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Lassa viral haemorrhagic fever is an acute illness of 1-4 weeks duration that occurs in West Africa. Though first described in the 1950s, the virus causing the disease was not identified until 1969. The virus is a single-stranded RNA virus belonging to the virus family Arenaviridae. Lassa fever is known to be endemic in Guinea (Conakry), Liberia, Sierra Leone and parts of Nigeria, but probably exists in other West African countries as well.

Signs and symptoms

About 80% of human infections are asymptomatic; the remaining cases have severe multi-system disease, where the virus affects several organs in the body, such as the liver, spleen and kidneys. The incubation period of Lassa fever ranges from 6-21 days. The onset of the disease 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 may follow. Severe cases may progress to show facial swelling, fluid in the lung cavity, bleeding from mouth, nose, vagina or gastrointestinal tract, and low blood pressure. Protein may be noted in the urine. Shock, seizures, tremor, disorientation, and coma may be seen in the late stages. Deafness occurs in 25% of patients of whom half recover some function after 1-3 months. Transient hair loss and gait disturbance may occur during recovery.

Morbidity and mortality

Some studies indicate that 300 000 to 500 000 cases of Lassa fever and 5000 deaths occur yearly across West Africa. The overall case-fatality rate is 1%, up to 15% among hospitalized patients. 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 greater than 80% of cases during the third trimester.

Animal reservoir

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 infected with Lassa virus do not become ill, but they can shed the virus in their excreta (urine and faeces).

People at risk

Lassa fever occurs in all age groups and in both men and women. Persons at greatest risk are those living in rural areas where Mastomys are usually found, especially in areas of poor sanitation or crowded living conditions. Health care workers are at risk if proper barrier nursing and infection control practices are not maintained.

Transmission

Humans usually become infected with Lassa virus from exposure to excreta of infected Mastomys. Both direct exposure, (touching the excreta) and Lassa virus may also be spread between humans through direct contact with the blood, urine, faeces, or other bodily secretions of a person 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.

Diagnosis

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 many other diseases which cause fever, including malaria, shigellosis, typhoid fever, yellow fever and other viral haemorrhagic fevers.

Definitive diagnosis requires testing that is available only in highly specialized laboratories. Laboratory specimens may be hazardous and must be handled with extreme care. Lassa fever is diagnosed by detection of Lassa antigen, anti-Lassa antibodies, or virus isolation techniques.

Treatment and prophylaxis

The antiviral drug ribavirin is 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.

Prevention

Prevention of Lassa fever in the community centers 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.

Infection control

Family members and health care workers should always be careful to avoid contact with blood and body fluids while caring for sick persons. Routine barrier nursing precautions probably protect against transmission of Lassa virus in most circumstances. However, for added safety, patients suspected to have Lassa fever should be cared for under specific “isolation precautions,” which include the wearing of protective clothing such as masks, gloves, gowns, and face shields, and the systematic sterilization of contaminated equipment (see also detailed guidelines in “Infection Control for Viral Hemorrhagic Fevers in the African Health Care Setting”) ¹

Ongoing initiatives

Civil unrest in many of the countries where Lassa fever is endemic has impeded effective control. However, recent peace initiatives have opened new opportunities to combat the problem. 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. In addition, a new ward dedicated to the care of patients with Lassa fever is under construction in Sierra Leone, sponsored by the European Union.

International public health implications

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 exposures 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.