|Synonyms||Lassa hemorrhagic fever|
|Community education material for Lassa fever|
|Symptoms||Fever, headaches, bleeding |
|Usual onset||1–3 weeks following exposure |
|Causes||Lassa virus |
|Risk factors||Exposure to rodents in West Africa |
|Diagnostic method||Laboratory testing |
|Differential diagnosis||Ebola, malaria, typhoid fever |
|Treatment||Supportive, ribavirin |
|Prognosis||~1% risk of death |
|Frequency||~400,000 cases per year |
|Deaths||~5,000 deaths per year |
Lassa fever, also known as Lassa hemorrhagic fever (LHF), is a type of viral hemorrhagic fever caused by the Lassa virus.  Many of those infected by the virus do not develop symptoms.  When symptoms occur they typically include fever, weakness, headaches, vomiting, and muscle pains.  Less commonly there may be bleeding from the mouth or gastrointestinal tract.  The risk of death once infected is about one percent and frequently occurs within two weeks of the onset of symptoms.  Among those who survive about a quarter have hearing loss, which improves over time in about half. 
The disease is usually initially spread to people via contact with the urine or feces of an infected multimammate rat.  Spread can then occur via direct contact between people.  Diagnosis based on symptoms is difficult.  Confirmation is by laboratory testing to detect the virus's RNA, antibodies for the virus, or the virus itself in cell culture.  Other conditions that may present similarly include Ebola, malaria, typhoid fever, and yellow fever.  The Lassa virus is a member of the Arenaviridae family of viruses. 
There is no vaccine.  Prevention requires isolating those who are infected and decreasing contact with the rats.  Other efforts to control the spread of disease include having a cat to hunt vermin, and storing food in sealed containers.  Treatment is directed at addressing dehydration and improving symptoms.  The antiviral medication ribavirin may be useful when given early.  These measures improve outcomes. 
Descriptions of the disease date from the 1950s.  The virus was first described in 1969 from a case in the town of Lassa, in Borno State, Nigeria.   Lassa fever is relatively common in West Africa including the countries of Nigeria, Liberia, Sierra Leone, Guinea, and Ghana.   There are about 300,000 to 500,000 cases which result in 5,000 deaths a year. 
In 80% of cases, the disease is asymptomatic, but in the remaining 20%, it takes a complicated course. The virus is estimated to be responsible for about 5,000 deaths annually. The fever accounts for up to one-third of deaths in hospitals within the affected regions and 10 to 16% of total cases. 
After an incubation period of six to 21 days, an acute illness with multiorgan involvement develops. Nonspecific symptoms include fever, facial swelling, and muscle fatigue, as well as conjunctivitis and mucosal bleeding. The other symptoms arising from the affected organs are:
- Gastrointestinal tract
- Cardiovascular system
- Respiratory tract
- Nervous system
High risk groups include pregnant women and newborns. The latter may have an 87% fatality rate. The "Swollen baby syndrome" occurs in newborns, infants and toddlers with pitting edema, abdominal distension and hemorrhage. 
Lassa virus is a member of the Arenaviridae, a family of negative-sense, single-stranded RNA viruses.  Specifically it is an old world arenavirus, which is enveloped, single-stranded, and bi-segmented RNA. This virus has a both a large and a small genome section, with four lineages identified to date: Josiah (Sierra Leone), GA391 (Nigeria), LP (Nigeria) and strain AV. 
Lassa virus commonly spreads to humans from other animals, specifically the natal multimammate rat or African rat, also called the natal multimammate mouse (Mastomys natalensis).  This is probably the most common mouse in equatorial Africa, common in human households and eaten as a delicacy in some areas.  
The multimammate rat can quickly produce a large number of offspring, tends to colonize human settlements increasing the risk of rodent-human contact, and is found throughout the west, central and eastern parts of the African continent.  Once the rat has become a carrier, it will excrete the virus throughout the rest of its lifetime through feces and urine creating ample opportunity for exposure.  The virus is probably transmitted by contact with the feces or urine of animals accessing grain stores in residences. 
Individuals who are at a higher risk of contracting the infection are those who live in rural areas where Mastromys are discovered, and where sanitation is not prevalent. Infection typically occurs by direct or indirect exposure to animal excrement through the respiratory or gastrointestinal tracts. Inhalation of tiny particles of infectious material (aerosol) is believed to be the most significant means of exposure. It is possible to acquire the infection through broken skin or mucous membranes that are directly exposed to infectious material. Transmission from person to person has been established, presenting a disease risk for healthcare workers. The virus is present in urine for between three and nine weeks after infection, and it can be transmitted in semen for up to three months after becoming infected.  
A range of laboratory investigations are performed, where possible, to diagnose the disease and assess its course and complications. The confidence of a diagnosis can be compromised if laboratory tests are not available. One comprising factor is the number of febrile illnesses present in Africa, such as malaria or typhoid fever that could potentially exhibit similar symptoms, particularly for non-specific manifestations of Lassa fever.  In cases with abdominal pain, in countries where Lassa is common, Lassa fever is often misdiagnosed as appendicitis and intussusception which delays treatment with the antiviral ribavirin.  In West Africa, where Lassa is most prevalent, it is difficult for doctors to diagnose due to the absence of proper equipment to perform tests. 
