Overview of Lassa Fever

Lassa has been spreading in Nigeria

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

Lassa, it is thought, leads to 5,000 deaths a year and between 100,000 to 300,000 infections every year. It is thought to lead to death in 1% of those who are infected.

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What Does Lassa Cause?

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, or back pain, vomiting, diarrhea, facial swelling, conjunctivitis, tremors, confusion, or encephalitis. Shock can occur. Some hearing loss occurs in one-third of those with symptoms. Those who have severe symptoms are at risk for death. Overall, about 1% 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 of 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?

Lassa fever 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, raising the question of whether it has spread there (though cross-reactive antibodies cannot always be ruled out).

The Sierra Leone hospital that became an early Ebola hospital in Kenema was first a Lassa hospital. In some parts of Liberia and Sierra Leone, as many as 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). The areas where Lassa is found are the areas where this rat is found. This exposure need not be directly to the rat; infection can also result from exposure to rat droppings, urine, or saliva. Person-to-person transmission may also occur after exposure to the Lassa virus in the blood, urine, feces, or other bodily secretions of an infected individual.

Can Lassa Spread in Hospitals?

Lassa can spread in the hospital. It can spread if protection, such as gloves and gowns, is 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.

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, however, 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 a fetus or infant appears to improve the health of a pregnant woman who has Lassa fever.

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. Antibody tests may also be used.

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

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

Is There a Vaccine?

There is currently no vaccine.

Is This Seen in the U.S.?

The last U.S. case was someone who returned from Liberia to New Jersey with Lassa. The diagnosis was delayed despite monitoring procedures due to the ongoing Ebola outbreak at the time.

It is rarely found in the U.S., and no transmission in the U.S. has ever been documented.

What Sort of Virus Is Lassa?

Lassa is an arenavirus. Its genome consists of two single-stranded RNA segments.

It is thought the virus may have resulted in changes in human 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.

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