Lassa fever is a serious illness caused by the Lassa virus. People usually get infected through contact with the urine, feces, saliva, or nesting materials of infected multimammate rats (Mastomys natalensis), which often live in or around homes in West Africa. These rats are the natural carriers of the virus.
The disease is common in several West African countries, including Nigeria, Sierra Leone, Liberia, and Guinea, and smaller outbreaks have also occurred in nearby areas. Each year, it is estimated that between 100,000 and 300,000 people become infected, and about 5,000 people die. However, the true number may be higher because many cases go undiagnosed due to limited testing and because the symptoms are similar to other diseases like malaria and typhoid fever.
Lassa fever can also spread from person to person, especially in hospitals where proper protective gear and hygiene practices are not followed. This type of hospital-acquired infection has led to outbreaks among healthcare workers.
The symptoms of Lassa fever can vary widely. About 80% of infected people have no symptoms or only mild ones such as fever, tiredness, and headache. In more serious cases, the disease can cause swelling of the face, fluid in the chest or around the heart, internal bleeding, low blood pressure, trouble breathing, nervous system problems, and even failure of multiple organs. Pregnant women, especially in late pregnancy, are at much higher risk of death, and miscarriage is common. In hospitals, the death rate among seriously ill patients can be over 15%, and sometimes even higher during outbreaks.
The main treatment is an antiviral drug called ribavirin, which works best when given early in the illness. Patients also need supportive care, such as fluids, oxygen, and treatment for complications, to help them recover. Unfortunately, many health facilities in affected areas do not have easy access to this treatment.
Currently, there is no approved vaccine for general use, but several potential vaccines are being developed and tested.
Epidemiology
Lassa fever is endemic in several West African countries, including Nigeria, Sierra Leone, Liberia, and Guinea. It is estimated to infect 100,000 to 300,000 people annually, with approximately 5,000 deaths.
About 80% of Lassa fever infections are either mild or asymptomatic, meaning that many people who contract the virus may not show noticeable symptoms and may remain unaware they are infected. As a result, numerous cases go undetected and unreported, complicating efforts to understand the true scale of the disease.
Among those who do develop symptoms, the case fatality rate is estimated to be around 1–2%. However, this rate can rise significantly to 15–20% or higher among hospitalized patients, particularly those with severe complications.
Lassa fever cases tend to spike during the dry season, typically from November to April or May. During this time, rodents are more likely to enter homes in search of food and shelter due to reduced vegetation and environmental changes. This seasonal behavior increases the likelihood of human exposure to contaminated rodent waste, especially around food storage areas and living spaces.
Risk of infection is higher in rural and peri-urban areas, where people often live in closer contact with rodent habitats and housing structures tend to be less secure. Poorly constructed buildings, such as homes with cracks, rodent burrows, and multiple rooms, are more vulnerable to rodent infestation. These conditions make it easier for the virus to spread from rodents to humans, increasing the likelihood of transmission..
Causes and Transmission
Lassa fever is caused by the Lassa virus and spreads primarily through contact with infected rodents or their excretions. Human-to-human transmission also occurs, especially in healthcare settings. Various factors influence the risk of infection, including environmental and behavioral elements.
Source of Infection
The primary reservoir of the Lassa virus is the Mastomys rodent, commonly known as the multimammate rat. These rodents are often found in rural and urban households, especially in regions with poor housing infrastructure. They carry the virus asymptomatically and shed it through urine, feces, and saliva, which can contaminate food, cooking utensils, floors, and other surfaces.
Humans can become infected by ingesting contaminated food, inhaling dust particles mixed with rodent excreta, or through broken skin and mucous membranes (such as the eyes, nose, or mouth) after touching contaminated objects. Handling or preparing rodents for food, a practice in some communities, can also lead to direct exposure.
Modes of Transmission
The primary route of transmission is direct contact with rodent excretions or ingestion of contaminated food. For example, food items left uncovered can become contaminated with rodent urine or droppings, leading to infection when consumed.
Person-to-person transmission occurs through contact with bodily fluids such as blood, saliva, urine, vomit, or breast milk from an infected person. This route is particularly concerning in healthcare environments, where exposure to infected patients without proper use of personal protective equipment (PPE) increases the risk for healthcare workers.
The virus can also be transmitted through contaminated medical equipment, including reuse of needles, surgical tools, or accidental needle-stick injuries. Though airborne transmission through aerosolized particles has been suggested in laboratory settings, it is not considered a major route of infection under normal circumstances.
Risk Factors
Individuals living in poorly constructed homes that allow easy rodent access such as buildings with cracks, open eaves, or holes are at increased risk. Rural populations are particularly vulnerable due to frequent interaction with natural rodent habitats during farming, hunting, or bush clearing activities.
Improper food storage (e.g., storing food on the ground or in open containers) and inadequate waste disposal practices further raise the risk of household contamination. In some regions, consumption of rats as food may also contribute to exposure.
