Lassa Virus

Lassa fever

Lassa fever is a viral hemorrhagic fever (VHF) that is caused by infection with Lassa virus. It is endemic to a number of countries in West Africa, including Nigeria, Ghana, Guinea, Sierra Leone, and Liberia. In these countries, several hundred thousand infections and about 5,000 deaths occur each year. 

The reservoir for the Lassa virus is a rodent called the multimammate rat (Mastomys genus). The virus is transmitted to humans by contact with the excreta or body fluids of infected rodents or humans. The virus can also be transmitted by consumption of contaminated food or contact with contaminated household items. Person-to-person transmission of Lassa virus is rare, but can occur in health care settings if adequate precautions are not taken. 

The incubation period between exposure and onset of symptoms is 1 to 3 weeks. According to the World Health Organization 80% of people who contract the virus have no or only mild symptoms such as headache, fatigue, and low-grade fever. Those patients who develop severe illness can have respiratory distress, hypotension, proteinuria, vomiting, bleeding, edema, and shock. About 15%of people with severe Lassa disease die from the condition. Treatment with ribavirin lowers the fatality risk to less than 5% if started in patients during the first 6 days of illness. 

On October 28, 2024, the Iowa Department of Health and Human Services reported a person who recently returned to the state from Liberia had died after contracting Lassa fever. This was the ninth case of Lassa fever diagnosed in the United States since 1969. 

Lassa virus is designated as a category A pathogen by the National Institute of Allergy and Infectious Diseases. Biosafety level 4 (BSL-4) precautions are recommended for handling potentially infectious specimens. Real-time PCR has become the clinical standard for Lassa fever diagnosis. It can be performed rapidly on inactivated specimens under BSL-2 conditions. False negative reactions might occur due to the high degree of genetic diversity among viruses. 

References

Von Stein DL, Barger A, Hennenfent A, et al. Notes from the Field: Response to a Case of Travel-Associated Lassa Fever — Iowa, October–November 2024. MMWR Morb Mortal Wkly Rep 2025;74:194–196. DOI: http://dx.doi.org/10.15585/mmwr.mm7411a3

Raabe V and Koehler J. Laboratory Diagnosis of Lassa fever. J Clin Microbiol 2017;55(6):1629-1637. doi: 10.1128/JCM.00170-17