Lassa fever: Acute viral haemorrhagic illness caused by Lassa virus
Lassa fever: Viral hemorrhagic disease endemic in West Africa
Dr. Chioma Okonkwo (Ondo State, Nigeria) was working the night shift at the Infectious Disease Hospital in Ondo City when the ambulance arrived in January 2018. The patient, a 32-year-old pregnant woman named Blessing, was in her third trimester and critically ill. High fever, severe weakness, bleeding from her gums and nose, confusion. Her husband, desperate and terrified, explained she’d been sick for five days before seeking medical care.
“I examined Blessing and immediately suspected Lassa fever. The symptoms—fever, weakness, bleeding—fit the pattern. But what made my heart sink was that she was 32 weeks pregnant. Lassa fever in late pregnancy is devastating—over 80% of mothers die, and even those who survive usually lose their babies. I’d seen it before, watched pregnant mothers and their unborn children die from this terrible disease,” Dr. Okonkwo recalled.
“We immediately isolated Blessing, started supportive care—intravenous fluids, oxygen, monitoring vital signs. We collected blood samples and sent them to the national reference laboratory in Lagos for testing. We contacted the State Ministry of Health, who notified WHO and the Nigeria Centre for Disease Control. Within hours, we had additional personal protective equipment, infection control supplies, and technical guidance. But for Blessing, it was already too late,” Dr. Okonkwo explained.
“The laboratory results came back positive for Lassa virus. By then, Blessing had deteriorated further—her blood pressure dropped dangerously low, she developed seizures, went into a coma. Despite everything we did, she died 36 hours after admission. Her baby died with her. Her husband collapsed when I told him. He’d lost his wife and unborn child to Lassa fever—a preventable disease transmitted by rats contaminating food with their urine and feces. It was heartbreaking and completely preventable with early detection and treatment,” Dr. Okonkwo continued, her voice breaking.
According to WHO, 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 faeces 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.
For more on viral hemorrhagic fevers, see our articles on infectious diseases and outbreak response at ObserverVoice.com.
Transmission and Clinical Features
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.
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. The incubation period of Lassa fever ranges from 2–21 days. The onset, 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 recovered patients. 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 in fatal cases. The disease is severe late in pregnancy, with maternal death and/or foetal loss in more than 80% of cases during the third trimester.
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, malaria, shigellosis, typhoid fever and yellow fever.
WHO provides Lassa fever fact sheet and Latest Lassa fever outbreak news. WHO offers Introduction to Lassa fever training. Related topics include Crimean-Congo haemorrhagic fever, Ebola virus disease, Nipah virus infection, and Rift Valley fever. For more on disease surveillance, see our article on epidemic preparedness at ObserverVoice.com.
Treatment and Prevention
There is currently no licensed vaccine for Lassa fever, but several potential vaccines are in development. Despite not being licensed as a treatment for Lassa fever the antiviral drug ribavirin has been used in several countries as a therapeutic agent. However, there is still a need for a safe treatment with proven efficacy, and a range of potential treatments including immune therapies and drug therapies are currently being evaluated. No evidence supports the role of ribavirin as a treatment for Lassa fever.
Prevention of Lassa fever relies on promoting good “community hygiene” to discourage rodents from entering homes. Effective measures include storing 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.
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. Health care staff should apply standard infection prevention and control precautions.
WHO’s R&D Blueprint and R&D Blueprint and Lassa Fever coordinate research. WHO provides Risk communications guidance.
Publications and Guidelines
December 2019 WHO published Introduction to Lassa fever. February 2018 WHO published How to safely collect blood samples by phlebotomy from patients suspected to be infected with Lassa fever. February 2018 WHO published How to safely ship human blood samples from Lassa cases within a country by road, rail and sea. December 2014 WHO published Interim infection prevention and control guidance for care of patients with suspected or confirmed filovirus haemorrhagic fever.
August 2024 WHO published Arenaviridae – Landscape of vaccines and therapeutics licensed or under development. January 2019 WHO published Lassa Fever Research and Development (R&D) Roadmap. September 2018 WHO published Disease Commodity Packages – Lassa fever. May 2018 WHO published Lassa Fever R&D Roadmap – DRAFT FOR PUBLIC COMMENT.
