By Stan Deresinski, MD, FACP, FIDSA

Clinical Professor of Medicine, Stanford University

Dr. Deresinski reports no financial relationships relevant to this field of study.

SYNOPSIS: Outbreaks of Lassa fever are occurring in Nigeria and several other West African nations, some of which also are endemic for Ebola virus infections.

SOURCE: Lassa Fever – West Africa. ProMED Mail June 6, 2017. Available at:

At the end of December 2016, a report appeared on ProMED indicating that a nurse in Ogun State, Nigeria, who had cared for a patient with suspected Lassa fever had died, as had the morgue attendant who had handled the patient’s corpse. Since Lassa fever is endemic in Nigeria, as it is in several other countries of West Africa, this was not surprising. In fact, in October 2016, the Ministry of Health stated that there had been 10 cases reported so far in that year in Ogun State. By the last day of the year, however, the Ministry of Health had to raise the threat level for fear of an epidemic.

Additional cases occurred in other Nigerian states — a total of nine states by February. By mid-February, 196 suspected and 53 laboratory-confirmed cases and 31 deaths had been detected nationally. The case fatality rate for confirmed cases was 53.4%.

By the end of February, two fatal cases were reported in the adjacent country of Benin as well as additional cases in the more distant countries of Togo, Burkina Faso, Sierra Leone, and Liberia. Meanwhile, cases continued to occur in Nigeria, with 262 cases reported by June 7, 2017, 59 of which were laboratory confirmed; 48 patients died.


Lassa fever is caused by a single-stranded RNA virus of the Arenavirus family, which includes, among others, lymphocytic choriomeningitis virus, and Junin, Guanarito, and Machupo viruses. Its eponymous name derives from its first identification in association with the village of Lassa in northwestern Nigeria.

Rodents are the reservoir of Lassa fever virus. Humans are infected by exposure to rat excreta either directly or by inhalation of contaminated dust or, in some cases, by eating rats. Human-to-human transmission appears to be limited to direct contact with secretions, but the period of risk is potentially prolonged (as with Ebola virus), with excretion of the virus in urine lasting for as long as six weeks and in semen for as long as three months. Nonetheless, the experience with cases exported to the more developed world indicate that the risk of person-to-person transmission is very low.

Exposure to the virus in the most highly endemic countries of Nigeria, Liberia, and Sierra Leone is frequent, with seroprevalence rates ranging from 7% to 20%. It is estimated that the number of cases may be as high as 300,000 per year and that these result in 5,000 deaths. The geographic overlap in some countries (e.g., Liberia) with Ebola virus makes clinical confusion with this viral infection an ongoing issue.

The usual incubation period is seven to 10 days, but it may be as short as three days and as long as 21 days. However, approximately four-fifths of infections are asymptomatic. In those with clinically evident illness, the severity may range from a mild illness with non-specific symptoms to full-blown hemorrhagic fever. While the overall mortality rate is only 1-2%, that of patients hospitalized in Africa is approximately 20%. The diagnosis is best made by PCR, when available, and patients are treated with ribavirin, also when available. Within the endemic areas, the differential diagnosis includes all the usual suspects, especially malaria, yellow fever, and Ebola virus infection.

In the endemic area, the major method of prevention is rodent control, although recent evidence suggests that it may be possible to develop a vaccine. Transmission to healthcare workers may occur, and the World Health Organization recommends the following precautions1:

“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 metre) 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).”

These recommendations are of relevance outside the endemic area since more than two dozen infections have been identified in returning travelers.


  1. World Health Organization. Lassa fever – Nigeria. Disease outbreak news. May 27, 2016. Available at: Accessed June 12, 2017.
  2. Brosh-Nissimov T. Lassa fever: Another threat from West Africa. Disaster Mil Med 2016;2:8.