The FDA has yet to approve a widely validated laboratory test for Lassa, but there are tests that have been able to provide definitive proof of the presence of the LASV virus.  These tests include cell cultures, PCR, ELISA antigen assays, plaque neutralization assays, and immunofluorescence essays. However, immunofluorescence essays provide less definitive proof of Lassa infection.  An ELISA test for antigen and Immunoglobulin M antibodies give 88% sensitivity and 90% specificity for the presence of the infection. Other laboratory findings in Lassa fever include lymphocytopenia (low white blood cell count), thrombocytopenia (low platelets), and elevated aspartate transaminase levels in the blood. Lassa fever virus can also be found in cerebrospinal fluid. 
Control of the Mastomys rodent population is impractical, so measures focus on keeping rodents out of homes and food supplies, encouraging effective personal hygiene, storing grain and other foodstuffs in rodent-proof containers, and disposing of garbage far from the home to help sustain clean households. Gloves, masks, laboratory coats, and goggles are advised while in contact with an infected person, to avoid contact with blood and body fluids. These issues in many countries are monitored by a department of public health. In less developed countries, these types of organizations may not have the necessary means to effectively control outbreaks.
Researchers at the United States Army Medical Research Institute of Infectious Diseases facility, where military biologists study infectious diseases, have a promising vaccine candidate.  They have developed a replication-competent vaccine against Lassa virus based on recombinant vesicular stomatitis virus vectors expressing the Lassa virus glycoprotein. After a single intramuscular injection, test primates have survived lethal challenge, while showing no clinical symptoms. 
All persons suspected of Lassa fever infection should be admitted to isolation facilities and their body fluids and excreta properly disposed of.
Early and aggressive treatment using ribavirin was pioneered by Joe McCormick in 1979. After extensive testing, early administration was determined to be critical to success. Additionally, ribavirin is almost twice as effective when given intravenously as when taken by mouth.  Ribavirin is a prodrug which appears to interfere with viral replication by inhibiting RNA-dependent nucleic acid synthesis, although the precise mechanism of action is disputed.  The drug is relatively inexpensive, but the cost of the drug is still very high for many of those in West African states. Fluid replacement, blood transfusion, and fighting hypotension are usually required. Intravenous interferon therapy has also been used.[ citation needed]
When Lassa fever infects pregnant women late in their third trimester, inducing delivery is necessary for the mother to have a good chance of survival.  This is because the virus has an affinity for the placenta and other highly vascular tissues. The fetus has only a one in ten chance of survival no matter what course of action is taken; hence, the focus is always on saving the life of the mother.  Following delivery, women should receive the same treatment as other Lassa fever patients.
Work on a vaccine is continuing, with multiple approaches showing positive results in animal trials. 
About 15–20% of hospitalized Lassa fever patients will die from the illness. The overall mortality rate is estimated to be 1%, but during epidemics, mortality can climb as high as 50%. The mortality rate is greater than 80% when it occurs in pregnant women during their third trimester; fetal death also occurs in nearly all those cases. Abortion decreases the risk of death to the mother.  Some survivors experience lasting effects of the disease,  and can include partial or complete deafness. 
The study of the epidemiology of Lassa fever is complicated by a lengthy incubation period, which may be up to three weeks.  Incubation periods as long as Lassa fever may affect spatial clustering of the disease by limiting the understanding of the incidence and distribution of the disease. The spatial clustering for this disease is still in development as a lack of easy-available diagnosis, limited public health surveillance infrastructure, and high clustering of incidence near high intensity sampling make for an incomplete look at the impact of Lassa in this region. 
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, 10-16% of people in Sierra Leone and Liberia admitted to hospital had the virus.  The case fatality rate for those who are hospitalized for the disease is about 15-20%. Research showed a twofold increase risk of infection for those living in close proximity to someone with infection symptoms within the last year.
The high risk areas cannot be well defined by any known biogeographical or environmental breaks except for the multimammate rat, particularly Guinea (Kindia, Faranah and Nzerekore regions), Liberia (mostly in Lofa, Bong, and Nimba counties), Nigeria (everywhere) and Sierra Leone (typically from Kenema and Kailahun districts). It is less common in the Central African Republic, Mali, Senegal and other nearby countries, and less common yet in Ghana and the Democratic Republic of the Congo. Benin had its first confirmed cases in 2014, and Togo had its first confirmed cases in 2016. 
As of 2013, the spread of Lassa outside of West Africa had been very limited. Twenty to thirty cases had been described in Europe, as being caused by importation through infected individuals.  These cases found outside of West Africa were found to have a high fatality risk because of the delay of diagnosis and treatment due to being unaware of the risk associated with the symptoms.  Imported cases have not manifested in larger epidemics outside of Africa due to a lack of human to human transmission in hospital settings. An exception had occurred in 2003 when a healthcare worker became infected before the patient showed clear symptoms. 
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 recorded.    As of February 25, 2018, there were 1081 suspected cases and 90 reported deaths; 317 of the cases and 72 deaths were confirmed as Lassa. 
The Lassa virus is one of several viruses identified by WHO as a likely cause of a future epidemic. They therefore list it for urgent research and development to develop new diagnostic tests, vaccines, and medicines.  
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