January 2016 WHO published Clinical management of patients with viral haemorrhagic fever: A pocket guide for front-line health workers. June 2016 WHO published Rapid case management form. July 2009 WHO published Hand Hygiene: Why, How & When?. October 2007 WHO published Standard precautions in health care. December 2019 WHO published WHO outbreak communication planning guide. July 2012 WHO published Communication for behavioural impact (COMBI) toolkit.
WHO Activities and Feature Stories
WHO works on Prioritizing diseases for research and development in emergency contexts, Streamlining operational R&D response during outbreaks, Developing norms and standards in the epidemic context, Improving coordination and fostering an enabling environment, and Accelerating R&D processes.
April 2018 WHO featured On the trail of Lassa fever in southern Nigeria. March 2018 WHO featured On the frontlines of the fight against Lassa fever in Nigeria. WHO provides infographics showing Geographic distribution of Lassa fever in West African affected countries and Lassa Fever infographic.
Hope Through Prevention and Early Treatment
Dr. Okonkwo’s experience with Blessing’s death galvanized her commitment to Lassa fever prevention and control. She became a vocal advocate for community education, healthcare worker training, and improved diagnostic capacity in Nigeria.
“After Blessing died, I couldn’t stop thinking about how preventable her death was. If she’d sought care earlier—within the first 2-3 days of symptoms—we could have started supportive treatment immediately. Early rehydration and symptom management dramatically improve survival. If our community had better knowledge about Lassa fever—recognizing symptoms, seeking care promptly, preventing rodent contamination of food—Blessing might still be alive,” Dr. Okonkwo explained.
“I partnered with the State Ministry of Health and WHO to launch community education campaigns. We teach families to store food in rodent-proof containers, dispose of garbage properly, maintain clean homes, and keep cats to control rodent populations. We emphasize that Mastomys rats—the animals that carry Lassa virus—are everywhere in West Africa. You can’t eliminate them, but you can prevent them from contaminating your food and living spaces,” Dr. Okonkwo continued.
“We also train healthcare workers on infection prevention and control. Person-to-person transmission happens in healthcare settings when staff don’t use proper protective equipment or follow standard precautions. Several Nigerian healthcare workers have died from Lassa fever contracted while caring for patients. WHO’s guidance on safe blood sample collection, infection control measures, and early case detection has been essential. WHO helped Nigeria establish specialized Lassa fever treatment centers with isolation facilities, trained staff, and proper equipment,” Dr. Okonkwo emphasized.
“The 2018 Lassa fever outbreak in Nigeria was the worst on record—633 confirmed cases, 171 deaths. But WHO’s support—coordinating response efforts, providing technical guidance, supplying diagnostic equipment, training healthcare workers—helped Nigeria mount an effective response. Case-fatality rates declined as more patients received early supportive care. WHO’s R&D Blueprint is accelerating vaccine and treatment development. Several promising vaccines are in clinical trials. One day, we may be able to vaccinate people in endemic areas, preventing Lassa fever entirely,” Dr. Okonkwo noted.
“Blessing’s death wasn’t in vain if it motivates action. Every pregnant woman in West Africa deserves to know that Lassa fever is extremely dangerous during pregnancy—80% maternal death rate in the third trimester. Every family deserves education on preventing rodent contamination. Every healthcare facility deserves proper infection control supplies. WHO’s leadership on Lassa fever—research coordination, outbreak response, prevention guidance, healthcare worker training—is saving lives across West Africa. My mission now is ensuring Blessing’s story drives the changes needed to prevent other mothers from dying needlessly from this preventable disease.”
For more information, visit WHO’s Lassa fever topic page or explore related content at ObserverVoice.com.
Frequently Asked Questions (FAQs)
Lassa fever is acute viral haemorrhagic illness caused by Lassa virus, member of arenavirus family of viruses. Humans usually become infected with Lassa virus through exposure to food or household items contaminated with urine or faeces of infected Mastomys rats. Disease is endemic in 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 absence of adequate infection prevention and control measures. About 80% of people who become infected with Lassa virus have no symptoms. 1 in 5 infections result in severe disease, where virus affects several organs such as liver, spleen and kidneys. WHO provides Lassa fever fact sheet and Introduction to Lassa fever training. Related: Crimean-Congo haemorrhagic fever, Ebola virus disease.
