high seroprevalences have been recorded in Nigeria, Sierra Leone, Guinea and Liberia. After 7–10 days of incubation, humans develop fever, malaise, headache, myalgias and sore throat, although most have no recognized symptoms. Acute cases with diarrhea, edema and bleeding from mucosal surfaces affect 0.5% of the human population, with death occurring 10–15 days after onset. The disease has two geographically endemic areas separated by 2000 km with little evidence of virus circulation in Benin, Ghana, Côte d’Ivoire and Mali. The reservoir is the multimammate mouse, Mastomys natalensis, a rodent widely distributed in Africa. In West Africa, this species is mainly commensal, with populations fluctuating over time and space in rural localities. They are more abundant inside human habitation during the dry season than during the rainy season, increasing the risk of transmission to humans in the dry season. People living in villages are infected by contact with rodent excreta, or by inhaling viral particles. There is also evidence of human-to-human transmission in rural and urban localities. A human-to-rodent transmission is also discussed in this chapter as a consequence of the virus excretion in human saliva and urine, and the virus surviving outside the host. The genome of Lassa virus is bisegmented, with a small fragment coding for the glycoprotein and the nucleoprotein, and a long segment coding for the polymerase and the matrix protein. Today, 11 strains are completely sequenced, and phylogenies reveal large virus diversity in Nigeria with three lineages, while only one exists in Sierra Leone-Guinea-Liberia. Another lineage from Ghana-Côte d’Ivoire-Mali would be intermediate. The phylogeny reveals that Lassa virus emerged 700–900 years ago in Nigeria and then in Sierra Leone 150–200 years ago. This time scale does not support the co-evolution concept since M. natalensis emerged 2.3 million years ago. Recent findings such as new arenaviruses in other African rodents and in snakes argue preferably toward the host-switching concept. The recent emergence in Sierra Leone, the absence of virus positive Mastomys between the two endemic zones and poor virus diversity in the Mano River area point in the direction of a unique import of Lassa virus from Nigeria to Sierra Leone during the 19th century. The hypothesis of human displacements through the Atlantic slave trade and its abolition in 1807 concludes this chapter.