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Lassa fever is an acute viral hemorrhagic fever first described in 1969 in the town of Lassa, in Borno State, Nigeria located in the Yedseram river valley at the south end of Lake Chad.[1] Clinical cases of the disease had been known for over a decade earlier but not connected with this viral pathogen. The infection is endemic in West African countries, and causes 300-500,000 cases annually with ~5,000 deaths.[2] Outbreaks of the disease have been observed in Nigeria, Liberia, Sierra Leone, Guinea, and the Central African Republic, but it is believed that human infections also exist in Democratic Republic of the Congo, Mali, and Senegal.[citation needed] Its primary animal host is the Natal Multimammate Mouse (Mastomys natalensis), an animal indigenous to most of Sub-Saharan Africa.[3] Although the rodents are also a source of protein for peoples of these areas, the virus is probably transmitted by the contact with the feces and urine of animals accessing grain stores in residences.[3]
Lassa fever is caused by the Lassa virus, a member of the Arenaviridae family; it is an enveloped, single-stranded, bisegmented RNA virus.[2]
Replication for Lassa virus is very rapid, while also demonstrating temporal control in replication. There are two genome segments. The first step involved is making mRNA copies of the – sense genome. This ensures that there is adequate proteins, which are required for replication. The N and L proteins are made from the mRNA produced. The – sense genome then makes viral complementary RNA (vcRNA) copies of itself which are + sense. The vcRNA is a template for producing – sense progeny but mRNA is also synthesized from it. The mRNA synthesized from vcRNA translate the G (spike) proteins and Z proteins. Thus, with this temporal control, the spike proteins are produced last, making the infection further undetected by the host immune system.
Lassa virus will infect almost every tissue in the human body. It starts with the mucosa, intestine, lungs and urinary system, and then progresses to the vascular system.
Lassa virus is zoonotic (transmitted from animals), in that it spreads to man from rodents, specifically multi-mammate rats (Mastomys natalensis). This is probably the most common rodent in equatorial Africa, ubiquitous in human households and eaten as a delicacy in some areas.[citation needed] In these rats infection is in a persistent asymptomatic state. The virus is shed in their excreta (urine and feces), which can be aerosolized. In fatal cases, Lassa fever is characterized by impaired or delayed cellular immunity leading to fulminant viremia.
Infection in humans typically occurs via exposure to animal excrement through the respiratory or gastrointestinal tracts. Inhalation of tiny particles of infective material (aerosol) is believed to be the most significant means of exposure. It is possible to acquire the infection through broken skin or mucous membranes that are directly exposed to infective material. Transmission from person to person has also been established, presenting a disease risk for healthcare workers. Frequency of transmission via sexual contact has not been established.
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