Overview Symptoms and treatment Prevention and control
Lassa fever is an acute viral haemorrhagic illness caused by Lassa virus, a member of the arenavirus family of viruses.
Humans usually become infected with Lassa virus through exposure to food or household items contaminated with urine or feces of infected Mastomys rats. The disease is endemic in the rodent population in parts of West Africa.
Lassa fever is known to be endemic in Benin, Ghana, Guinea, Liberia, Mali, Sierra Leone, Togo and Nigeria, but probably exists in other West African countries as well.
Person-to-person infections and laboratory transmission can also occur, particularly in health care settings in the absence of adequate infection prevention and control measures.
Diagnosis and prompt treatment are essential. The overall case-fatality rate is 1%. Among patients who are hospitalized with severe clinical presentation of Lassa fever, case-fatality is estimated at around 15%. Early supportive care with rehydration and symptomatic treatment improves survival.
Though first described in the 1950s, the virus causing Lassa fever disease was not identified until 1969. The virus is a single-stranded RNA virus belonging to the virus family Arenaviridae.
About 80% of people who become infected with Lassa virus have no symptoms. 1 in 5 infections result in severe disease, where the virus affects several organs such as the liver, spleen and kidneys.
Symptoms and Treatment
The incubation period of Lassa fever ranges from 2–21 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.
Protein may be noted in the urine. Shock, seizures, tremor, disorientation, and coma may be seen in the later stages. Deafness occurs in 25% of patients who recover from the disease. In half of these cases, hearing returns partially after 1–3 months. Transient hair loss and gait disturbance may occur during recovery.
Death usually occurs within 14 days of onset in fatal cases. The disease is especially severe late in pregnancy, with maternal death and/or fetal loss occurring in more than 80% of cases during the third trimester.
There is currently no specific antiviral therapy against Lassa Fever approved by the United States Food and Drug Administration or the European Medical Agency. Despite not being licensed as a treatment for Lassa fever the antiviral drug ribavirin has been used in several countries as a therapeutic agent. Hovever, there is still a need for a safe treatment with proven efficacy against Lassa fever, and a range of potential treatments including immune therapies and drug therapies are currently being evaluated. There is no evidence to support the role of ribavirin as post-exposure prophylactic treatment for Lassa fever.
There is currently no licensed vaccine for Lassa fever, but several potential vaccines are in development.
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, as well as from other diseases that cause fever, including malaria, shigellosis, typhoid fever and yellow fever.
Definitive diagnosis requires testing that is available only in reference laboratories. Laboratory specimens may be hazardous and must be handled with extreme care. Lassa virus infections can only be diagnosed definitively in the laboratory using the following tests:
reverse transcriptase polymerase chain reaction (RT-PCR) assay
antibody enzyme-linked immunosorbent assay (ELISA)
antigen detection tests
virus isolation by cell culture.
Prevention and Control
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 rats are so abundant in endemic areas, it is not possible to eliminate them from the environment. Family members should always be careful to avoid contact with blood and body fluids while caring for sick persons.
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.
Health-care workers caring for patients with suspected or confirmed Lassa fever should apply extra infection control measures to prevent contact with the patient's blood and body fluids and contaminated surfaces or materials such as clothing and bedding. When in close contact (within 1 meter) of patients with Lassa fever, health-care workers should wear face protection (a face shield or a medical mask and goggles), a clean, non-sterile long-sleeved gown, and gloves (sterile gloves for some procedures).
Laboratory workers are also at risk. Samples taken from humans and animals for investigation of Lassa virus infection should be handled by trained staff and processed in suitably equipped laboratories under maximum biological containment conditions.
On rare occasions, travellers from areas where Lassa fever is endemic, export the disease to other countries. 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 exposure in rural areas or hospitals in countries where Lassa fever is known to be endemic. Health-care workers seeing a patient suspected to have Lassa fever should immediately contact local and national experts for advice and to arrange for laboratory testing.