Lassa fever also known as hemorrhagic fever (LHF) is a type of viral hemorrhagic fever caused by the Lassa virus. Many of those infected by the virus do not develop symptoms.  When symptoms occur they are typically include fever, weakness, headaches, vomiting and muscle pains.

Though Lassa fever was first described in the 1950s, the virus causing Lassa fever 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 symptom. 1 in 5 infections result in severe disease, where the virus affects several organs such as the liver, spleen and kidneys. 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 they can shed the virus in their urine and faeces. Because the clinical course of the disease is so variable, detection of the disease in affected patients has been difficult. When presence of the disease is confirmed in a community, however prompt isolation of affected patients, good infection prevention and control practices as well as rigorous contact tracing can stop outbreaks.

In 1969, a series of sequential life threatening or fatal infections occurred among healthcare workers in Nigeria and the laboratory scientist ho isolate and characterized the agent causative agent. The diseases Lassa virus was named after the geographical location of the first recognized human case, which is in Lassa, Borno State, Nigeria. The new virus was shown to be related to lymphatic choriomeningitis and previously unclassified Neotropical viruses, including Argentine and Bolivian hemorrhagic fevers and a new taxonomic grouping, the Arena viruses was proposed. In 1970-1972, three further epidemics occurred in Nigeria, Liberia and Sierra Leone, the first two involved was nosocomial and the third was a community based outbreak, during which the rodent reservoir host was identified. In 1979 a long-term research project commenced in Sierra Leone which produced a rich body of data from prospectively designed studies on the clinical features, transmission and treatment of the disease.

Symptoms of Lassa fever

The incubation period of Lassa fever ranges from 6-21 days. The onset of the disease, when it is symptomatic, is usually gradual, starting with fever, general weakness and malaise. After few days, headaches, sore throat, muscle pain, chest pain, nausea, vomiting, diarrhea, cough and abdominal pain may follow. In severe cases facial swelling, fluid in the lung cavity, bleeding from the mouth, 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 survive the disease. In half of these cases, hearing returns partially after 1-3 months. Transient hair loss, 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.


 Lassa virus is typically transmitted by the urine or faeces of mastomys rats to humans.

Health workers, humans may be infected by direct contact with blood, body fluids, urine or stool of a patient with Lassa fever.


Lassa fever is most often by using enzyme-linked immunosorbent serologic assays (ELISA), which detect IgM and IgG antibodies as well as Lassa antigen.

Reverse transcription-polymerase chain reaction (RT-PCR) can be used in the early stage.

The virus itself may be cultured 7 to10 days, but this procedure should only be done in a high containment laboratory with good laboratory practices.

Immunohistochemistry, performed on formalin-fixed tissue specimen, can be used to make a post-mortem diagnosis.


The antiviral drug ribavirin seems to be an effective treatment for Lassa fever if given early on in the course of clinical illness. There is no evidence to support the role of ribavirin as post-exposure prophylactic treatment for Lassa fever.

Ribavirin given intravenously and early in the course of illness is an effective treatment in addition to support of fluid and electrolytes, oxygenation and blood pressure. In the only study to evaluate it in 1986, this treatment reduced mortality from 50% to 5% if given early in serious illness. Adverse effects include hemolytic anemia (rupture of red blood cells) if infused quickly.

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 foodstuff in rodent-proof containers, disposing of garbage far from the home, maintaining clean household 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.

In health care settings, staff should always apply standard infection prevention and control precautions when caring for patients’ regardless if 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 workers and individuals caring for patients with suspected or confirmed Lassa fever should apply extra infection measures to prevent contact with the patient’s blood and body fluid when in close contact (within 1 meter) of patients with Lassa fever, health care workers should wear face protection, a clean non-sterile long-sleeve 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, travelers from areas where Lassa fever is endemic export the disease to other countries. The diagnosis of Lassa fever should be considered in febrile patients returning from West Africa, especially if they have had exposures in rural areas or hospitals in countries where Lassa fever is known to be endemic. Health workers seeing patients suspected to have Lassa fever should immediately contact local and national experts for advice and to arrange for laboratory testing.

World Health Response (WHO) Response

The ministries of Health of Guinea, Liberia and Sierra Leone, WHO, the office of United States Disaster Assistance, the United Nations and other partners have worked together to establish the Mano River Union Lassa Fever Network. The program supports these countries in developing national prevention strategies and enhancing laboratory diseases. Training in laboratory diagnosis, clinical management and environmental control is also included.



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