Lassa Fever

Primary reference(s)

WHO, 2024a. Lassa fever. World Health Organization (WHO). Accessed 6 January 2025.

Annotations

Key relevant UN convention / multilateral treaty

International Health Regulations (2005), 3rd ed. (WHO, 2016b).

Drivers

Transmission is primarily from direct and indirect contact with the urine or faeces of infected Mastomys rats, usually from contaminated surfaces, food and water. Humans usually become infected with Lassa virus from exposure to urine or faeces of infected Mastomys rats. Lassa virus may also be spread between humans through direct contact with the blood, urine, faeces, or other bodily secretions of a person infected with Lassa fever. There is no epidemiological evidence supporting airborne spread between humans. Person-to-person transmission occurs in both community and health-care settings, where the virus may be spread by contaminated medical equipment, such as re-used needles. Lassa virus may persist in the semen of some males who recovered from the disease for up to a few months. However, to date, no transmission via exposure to infected semen had ever been documented (WHO, 2024a). 

Impacts

The incubation period of Lassa fever ranges from 6 to 21 days. The onset of the disease, when it is symptomatic, is usually gradual, starting with fever, general weakness, and malaise. After a few days, headache, sore throat, muscle pain, chest pain, nausea, vomiting, diarrhoea, cough, and abdominal pain may follow. In severe cases, facial swelling, fluid in the lung cavity, bleeding from the mouth, nose, vagina or gastrointestinal tract and low blood pressure may develop. Shock, seizures, tremors, disorientation, and coma may be seen in the later stages. Death usually occurs within 14 days of onset in fatal cases. Deafness occurs in 25% of patients who survive the disease. In half of these cases, hearing returns partially after one to three months (WHO, 2024a). 

Transient hair loss and gait disturbance may occur during recovery. During pregnancy, Lassa fever can cause high maternal and foetal mortality, especially late in pregnancy. In the third trimester, foetal death and maternal death rates can exceed 80% and 30% respectively (WHO, 2024a). 

Because the symptoms of Lassa fever are so varied and non-specific, clinical diagnosis is often difficult, especially early in the course of the disease. Lassa fever is difficult to distinguish from other viral haemorrhagic fevers such as Ebola virus disease, malaria, shigellosis, typhoid fever and yellow fever (WHO, 2024b).

Multi-hazard context

Person-to-person transmission can also occur, particularly in health care settings lacking adequate infection prevention and control measures. The overall case fatality rate is 1% but observed case fatality rate among patients hospitalized with severe diseases of Lassa fever is 15% and above (WHO, 2024a).  

The disease is closely associated with inadequate community sanitation and with unsafe water or unsafe food storage where rats have easy access. Prevention of Lassa fever relies on promoting good ‘community hygiene’ to discourage rodents from entering homes. Effective measures include storing grain and other foodstuffs in rodent-proof containers, disposing of garbage far from the home, maintaining clean households and keeping cats. Because Mastomys are so abundant in endemic areas, it is not possible to completely eliminate them from the environment. Family members should always be careful to avoid contact with blood and body fluids while caring for sick persons (WHO, 2024a).

Prolonged epidemics cause serious economic and social disruption (WHO, 2024a). 

Risk Management

During an outbreak, health care professionals are at high risk of transmission, and these are associated with close contact to contaminated surfaces, sample collection, transport and laboratory investigations. In health-care settings, staff should always apply standard infection prevention and control precautions when caring for patients, regardless of their presumed diagnosis. These include basic hand hygiene, respiratory hygiene, use of personal protective equipment (to block splashes or other contact with infected materials), safe injection practices and safe burial practices. The WHO recommends standard infection prevention and control precautions (WHO, 2024a). 

On rare occasions, travellers returning from endemic areas were confirmed for Lassa fever. Although malaria, typhoid fever and many other tropical infections are much more common, the diagnosis of Lassa fever should be considered in febrile patients returning from West Africa, especially if they have had exposures in areas where Lassa fever is known to be endemic. Healthcare workers seeing a patient suspected to have Lassa fever should immediately contact local and national experts for advice and to arrange for laboratory testing (WHO, 2024a). 

The Ministries of Health of Guinea, Liberia and Sierra Leone, WHO, the Office of the United States Foreign Disaster Assistance, the United Nations, and other partners have worked together to establish the Mano River Union Lassa Fever Network. The programme supports these three countries in developing national prevention strategies and enhancing laboratory diagnostics for Lassa fever and other dangerous diseases. Training in laboratory diagnosis, clinical management, and environmental control is also included (WHO, 2020). 

With the WHO’s overall coordination and oversight, together with a task force of internationally recognized subject-matter experts, including scientists from the most affected countries, with specific expertise in arenaviruses and in one or more research and development areas, and with support from the Wellcome Trust, the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota has been tasked with facilitating the collaborative development of an “Lassa fever R&D RoadMap”. The roadmap prioritizes the development of countermeasures (diagnostics, therapeutics and vaccines) that are most needed by Lassa fever-affected countries and is the result of extensive consultations with the Lassa R&D roadmap task force, leading national and international experts and other key stakeholders (WHO, 2019).

Monitoring

WHO supports countries to conduct all-hazards strategic risk assessment in the contexts of health emergencies and disasters, which results in the development of a country risk profile. Empowered with the country risk profile, inclusive of a seasonal risk calendar, countries can anticipate potential emergencies before they occur to trigger early alerts and inform early actions (WHO, 2021).

WHO's Early Warning, Alert and Response System (EWARS) has been designed to improve disease outbreak detection in emergency settings, such as in countries in conflict or following a natural disaster. It is a simple and cost-effective way to rapidly set up a disease surveillance system. EWARS is deployed during an emergency as an adjunct to the national disease surveillance system. WHO works with Ministries of Health and health sector partners to train local health workers to use the system. After the emergency, EWARS should re-integrate back into the national system (WHO, 2023b).