Overview of Lassa Fever

Lassa has been spreading in Nigeria

The symptoms of Lassa bring to mind Ebola. They are both viral hemorrhagic fevers. They are both found in West Africa. But Lassa, the disease fewer have heard of, has led to more deaths over the last decade.

Each person infected with Lassa is less likely to die. Lassa, though, kills more people; it infects more. 

Lassa, it is thought, leads to 5000 deaths a year and between 100,000 to 300,000 infections a year, every year. It is thought to lead to death in only 1% who are infected, though these figures may imply there are more infections (or fewer deaths or a higher mortality rate).

Officially, Ebola is reported to have led to over 11,300 deaths and 28,600 infections from December 2013 to December 2015.

What Does Lassa Cause?

Likely in West Africa, where it is found, most people infected with Lassa are not diagnosed. Most (80%) have mild symptoms: slight fever, fatigue, headache. It's the other 20% who have more worrisome symptoms. They may develop bleeding (mucosal - gums, nose), severe abdominal/chest/back pain, vomiting, diarrhea, facial swelling, conjunctivitis, protein in the urine, possibly confusion (and encephalitis), tremors. Shock can occur. Some hearing loss occurs in 1/3 of those with symptoms. Those who have the severe disease are at risk for death. About 1% overall die.

Those in the hospital have a higher chance of dying because they may have been sicker to have been brought to the hospital or may have had a higher degree of exposure. About 15-20% of hospitalized patients die. Many patients have difficulty accessing care and so many cases are not diagnosed, even if they have symptoms.

There are sometimes epidemics, which see much higher death rates - up to 50%, as the virus and its transmission are amplified.

Women who are pregnant are more likely to die. Women pregnant in their third trimester are particularly at risk. Their fetuses usually do not result in live births; 95% do not.

Where Is Lassa Found?

It was first diagnosed in a place called Lassa in Borno state, Nigeria. It was first identified in 1969 when two missionary nurses died of it.

It is now found in Nigeria, Sierra Leone, Liberia, and Guinea. A few cases have been reported in southern Mali, southern Burkina Faso, Ghana, and Côte d’Ivoire. Antibodies to the virus have been identified in people in Togo and Benin, which raise the question of whether it is there (but cross-reactive antibodies cannot always be ruled out).

The Sierra Leone hospital that became an early Ebola hospital in Kenema was a Lassa hospital. In some parts of Liberia and Sierra Leone, it may even be as many of 10%-16% of hospitalized patients have Lassa.

How Do You Get Lassa?

Lassa Fever develops 1-3 weeks after exposure. The exposure is usually to a "multimammate rat" (Mastomys natalensis). This exposure need not be directly to the rat; infection can also be due to exposure to rat droppings, urine, or saliva. The areas where Lassa is found are the areas where this rat is found.

Can Lassa Spread in Hospitals?

Lassa can spread in the hospital. It can spread if protection — like gloves and gowns are not used. It is not spread as easily as Ebola in hospitals. Contact with body fluids is needed for it to spread. It can also spread with needle sticks or if medical supplies are not properly disposed of after use or sterilized for reuse. Such reuse of medical materials may also happen outside of hospitals, leading to spread.

Dr. Khan, who died of Ebola after running the Lassa unit and later the Ebola unit at Kenema General Hospital, had taken his position after the previous doctor had died from Lassa. The previous doctor had a needlestick from a Lassa patient.

Is There Treatment?

Ribavirin, an antiviral drug, is used. It is most effective if given early. It is not a specific treatment for the virus and is not a cure-all drug.

Most treatment involves supportive management — making sure patients are hydrated and fed while providing oxygen and other treatment as needed.

Delivery of the fetus or infant appears to improve a pregnant mother's health.

Ribavirin has also been used as post-exposure prophylaxis. However, it will be difficult to ever fully study its effectiveness.

How Is It Diagnosed?

Diagnosis is based on PCR testing or also antibody tests are used.

It can be difficult to recognize Lassa clinically. Diagnosis will be initially through symptoms, patient history, and contacts. The symptoms, however, may be very non-specific and may be thought to be another febrile disease, like malaria even.

Enhanced case finding is important. Delay in presentation is associated with increased mortality.

Is There a Vaccine?

There is no vaccine.

Is This Seen in the US?

The last US case was someone who returned from Liberia to New Jersey with Lassa. The diagnosis was delayed despite monitoring procedures due to the on-going Ebola outbreak at the time.

What Sort of Virus Is Lassa?

Lassa is an arenavirus, Its genome consists of 2 single stranded RNA segments.

It is thought the virus may have resulted in changes in the human's genes passed along in Lassa-affected areas, much like how certain genes that reduce the severity of malaria have been passed along. There have also been a number of changes in what genetic material Lassa carries, as the virus has tried to evade host immune responses.

Are There Other Arenaviruses?

There are other rare hemorrhagic fever (HF) arenaviruses in South America: Junin (Argentine HF), Machupo (Bolivian HF), Guanarito (Venezuelan HF), Sabia (Brazilian HF), Chapare virus (in Bolivia)

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