Overview
Lassa fever is a viral illness carried by the multimammate rat, a rodent common in and around homes in West Africa. People usually catch it from food or household items soiled with rat urine or droppings, and it can also pass between people through body fluids.
Most infections are mild. A minority become severe, with bleeding, facial swelling and shock, and WHO reports that hearing loss can follow even a mild illness.
Disease Summary
- Category
- Infectious Disease
- Cause
- Lassa virus
- Transmission
- Contact with food or items soiled by rodent urine or droppings
- Body System
- Multiple organ systems
- Preventable
- Yes
- Curable
- No
Symptoms
Common Symptoms
WHO states that the illness begins 2 to 21 days after exposure. Roughly four out of five infections are mild or cause no symptoms at all.
Fever
The illness usually begins gradually with fever, which is why it is so often mistaken for malaria at first.
General Weakness and Malaise
WHO states onset typically begins with fever, general weakness, headache and a sense of being unwell.
Headache
Headache is present from the start in most cases and can become severe.
Sore Throat and Muscle Pain
WHO lists throat soreness and muscle pain as the illness progresses.
Chest Pain
Chest pain is listed by WHO among the features of progressing illness.
Nausea, Vomiting and Diarrhoea
Stomach and bowel symptoms are common as the illness advances.
Cough
A cough may develop alongside the other symptoms.
Abdominal Pain
Pain in the abdomen is listed by WHO among the symptoms of the developing illness.
Danger Signs
Seek urgent medical attention immediately if any of the following symptoms develop.
Bleeding
Bleeding from the gums, nose, eyes or elsewhere is listed first by CDC among severe symptoms and means immediate hospital care.
Facial Swelling
Swelling of the face is listed by both WHO and CDC as a feature of severe disease.
Breathing Difficulty
WHO lists fluid collecting in the lung cavity among the severe features.
Shock
Collapse with a weak pulse and cold skin is a medical emergency.
Sudden Hearing Loss
WHO reports sudden hearing loss in up to 25% of recovered patients, and CDC reports some degree of deafness in about one in three cases. It can follow mild illness too.
Confusion or Other Neurological Signs
WHO lists neurological signs among the features of severe disease and among the lasting effects in survivors.
Diagnosis
Exposure and Travel History
Illness beginning 2 to 21 days after being in an endemic area, or after contact with a suspected case or their body fluids, raises suspicion immediately.
Recognising the Pattern of Deterioration
CDC describes progression from dry symptoms of fever and aches to wet symptoms of diarrhoea, vomiting and bleeding as the marker that illness is becoming severe.
Excluding Malaria First
WHO states Lassa fever is hard to distinguish from malaria, typhoid fever, shigellosis, yellow fever and other viral haemorrhagic fevers, and malaria must be tested for in every case.
Referral to a Designated Treatment Centre
WHO advises that health workers in endemic areas keep a high index of suspicion during epidemic seasons and refer suspected cases to designated treatment centres.
Testing Under Containment
WHO states that samples from patients are an extreme biohazard risk, and that testing on non-inactivated samples must be done under maximum biological containment.
Laboratory Tests
Malaria Test
Done first in every febrile patient in an endemic area, because malaria is common, looks the same early on, and can kill quickly if missed.
Reverse Transcriptase PCR
The test that confirms acute Lassa fever by detecting the genetic material of the virus. It is performed in a high-containment laboratory.
Antibody ELISA
Detects the antibodies the body makes against Lassa virus. Named by WHO among the confirmation methods.
Antigen Detection Test
Detects viral protein during active infection. Named by WHO among the confirmation methods.
Virus Isolation by Cell Culture
The most definitive method and the slowest. It requires maximum containment facilities.
Treatment
Early Intensive Supportive Care
WHO states that early intensive supportive care including fluid management and treatment of specific symptoms improves the chance of survival. This is the mainstay of treatment.
Ribavirin
CDC states that this antiviral has been used to treat patients successfully and is most effective when given soon after symptoms start. WHO is more cautious, stating that no antiviral is approved for Lassa fever and that there are considerable uncertainties about how well ribavirin works.
Care in a Designated Treatment Centre
Treatment centres combine the fluid and organ support severe cases need with the infection control that protects other patients and staff.
Infection Prevention and Control
WHO advises hand hygiene, respiratory hygiene, personal protective equipment and safe injection practices, with extra precautions to prevent contact with blood and body fluids.
Hearing Assessment and Follow-Up
Because hearing loss is common and often permanent, hearing should be checked during recovery so that support can be arranged early.
Prevention
Store grain and food in rodent-proof containers
WHO lists this first among community prevention measures, because contaminated food is the main route of infection.
Keep the household clean and dispose of rubbish away from the home
WHO advises disposing of waste far from the house so that rodents are not drawn towards living and cooking areas.
Cook food thoroughly
WHO lists safe food preparation, including thorough cooking, among the core preventive measures.
Keep rodents out of living and sleeping areas
Blocking entry points and keeping food covered reduces the contamination of surfaces and utensils.
Infection prevention in health facilities
WHO states person-to-person spread happens particularly in health facilities without adequate infection prevention and control, making this a core protective measure for health workers.
Seek care early during outbreak season
WHO advises a high index of suspicion during epidemic seasons and prompt referral of suspected cases.
