Lassa fever or Lassa hemorrhagic fever (LHF) is an acute viral hemorrhagic fever caused by the Lassa virus and first described in 1969 in the town of Lassa, in Borno State, Nigeria. Lassa virus is a member of the Arenaviridae virus family. Similar to ebola, clinical cases of Lassa fever had been known for over a decade, but had not been connected with a viral pathogen.
The primary animal host of the Lassa virus is the Natal multimammate mouse (Mastomys natalensis), an animal found in most of sub-Saharan Africa. The virus is probably transmitted by contact with the feces or urine of animals accessing grain stores in residences.Given its high rate of incidence, Lassa fever is a major problem in affected countries.
Lassa fever occurs commonly in West Africa. It results in 300,000 to 500,000 cases annually and causes about 5,000 deaths each year.Outbreaks of the disease have been observed in Nigeria, Liberia, Sierra Leone, Guinea, and the Central African Republic.
Signs and symptoms
In 80% of cases, the disease is asymptomatic, but in the remaining 20%, it takes a complicated course. The virus is estimated to be responsible for about 5,000 deaths annually. The fever accounts for up to one-third of deaths in hospitals within the affected regions and 10 to 16% of total cases.
After an incubation period of six to 21 days, an acute illness with multiorgan involvement develops. Nonspecific symptoms include fever, facial swelling, and muscle fatigue, as well as conjunctivitis and mucosal bleeding. The other symptoms arising from the affected organs are:
Dysphagia (difficulty swallowing)
- CARDIOVASCULAR SYSTEM
Tachycardia (abnormally high heart rate)
- RESPIRATORY TRACT
- NERVOUS SYSTEM
Unilateral or bilateral hearing deficit
Clinically, Lassa fever infections are difficult to distinguish from other viral hemorrhagic fevers such as Ebola and Marburg, and from more common febrile illnesses such as malaria.
The virus is excreted in urine for 3-9 weeks and in semen for three months.
Mastomys natalensis, the natural reservoir of the Lassa fever virus
Lassa virus is zoonotic (transmitted from animals), in that it spreads to humans from rodents, specifically multimammate mice (Mastomys natalensis).This is probably the most common mouse in equatorial Africa, ubiquitous in human households and eaten as a delicacy in some areas. In these rodents, 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 by exposure to animal excrement through the respiratory or gastrointestinal tracts. Inhalation of tiny particles of infectious 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 infectious material. Transmission from person to person has also been established, presenting a disease risk for healthcare workers. Frequency of transmission by sexual contact has not been established.