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Lassa fever is a type of viral hemorrhagic fever caused by the Lassa virus. Many, but not all, of those infected by the virus do not develop symptoms. The disease is usually first spread to people via contact with the urine or feces of an infected multimammate mouse; there is currently no vaccine available.
Signs and symptoms
Onset of symptoms is typically 7 to 21 days after exposure. These symptoms may include fever, tiredness, weakness, and headache. In 20 percent of people more severe symptoms such as bleeding gums, breathing problems, vomiting, chest pain, or very low blood pressure may occur.
Lassa viruses are enveloped, single-stranded, bisegmented, ambisense RNA viruses. Their genome is contained in two RNA segments that code for two proteins each, one in each sense, for a total of four viral proteins.
Lassa virus commonly spreads to humans from other animals, specifically the natal multimammate mouse or African rat, also called the natal multimammate rat, Mastomys natalensis. One of the most common mouse in equatorial Africa, common in human households and eaten as a delicacy in some areas.
In terms of the mechanism of Lassa fever, currently the chain of events that occur during disease development is not known. However, according to one review in 2012, a possible pathogenesis could be infection-triggered induction of uncontrolled cytokine expression. According to the same review the hypothesis would be supported by a case in which - proinflammatory cytokines, interferon (gamma) and tumor necrosis factor (alpha) all rose to very high levels before death of an infected individual with the virus.
The evaluation of this infection is done by enzyme-linked immunosorbent serologic assays, which detect certain antibodies; RT-PCR can be used as well to detect the disease. In cases with abdominal pain, in countries where Lassa is common, Lassa fever is often misdiagnosed as appendicitis and intussusception which delays treatment.
Treatment is directed at addressing dehydration and improving symptoms, all persons suspected of Lassa fever infection should be admitted to isolation facilities and their body fluids and excreta properly disposed of. The antiviral medication ribavirin has been recommended, but evidence to support its use is weak.
About 15 to 20 percent of hospitalized people with Lassa fever will die from the illness. The overall case fatality rate is estimated to be 1 percent, but during epidemics, mortality can climb as high as 50 percent.
Lassa high risk areas are near the western and eastern extremes of West Africa. As of 2018, the Lassa belt includes Guinea, Nigeria, Sierra Leone and Liberia. As of 2003, 10 to 16 percent of people in Sierra Leone and Liberia admitted to the hospital had the virus. The case fatality rate for those who are hospitalized for the disease is about 13 percent. 
An outbreak of Lassa fever occurred in Nigeria during 2018 and spread to 18 of the country's states; it was the largest outbreak of Lassa fever recorded. On 25 February 2018, there were 1081 suspected cases and 90 reported deaths; 317 of the cases and 72 deaths were confirmed as Lassa which increased to a total of 431 reported cases later in 2018.
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