Lassa fever

Lassa fever, also known as Lassa haemorrhagic fever, is an acute viral illness caused by the Lassa virus, an Old World arenavirus. It is a significant public health concern in West Africa, where the disease is endemic and responsible for hundreds of thousands of infections each year. While many infections produce mild or no symptoms, severe disease can occur, leading to multi-organ involvement and, in some cases, fatal outcomes.

Epidemiology and Background

Lassa fever is widespread in West African countries including Nigeria, Sierra Leone, Liberia, Guinea and Ghana. Estimates suggest 300,000 to 500,000 infections annually, resulting in approximately 5,000 deaths. The virus was first identified in 1969 following an outbreak in the town of Lassa in present-day Borno State, Nigeria, though descriptions of a similar illness date back to the 1950s. Endemic transmission is closely tied to the ecology of local rodent populations.

Cause and Virology

The Lassa virus belongs to the Arenaviridae family of enveloped, negative-sense, single-stranded RNA viruses. Its genome is bisegmented, consisting of a large and a small segment. At least seven genetic lineages have been identified across West Africa, each associated with particular geographical regions. The reservoir host is the Natal multimammate mouse (Mastomys natalensis), one of the most common rodent species in equatorial Africa. Once infected, these mice shed virus in urine and faeces throughout their lifespan.
Humans typically become infected after exposure to materials contaminated with rodent excreta, either through ingestion, inhalation of aerosolised particles or contact with broken skin. Person-to-person transmission can occur through contact with bodily fluids, posing danger to caregivers and healthcare workers. Viral shedding may persist in urine for several weeks and in semen for up to three months.

Signs and Symptoms

The incubation period ranges from 7 to 21 days. Many individuals remain asymptomatic or develop only mild symptoms, including:

• Fever and fatigue
• Headache and weakness
• General aches and malaise

Around 20 per cent of infections progress to severe disease marked by:

• Vomiting and chest or abdominal pain
• Respiratory difficulty
• Mucosal bleeding, such as from gums or the gastrointestinal tract
• Circulatory shock in advanced cases

A triad of sore throat, retrosternal pain, proteinuria and fever is considered particularly suggestive. Death typically occurs within two weeks of symptom onset in fatal cases. While overall mortality is about 1 per cent, among hospitalised patients it rises to 15–20 per cent, and is markedly higher in pregnant women, particularly in the third trimester. Survivors may experience hearing loss, which affects roughly one-quarter of cases; half of these individuals regain hearing within several months.
Infants and toddlers may develop swollen baby syndrome, characterised by oedema, abdominal distension and bleeding.

Diagnosis

Diagnosing Lassa fever on clinical grounds is difficult because symptoms resemble those of malaria, typhoid fever, yellow fever and Ebola virus disease. Laboratory confirmation may involve:

• PCR testing for viral RNA
• ELISA assays for antigen detection or IgM antibodies
• Cell culture, plaque neutralisation or immunofluorescence tests

In endemic regions diagnostic capacity may be limited, leading to misdiagnosis and delayed treatment. Other laboratory findings include lymphocytopenia, thrombocytopenia and elevated liver enzymes such as AST. The virus may also be detectable in cerebrospinal fluid in patients with neurological symptoms.

Prevention

Controlling rodent populations remains challenging; therefore preventive strategies focus on reducing contact with rodents:

• Keeping homes and food stores rodent-proof
• Storing grain in sealed containers
• Maintaining clean households and disposing of refuse away from living areas
• Encouraging sanitation and personal hygiene

For those caring for infected patients, protective equipment such as gloves, masks, gowns and eye protection is essential to minimise exposure to bodily fluids. Public health infrastructure is crucial but often limited in affected regions.
There is no licensed human vaccine as of 2023, though experimental vaccine candidates—such as recombinant vesicular stomatitis virus vectors expressing Lassa glycoproteins—have shown promising results in animal studies.

Treatment

Treatment centres on supportive care:

• Management of dehydration
• Correction of electrolyte disturbances
• Monitoring of organ function

The antiviral ribavirin has been used and is recommended by several guidelines, particularly when administered early, though evidence for its effectiveness remains limited and sometimes inconsistent. Supportive therapy often includes maintaining adequate oxygenation, managing bleeding and providing nutritional support.
Given the variability in clinical presentation and the risk of severe disease, early recognition and prompt medical intervention are critical for improving outcomes.

Significance

Lassa fever remains a major cause of infectious morbidity and mortality in West Africa. Its close link with rodent ecology, combined with challenges in diagnosis, treatment and prevention, has contributed to its persistence. Ongoing research into vaccines, antiviral therapies and improved surveillance holds promise for reducing the impact of this important viral haemorrhagic fever.