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Primary reference(s)

WHO, 2017. . Accessed 3 November 2020.

Additional scientific description

Although first described in the 1950s, the virus causing Lassa disease was not identified until 1969. The virus is a singlestranded RNA virus belonging to the virus family Arenaviridae. Lassa fever is a zoonotic disease, meaning that humans become infected from contact with infected animals. The animal reservoir, or host, of Lassa virus is a rodent of the genus Mastomys, commonly known as the ‘multimammate rat’. Mastomys rats infected with Lassa virus do not become ill, but can shed the virus in their urine and faeces (WHO, 2017). People at greatest risk are those living in rural areas where Mastomys are usually found, especially in communities with poor sanitation or crowded living conditions. Lassa fever occurs in all age groups and both sexes.

About 80% of people who become infected with Lassa virus have no symptoms. One in five infections results in severe disease, where the virus affects several organs such as the liver, spleen and kidneys. Because the clinical course of the disease is so variable, detection of the disease in affected patients has been difficult. When the disease is confirmed to be present in a community, however, prompt isolation of affected patients, good infection prevention and control practices, and rigorous contact tracing can stop outbreaks (WHO, 2017).

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, tremor, 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. Transient hair loss and gait disturbance may occur during recovery. The disease is especially severe late in pregnancy, with maternal death and/or foetal loss occurring in more than 80% of cases during the third trimester (WHO, 2017).

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. Sexual transmission of Lassa virus has been reported (WHO, 2017).

Lassa fever is difficult to diagnose because the symptoms are non-specific. Definitive diagnosis requires viral and serological testing available only in reference laboratories. Isolation of the virus is usually done from blood, urine or throat washings (WHO, 2017).

• Lassa fever can cause prolonged epidemics and is endemic in Benin, Ghana, Guinea, Liberia, Mali, Sierra Leone and Nigeria (WHO, 2016a). Two recent outbreaks are summarised below: Liberia: Lassa fever is endemic. The World Health Organization (WHO) reported that from 1 January 2017 through 23 January 2018, 91 suspected cases were reported from six counties: Bong, Grand Bassa, Grand Kru, Lofa, Margibi, and Nimba. Thirtythree of these cases were laboratory confirmed, including 15 deaths (case fatality rate for confirmed cases = 45.4%) (WHO, 2018). 
• Nigeria: The WHO reported that from 1 January through 9 February 2020, 472 laboratory confirmed cases including 70 deaths (case fatality ratio = 14.8%) were reported in 26 out of 36 Nigerian states and the Federal Capital Territory. Of the 472 confirmed cases, 75% were reported from three states: Edo (167 cases), Ondo (156 cases) and Ebonyi (30 cases). The other states reporting cases include: Taraba (25), Bauchi (14), Plateau (13), Kogi (13), Delta (12), Nasarawa (4), Kano (4), Rivers (4), Enugu (4), Borno (3), Kaduna (3), Katsina (3), Benue (2), Adamawa (2), Sokoto (2), Osun (2), Abia (2), Kebbi (2), Gombe (1), Oyo (1), Anambra (1), FCT (1), and Ogun (1) (WHO, 2020).

The Centers for Disease Control and Prevention has published information on case definitions and classification for viral haemorrhagic fevers (CDC, 2011).

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Prolonged epidemics cause serious economic and social disruption (WHO, 2017).

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, 2017).

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, 2017).

There have been limited cases of exportation of the virus to the UK, Sweden and Germany and other European countries (WHO, 2016a).

The Ministries of Health of Guinea, Liberia and Sierra Leone, WHO, the Office of 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, 2017).

The WHO has published recommendations for prevention and control (WHO, 2017).