Healthcare workers are at high risk of infection, especially when caring for patients without adequate PPE or in facilities with limited infection control measures. Outbreaks among medical staff have been documented in multiple Lassa-endemic regions.
Other high-risk groups include immunocompromised individuals, pregnant women especially in the third trimester and people living in crowded conditions with poor access to clean water, sanitation, and healthcare services. These factors contribute to both the spread and severity of the disease in endemic areas.
Signs and Symptoms
Lassa fever presents with a range of clinical features that vary from mild to severe. Symptoms often develop gradually, and specific signs help distinguish the infection in affected individuals.
Incubation Period
The incubation period for Lassa fever—the time between exposure to the virus and the onset of symptoms—usually ranges from 6 to 21 days. During this phase, the infected person may appear healthy but can start shedding the virus, particularly in bodily fluids such as blood, saliva, and urine.
Because no symptoms are present during this time, individuals are unaware they are infected, which complicates early detection and increases the risk of unintentional virus transmission to others. The length of the incubation period may vary depending on how much virus a person is exposed to and how well their immune system responds.
Signs and Symptoms
The initial symptoms of Lassa fever are often nonspecific and may resemble those of common viral infections. These include:
- Fever
- General weakness or malaise
- Headache
- Sore throat
- Cough
- Muscle and joint pain
- Chest pain
These early symptoms often appear gradually and can last for several days, making Lassa fever difficult to distinguish from other endemic illnesses during the early stages.
In some individuals, especially those with weakened immune systems, the disease may progress to a more severe stage involving:
- Facial swelling
- Bleeding from gums, nose, or eyes (hemorrhaging)
- Difficulty breathing
- Abdominal pain
- Nausea, vomiting, and diarrhea
- Low blood pressure (hypotension)
- Fluid accumulation in the lungs or abdomen
Neurological symptoms may develop as the disease worsens. These can include:
- Tremors
- Confusion
- Seizures
- Disorientation
Complications
A major complication of Lassa fever is hearing loss, which affects roughly one-third of survivors. In some cases, the hearing impairment is permanent, even in patients who had only mild symptoms. Other long-term effects may include neurological issues, fatigue, and mood disturbances.
Severe cases can lead to multi-organ failure, most commonly involving the liver, kidneys, heart, and lungs. These complications significantly increase the risk of fatal outcomes, especially in patients who do not receive early medical intervention.
Pregnant women are at particularly high risk, especially during the third trimester. Infections in pregnancy often result in fetal loss, and maternal death rates are significantly elevated in this group.
Diagnosis of Lassa Fever
Diagnosis relies on recognizing the symptoms, confirming infection with laboratory tests, and excluding other diseases with similar presentations.
Clinical Assessment
The symptoms of Lassa fever include fever, malaise, headache, and muscle pain. Symptoms often appear 1 to 3 weeks after exposure. Early signs are non-specific, making clinical diagnosis challenging.
Additional symptoms may include sore throat, vomiting, diarrhea, and chest or abdominal pain. In severe cases, hemorrhaging, respiratory distress, and neurologic signs such as seizures may develop. A detailed patient history, including exposure to rodents or endemic regions, is essential.
Healthcare providers should monitor vital signs and signs of bleeding. Clinical suspicion should be heightened in cases of unexplained febrile illness within endemic areas or travel history to those regions.
Laboratory Testing
Laboratory confirmation uses several diagnostic methods. The most common tests are:
- Reverse Transcription Polymerase Chain Reaction (RT-PCR): Detects viral RNA in blood, throat swabs, or tissue samples. It is highly sensitive and specific, especially effective in the early (acute) phase of illness when viral loads are highest.
- Enzyme-Linked Immunosorbent Assay (ELISA): Used to detect Lassa-specific antibodies (IgM and IgG) or viral antigens. IgM indicates a recent infection, while IgG suggests past exposure. Antigen detection ELISAs can also assist in early diagnosis.
- Virus Isolation: Involves culturing the virus from blood or tissue samples, but this method is rarely used due to its high biosafety risk and is limited to Biosafety Level 4 (BSL-4) laboratories.
Routine laboratory findings, while nonspecific, can support clinical suspicion. These may include:
- Leukopenia (low white blood cell count)
- Thrombocytopenia (low platelet count)
- Elevated liver enzymes (e.g., AST, ALT)
- Proteinuria (protein in the urine)
All blood samples must be handled under strict biosafety protocols to protect laboratory personnel and prevent accidental exposure. Testing should be conducted in designated reference laboratories capable of handling viral hemorrhagic fevers.