Risk Factors
People Living in Endemic Areas
WHO names Benin, Ghana, Guinea, Liberia, Mali, Nigeria and Sierra Leone as endemic, and states the virus probably exists in other West African countries too.
- Households where grain and food are stored in the open
- Homes where rodents enter living and cooking areas
- Communities where rodents are hunted or eaten
- Rural areas during the dry-season peak
Health Workers
WHO states that person-to-person transmission happens particularly where infection prevention and control is inadequate.
- Staff caring for patients before Lassa fever is suspected
- Laboratory staff handling untreated samples
- Facilities without reliable personal protective equipment
- Anyone in contact with the blood, urine or other fluids of a case
Pregnant Women
WHO reports that outcomes in pregnancy, particularly late pregnancy, are very poor.
- Women in the third trimester, where WHO reports fetal death rates can exceed 80%
- WHO reports maternal death rates in the third trimester can exceed 30%
- Fever in pregnancy in an endemic area needs urgent assessment
- Delivery care must include full infection prevention measures
Complications
Hearing Loss and Deafness
WHO reports sudden hearing loss in up to 25% of recovered patients. CDC reports some degree of deafness in about one in three cases, often permanent, and notes it follows mild as well as severe infection.
Bleeding and Shock
Both are listed among the features of severe disease and both are emergencies.
Fluid in the Lungs and Facial Swelling
WHO lists both among the severe features of the illness.
Loss of Pregnancy and Maternal Death
WHO reports third-trimester fetal death rates that can exceed 80% and maternal death rates that can exceed 30%.
Neurological Signs and Visual Impairment
WHO lists both among the lasting effects reported in survivors.
Joint Pain and Hair Loss
WHO lists these among the after-effects reported during recovery.
When to Seek Medical Care
Fever in an Endemic Area
Fever with severe headache, muscle or joint pain and weakness, starting 2 to 21 days after possible exposure, needs assessment the same day.
As Early as Possible
WHO states that early intensive supportive care improves survival, and CDC states ribavirin works best when given soon after symptoms start.
Any Bleeding, Facial Swelling or Breathing Difficulty
These mark severe disease and require emergency hospital care.
Any Sudden Change in Hearing
Hearing loss can appear during or after the illness, including after a mild one, and should be assessed straight away.
Fever During Pregnancy
Given the outcomes WHO reports in the third trimester, fever in pregnancy in an endemic area is an emergency.
Prognosis
Most Infections Are Mild
WHO reports that approximately 80% of human infections are mild or cause no symptoms at all.
Severe Disease Carries High Mortality
WHO states the overall case fatality rate is 1%, rising to 15% and above among patients hospitalised with severe disease.
Hearing Loss Is a Common Lasting Effect
WHO reports it in up to 25% of recovered patients and CDC in about one in three cases, often permanently.
Early Care Changes the Outcome
WHO states that early intensive supportive care with fluid management and treatment of symptoms improves the chance of survival.
Differential Diagnosis
Malaria
The most important alternative. It shares fever, headache and body aches and must be tested for in every case, because it is common and can kill quickly.
Typhoid Fever
Shares prolonged fever, headache and abdominal symptoms, and is separated by blood culture rather than by clinical features.
Shigellosis
Causes fever with bloody diarrhoea and abdominal pain, but lacks the facial swelling, lung fluid and bleeding of severe Lassa fever.
Yellow Fever
Another haemorrhagic illness with jaundice and bleeding, separated by specific testing and by vaccination history, since a yellow fever vaccine exists.
Other Viral Haemorrhagic Fevers
Including Ebola virus disease. These share the progression from fever and aches to vomiting, diarrhoea and bleeding, and are separated only by specific laboratory testing.
References
World Health Organization: Lassa fever
WHO fact sheet on transmission, symptoms, diagnosis, treatment and prevention.
CDC: About Lassa Fever
Overview of transmission routes, symptoms, hearing loss and treatment.
CDC: Screening and Evaluating an Ill Person for Viral Hemorrhagic Fevers
Exposure screening criteria and the progression of illness that marks deterioration.
WHO Regional Office for Africa: Lassa Fever
Regional overview of the burden of Lassa fever in West Africa.
Frequently Asked Questions
How do people catch Lassa fever?
WHO states the virus is passed to people mainly through contact with food or household items contaminated with the urine or droppings of the multimammate rat. It can also spread between people through blood, urine, faeces and other body fluids, particularly in health facilities without adequate infection control.
How dangerous is it?
WHO states the overall case fatality rate is 1%, but that among patients hospitalised with severe disease it is 15% and above. WHO also reports that approximately 80% of infections are mild or cause no symptoms.
Can Lassa fever cause permanent deafness?
Yes. WHO reports sudden hearing loss in up to 25% of recovered patients. CDC reports that about one in three cases experience some level of deafness, that it is often permanent, and that it occurs after mild infections as well as severe ones.
Is there a medicine or a vaccine?
WHO states there is no antiviral drug approved for Lassa fever and no licensed vaccine, though several candidate vaccines are in development. CDC states ribavirin has been used successfully and works best when given soon after symptoms start. WHO emphasises that early intensive supportive care improves survival.
How can I protect my household?
WHO advises storing grain and food in rodent-proof containers, keeping the household clean and disposing of rubbish far from the home, and preparing food safely including cooking it thoroughly. Keeping rodents out of food storage and sleeping areas is the central measure.