Lassa Fever Virus

Lassa fever virus is an enveloped, single-stranded RNA virus that belongs to the Arenaviridae family. It is responsible for a viral hemorrhagic illness called Lassa fever, named after the first outbreak in the Nigerian town of Lassa in 1969.   

Lassa fever poses a significant public health concern in West Africa, where the virus is estimated to affect 100,000 to 300,000 people each year and is responsible for around 5,000 deaths. The highest incidence is in Sierra Leone, Liberia, Guinea, and Nigeria, but neighboring countries like Mali, Togo, and Benin are also at an increased risk. Over the past few decades, several outbreaks have occurred in non-endemic countries, including England, Germany, and the United States due to imported cases.

Lassa fever is a zoonotic disease, meaning that it is transmitted to humans through contact with infected animals. The reservoir animal of the Lassa virus (i.e., the natural host that harbors the pathogen and can transmit it to susceptible hosts) is a rodent known as the multimammate rat, which is endemic to the African continent. Multimammate rats are often found in rural areas and communities with poor sanitation or crowded living conditions, and they often live inside homes where they scavenge for food. 

Multimammate rats infected with the Lassa virus do not become ill, but they can shed the virus in their urine and feces. Transmission to humans can occur by handling infected rats, ingesting contaminated food or water, and inhaling air particles contaminated with the rodent’s feces (e.g., when dusting or sweeping the floor). In some areas, small rodents are consumed as food, which represents another mechanism of transmission.

The Lassa fever virus can also be transmitted from person to person through exposure to blood, saliva, and other bodily fluids of infected individuals. This mode of transmission is common in family members who take care of sick individuals at home, as well as in healthcare settings where protective equipment is not readily available. Transmission through contaminated medical equipment, such as reused needles or syringes, may also occur.

Signs and symptoms of Lassa fever typically begin 1 to 3 weeks after contact with the virus. Around 80% of people who become infected with the Lassa virus are asymptomatic or have mild symptoms, such as low-grade fever, weakness, and malaise, that often go unreported, contributing to the spread of the virus.  

After a few days of mild illness, around 20% of infected individuals develop more severe disease with high fever, headache, muscle pain, weakness, diarrhea, and vomiting. In severe cases, there may be bleeding of mucosal membranes like the nose or conjunctiva of the eye; gastrointestinal bleeding; and petechiae, which are superficial skin bleeds. Additional signs of severity include low blood pressure, facial and neck swelling, abdominal and chest pain, and respiratory distress. Neurological symptoms, like tremors, seizures, altered mental status, and coma may develop in critically ill individuals. In fatal cases, death typically occurs within 10 to 14 days of symptom onset due to multi-organ failure.  

The overall case-fatality rate of Lassa fever is approximately 1%. However, in people who are admitted to hospitals due to severe disease, mortality rises to 15%. Pregnant individuals, specifically during the third trimester, are particularly at risk of severe disease. 

In individuals who recover from the infection, the fever usually subsides in the following 1 to 3 weeks, although weakness and neurological symptoms may persist for months. The most common complication is deafness, which occurs in around 25% of individuals and can sometimes be permanent. 

Diagnosis of Lassa fever can be challenging due to the long incubation period of the virus and non-specific symptoms. Differential diagnosis includes other viral hemorrhagic fevers, such as yellow fever, malaria, typhoid fever, Ebola, and other viral and bacterial diseases. 

Definitive diagnosis requires testing with specific equipment that is only available in some laboratories. All samples should be handled with extreme caution and by trained personnel to avoid transmission. Diagnosis may be achieved with serologic tests, like ELISA, that look for the presence of antibodies against the Lassa virus in the blood. Another way to diagnose the infection is by identifying the Lassa virus in biological samples, which can be done by detecting its genetic material or isolating the virus in cell cultures.  

There is currently no specific treatment for Lassa fever other than supportive management, which may include rehydration with IV fluids, oxygen supplementation, and analgesic medications (e.g., acetaminophen or paracetamol), as needed. An antiviral medication called Ribavirin has shown some success in people with Lassa fever, especially if administered early in the disease course.  

Preventive measures, such as storing grains and other food in rodent-proof containers, disposing of garbage far from home, and maintaining clean households, may help reduce the presence of rodents at home. Consuming meat from rodents is also not advised due to the high risk of infection. Finally, person-to-person transmission can be prevented by wearing protective equipment (e.g., sterile gloves, face masks, gowns, and goggles) and using appropriate isolation precautions during infection.