Incubation period of Lassa fever ranges from 2–21 days. Onset, when symptomatic, is usually gradual, starting with fever, general weakness and malaise. After 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 lung cavity, bleeding from mouth, nose, vagina or gastrointestinal tract and low blood pressure may develop. Shock, seizures, tremor, disorientation and coma may be seen in later stages. Deafness occurs in 25% of recovered patients. Death usually occurs within 14 days in fatal cases. Disease is severe late in pregnancy, with maternal death and/or foetal loss in more than 80% of cases during third trimester. Overall case-fatality rate is 1%. Among patients who are hospitalized with severe clinical presentation, case-fatality estimated at around 15%. Early supportive care with rehydration and symptomatic treatment improves survival. December 2019 WHO published Introduction to Lassa fever. January 2016 WHO published Clinical management pocket guide. Related: Nipah virus infection, Rift Valley fever.
There is currently no licensed vaccine for Lassa fever, but several potential vaccines are in development. Despite not being licensed as treatment for Lassa fever antiviral drug ribavirin has been used in several countries as therapeutic agent. However, there is still need for safe treatment with proven efficacy, and range of potential treatments including immune therapies and drug therapies are currently being evaluated. No evidence supports role of ribavirin as treatment for Lassa fever. Prevention of Lassa fever relies on promoting good “community hygiene” to discourage rodents from entering homes. Effective measures include storing foodstuffs in rodent-proof containers, disposing of garbage far from home, maintaining clean households and keeping cats. Because Mastomys rats are so abundant in endemic areas, it is not possible to eliminate them from environment. Family members should always be careful to avoid contact with blood and body fluids while caring for sick persons. August 2024 WHO published Arenaviridae vaccines and therapeutics landscape. January 2019 WHO published Lassa Fever R&D Roadmap. Related: Ebola virus disease.
Health-care workers seeing patient suspected to have Lassa fever should immediately contact local and national experts for advice and to arrange for laboratory testing. Health care staff should apply standard infection prevention and control precautions. Person-to-person infections and laboratory transmission can occur, particularly in health care settings in absence of adequate infection prevention and control measures. February 2018 WHO published How to safely collect blood samples from patients suspected to be infected with Lassa fever. February 2018 WHO published How to safely ship human blood samples from Lassa cases. December 2014 WHO published Interim infection prevention and control guidance for filovirus haemorrhagic fever. July 2009 WHO published Hand Hygiene: Why, How & When?. October 2007 WHO published Standard precautions in health care. Related: Crimean-Congo haemorrhagic fever.
WHO works on Prioritizing diseases for research and development in emergency contexts, Streamlining operational R&D response during outbreaks, Developing norms and standards in epidemic context, Improving coordination and fostering enabling environment, and Accelerating R&D processes. WHO’s R&D Blueprint and R&D Blueprint and Lassa Fever coordinate research. WHO provides Risk communications guidance and Latest Lassa fever outbreak news. April 2018 WHO featured On the trail of Lassa fever in southern Nigeria. March 2018 WHO featured On the frontlines of fight against Lassa fever in Nigeria. September 2018 WHO published Disease Commodity Packages – Lassa fever. December 2019 WHO published WHO outbreak communication planning guide. Related: Nipah virus infection, Rift Valley fever.
- WHO Lassa Fever Topic Page
- WHO R&D Blueprint – Lassa Fever
- Introduction to Lassa Fever Training
- Lassa Fever R&D Roadmap
- WHO Outbreak Communication Planning Guide
Disclaimer: This article is an adaptation of publicly available information from WHO’s Lassa fever
health topic page (WHO, Geneva. Licence: CC BYNC-SA 3.0 IGO). WHO is not responsible for the
content or accuracy of this adaptation. This content is for informational and educational purposes
only and does not constitute medical advice. ObserverVoice.com is a news and information platform
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