Lassa Fever

Introduction to Lassa Fever

Lassa fever (LF) is an acute viral haemorrhagic fever, which is endemic to parts of West Africa, including Nigeria, Sierra Leone, Guinea, and Liberia. It is likely that the reach of the virus extends across the region and beyond the borders of these countries. The Lassa virus is a member of the family Arenaviridae, RNA viruses which cause diseases in both the Old and the New world.

Mean predicted Lassa risk map for West Africa

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Mean predicted Lassa risk map for West Africa from the Model 2 series with two absence and one presence clusters, with positive localities indicated by stars.

LF is a zoonotic disease. The main animal reservoir of the Lassa virus is the multimammate rat, Mastomys natalensis, which can be found throughout sub-Saharan Africa, but the virus has also been identified in other small mammals across West Africa.

Transmission to humans occurs mostly during the dry season, in or near households (domestic or peri-domestic transmission) either by direct contact with the rat or indirect contact with contaminated food or household items. As such, it primarily concerns children and women. Human-to-human transmission is also possible, with healthcare staff working in resource-limited settings being at increased risk, due to the frequency and proximity of their interactions with Lassa fever patients and the suboptimal supply of high-quality protective equipment.

The true extent and incidence of infections and illnesses is not known. Nigeria reports the highest number of Lassa fever cases, due to a robust surveillance programme and reporting system coordinated via the Nigeria Centres for Disease Control (NCDC). In 2020, it recorded 1189 laboratory-confirmed cases of Lassa fever with focal points in Ondo, Edo and Ebonyi states.

However, surveillance of Lassa fever is challenging due to suboptimal availability of diagnostic tests (and no validated point-of-care pan-Lassa diagnostic test), need for capacity strengthening in laboratories and non-specific clinical presentation – making it particularly challenging to identify mild community cases. The true toll of Lassa fever infections could therefore be much higher than figures currently suggest – it is estimated that there are up to 500,000 new infections and 10,000 deaths per year across West Africa. The ‘Enable’ programme of the Coalition for Epidemic Preparedness Innovations (CEPI) is expected to provide useful information on the real incidence of LF in the region.

It is estimated that 80% of infections go unnoticed or cause only mild symptoms. Symptom onset typically occurs within 6 – 21 days of infection and is most commonly characterised by the presence of fever, headache, vomiting and abdominal pain. A small number of patients present with more severe, life-threatening forms of illness including haemorrhage, seizure, shock and breathing difficulty.

In hospitalised cases, the case fatality rate (CFR) is reported to be 2427% overall and 34% for pregnant women, rising in the third trimester to 80% and 95% for the foetus [7-9] – however in a recent cohort study taking place in a research setting, the CFR for hospitalised cases was 13% for adults and 6% for children. Mortality may be higher during outbreaks. The Lassa virus shows high genetic variability, with up to 8 lineages identified so far with varying virulence.

The primary treatment for Lassa fever is ribavirin in conjunction with supportive care. This recommendation is largely based on the results of a single prospective clinical trial that was conducted in the 1980s, which has since generated concern about its methods, analysis and safety when used to treat mild cases of LF. No further clinical trials evaluating ribavirin or new therapeutics have been conducted in the subsequent 40 years.