Differential Diagnosis
Lassa fever shares symptoms with several other endemic diseases, making differential diagnosis a critical step in clinical evaluation. Diseases that must be considered and ruled out include:
- Malaria
- Typhoid fever
- Yellow fever
- Dengue fever
- Ebola virus disease and other viral hemorrhagic fevers
- Sepsis of unknown origin
The process of distinguishing Lassa fever from these conditions depends on:
- Exposure history (e.g., rodent contact, travel to endemic areas)
- Seasonal patterns
- Epidemiological context (e.g., ongoing outbreaks)
- Specific clinical features
- Laboratory confirmation
Failure to correctly diagnose Lassa fever can delay life-saving treatment and may lead to further spread of the virus, especially in healthcare settings.
Treatment and Management
Effective treatment of Lassa fever involves a combination of targeted antiviral medications, supportive therapies, and careful monitoring of complications.
Antiviral Therapy
Ribavirin remains the cornerstone of antiviral treatment for Lassa fever. It is most effective when administered within the first six days of symptom onset, as it inhibits viral replication and can significantly reduce the risk of death. Treatment typically begins with a high intravenous loading dose, followed by oral or continued IV doses over a 10-day course.
Dosing must be adjusted based on patient-specific factors such as renal or hepatic function, especially in those with pre-existing conditions or developing complications. Ribavirin can cause adverse effects, including hemolytic anemia, liver enzyme elevations, and gastrointestinal discomfort. Therefore, regular monitoring of blood counts and liver function tests is necessary throughout the treatment period.
Currently, there are no other approved antiviral medications specifically targeting Lassa virus.
Supportive Care
Supportive care addresses dehydration, fever, and electrolyte imbalances. Intravenous fluids are commonly given to maintain hydration and correct electrolyte disturbances.
Antipyretics like acetaminophen are used for fever and pain relief, while NSAIDs are generally avoided due to their association with increased bleeding risk. Nutritional support is important to counter the catabolic effects of systemic infection, and oxygen supplementation may be needed for patients with respiratory distress.
Infection control measures including isolation, use of personal protective equipment (PPE), and strict hygiene protocols are vital to prevent nosocomial transmission during the care of infected individuals.
Management of Complications
Lassa fever can lead to life-threatening complications such as internal bleeding, septic shock, acute kidney injury, and multi-organ dysfunction. These cases require intensive supportive therapy in specialized or well-equipped facilities.
Severe hemorrhagic presentations may necessitate blood transfusions, while acute kidney injury could require renal replacement therapy such as dialysis. Bacterial superinfections—common in hospitalized patients—should be promptly treated with empiric broad-spectrum antibiotics, pending culture results.
Neurological complications, such as encephalopathy or hearing loss, are not uncommon and may appear during the recovery phase. These require neurologic assessment and rehabilitative care. Permanent hearing impairment is a known sequela of Lassa fever, affecting a significant proportion of survivors, particularly those with severe disease. Early detection of complications improves survival chances through targeted supportive measures.
Prevention and Control
Effective prevention and control of Lassa fever depend on reducing exposure to the rodent host, improving hygiene practices, and early identification of cases.
Public Health Strategies
- Encourage communities to store food and water in tightly sealed, rodent-proof containers to prevent contamination by Mastomys rats.
- Promote regular cleaning of homes, kitchens, and surrounding areas to reduce food scraps and shelter sources that attract rodents.
- Advocate for safe garbage disposal practices, discouraging open dumping and promoting covered bins to limit rodent access.
- Raise awareness about sealing holes in walls, floors, and roofs to block entry points for rodents into homes.
- Recommend clearing bushy surroundings and avoiding stacking firewood near living areas, which serve as hiding spots for rodents.
- Deploy community-based rodent control programs including strategic placement of traps and use of approved rodenticides.
- Establish robust surveillance systems to detect and report Lassa fever cases promptly for rapid containment.
- Train local health workers and community volunteers to identify early warning signs and report suspected outbreaks.
Hospital and Community Precautions
- Enforce the use of personal protective equipment (PPE) such as gloves, face masks, gowns, and goggles for healthcare workers handling suspected Lassa cases.
- Mandate the isolation of confirmed and suspected patients in designated treatment units to prevent nosocomial transmission.
- Implement rigorous hand hygiene protocols using soap or alcohol-based hand rubs in all healthcare settings.
- Ensure safe and effective sterilization or disposal of contaminated medical instruments and hospital waste.
- Train health personnel in proper doffing and disposal techniques for PPE to avoid accidental exposure.
- Encourage caregivers at the community level to wear gloves and maintain hygiene when tending to sick relatives.
- Promote culturally sensitive but medically safe burial practices to prevent exposure to infectious body fluids.
- Educate the public on recognizing Lassa symptoms and seeking early medical help rather than resorting to self-treatment.
Vaccination Research
Currently, no licensed vaccine exists for Lassa fever, but several candidates are in development. Research focuses on live-attenuated, recombinant viral vector, and DNA-based vaccines.
Preclinical trials have shown promising immune responses in animals, and phase 1 human trials are underway to assess safety and efficacy. Efforts aim to produce a vaccine suitable for endemic regions.
Funding and collaboration among international health organizations and research institutions are critical to accelerate vaccine development and availability.