Transmission and Infection
Lassa virus is zoonotic meaning it is transmitted to humans from animals. This particular virus is transmitted by Mastomys natalensis, probably the most common rodent in equatorial Africa, found predominantly in rural areas and ubiquitous in human households. In these rodents, infection is in a persistent asymptomatic state. The virus is shed in their excreta (urine and feces), which can be aerosolized and inhaled by humans. Infection in humans typically occurs via exposure to animal excrement through the respiratory or gastrointestinal tracts. Inhalation of tiny particles of the infectious agent is believed to be the most significant means of exposure. It is also possible to acquire the infection through broken skin or mucous membranes that are directly exposed to infective material. In some regions which are considered to be Lassa endemic, rats are consumed as food, which in turn can lead to infection.
Secondary transmission from person to person has also been established, presenting a disease risk for healthcare workers. It can be contracted by an airborne route or with direct contact with infected human blood, urine, semen or vaginal fluids. Transmission through breast milk has also been observed.
People of all ages are susceptible to infection. Despite healthcare workers being at high risk of infection, contact in households with persons ill, or recently ill with Lassa fever, as well as sexual contact with someone convalescent with Lassa fever are all also important risk factors for human-to-human transmission. The disease is mild or has no observable symptoms in up to 80% of people infected, but 20% develop a severe multisystem disease. Even after recovery, the virus remains in body fluids for long periods of time. It is excreted in urine for three to nine weeks after infection and in semen for up to three months.
Lassa fever in pregnant women is associated with infection of the fetus and loss of the fetus or newborn in 90% of the cases. The risk of death is also significantly higher for pregnant mothers in the third trimester and evacuation of the uterus significantly improves the mother's odds of survival.
Figure 1. Map of Africa showing the endemic area for Lassa fever. Although the disease is thought to exist throughout West Africa (light gray), laboratory confirmed cases are mostly observed in Guinea, Sierra Leone, Liberia and Nigeria (dark gray).
The dissemination of the infection can be determined by the prevalence of antibodies to the virus in various populations. Seropositivity (a significant level of serum antibodies indicating previous exposure to the infectious agent) has been found in Sierra Leone, Guinea and Nigeria, as well as the Central African Republic, Democratic Republic of Congo, Mali and Senegal. Increase in international travel and the possibility of Lassa virus being used as a biological weapon increases the potential for harm beyond the current endemic regions. There have also been sporadic cases in travellers returning from endemic regions to Europe and North America. Most recently a young woman in Sweden was diagnosed and treated for Lassa fever.