Lassa fever is caused by infection with the Lassa virus, a member of the arenavirus family. This virus is primarily transmitted to humans from a natural animal reservoir, the multimammate rat (Mastomys natalensis), which is common in West Africa. These rodents carry the virus without showing symptoms and shed it in their urine and feces, contaminating the environment. Humans typically become infected through direct or indirect contact with these rodent excretions, such as by touching contaminated surfaces, inhaling tiny particles in the air, or consuming food contaminated by the rodents.
The virus can also spread from person to person, but this requires close contact with the bodily fluids of an infected individual. Such fluids include blood, saliva, urine, vomit, or other secretions. Human-to-human transmission is most common in healthcare settings where protective measures are inadequate, as well as within households caring for sick relatives. However, people are not contagious during the incubation period; transmission occurs only when symptoms develop and the virus is present in bodily fluids.
Environmental and social factors play a significant role in the spread of Lassa fever. The multimammate rat thrives in areas where human settlements provide ample food and shelter, such as homes, farms, and grain storage facilities. Poor housing conditions and inadequate sanitation increase the likelihood of rodent infestation and human exposure. Seasonal changes, particularly during the dry season, can influence rodent behavior and virus transmission patterns, often leading to seasonal outbreaks.
Rapid urbanization and changes in land use also affect the dynamics of Lassa fever transmission. As people move into new areas or expand agricultural activities, they may come into closer contact with infected rodents. Climate change, by altering rainfall and temperature patterns, can shift the habitats of these rodents and potentially expand the geographic range of the virus, putting more people at risk.
Once a person is infected, the virus replicates in the body, causing symptoms that range from mild fever and malaise to severe hemorrhagic manifestations and multi-organ failure. The severity of illness varies widely, with some individuals experiencing no symptoms at all, while others develop life-threatening disease.
In summary, Lassa fever is caused by the Lassa virus, which is maintained in nature by multimammate rats. Human infection occurs mainly through contact with rodent excretions or, less commonly, through close contact with infected people’s bodily fluids. Environmental conditions, human behavior, and socio-economic factors all influence the risk and spread of this disease.
