Lassa Fever Virus

Epidemiology and History:

Lassa virus is a member of the Old World Arenaviruses, and, as such, is endemic in Western Africa. However, the full extent of Lassa’s endemicity is unknown due to poor mean of contact and communication with the rural villages where Lassa is surely seen, a factor that prevents both reporting and treatment of infection. Some researchers estimate the number of cases of Lassa infection at 100,000-300,000, though the number that are reported and treated in hospitals is significantly lower. The virus was first isolated in 1969 from a nurse in a Nigerian mission hospital. At least two cases of Lassa fever have occurred in the United States, the first in Chicago, the second in New Jersey. Both individuals who had recently been traveling in endemic areas of Africa, one in Nigeria, the other in Liberia, and succumbed to the fever after returning to the United States. Several incidences of infection in the lab have occurred with Lassa virus, as in the case of well-known virologist Dr. Jordi Casals, who became infected with the virus after studying samples from the original Nigerian case.


Lassa virus is typically spread through aerosolized virus particles, via either infected rodents (Mastomys natalensis) or close contact with infected individuals. Additionally, contact with infected bodily fluids, including blood, urine, and vomit, has been known to spread the virus. Virus particles have been isolated from the semen of infected individuals up to six weeks following acute symptoms, and transmission via sexual contact has been implicated in several cases of New and Old World arenaviruses.


The incubation period for Lassa virus is between 5 and 21 days, with symptoms typically appearing 10 days after infection.


Traditional laboratory tests provide little help in the way of diagnosis. Leukocyte levels and platelet counts are not useful means of diagnosis. Albuminuria is common. AST levels parallel the amount of virus in the blood, which is a useful factor in determining prognosis. The greater the amount of virus in the blood, the more likely the associated disease will be fatal. Chest X-rays may show some abnormalities, such as pleural effusions, but are most commonly normal. Lassa virus is easily isolated from the blood during the febrile stage of the illness, and CF, IFA, and ELISA may all be used for detecting viral antibodies.


Fever and malaise generally appear 10 days after infection. As the disease progresses, increased fever and myalgia are typical, accompanied by severe prostration. Gastro-intestinal manifestations, including nausea and vomiting, diarrhea, constipation, and abdominal pain may also appear. In 66% of individuals, sore throat accompanies infection. Cough and retrosternal pain are also common. Hemorrhagic symptoms develop is less than a third of individuals, but are associated with a significant raise in patient mortality. Neurologic phenomena are less common than the aforementioned symptoms, but are nevertheless important. Aseptic meningitis, encephalitis, and global encephalopathy with seizures have all been documented in cases of Lassa virus infection. Intriguingly, deafness is a common feature during late-stage disease or early convalescence, and may be either ephemeral or permanent. While serving as useful diagnostic tool, this manifestation also interrupts individuals’ re-acceptance into their community, and hence plays a role in disease stigmatization. When treated in a hospital setting, mortality rates are between 15% and 20%. However, this increases dramatically, up to 60%, in areas where appropriate medical care is unavailable. Fatal cases or Lassa fever rarely show any signs of remission, progressing from fever to shock and death in an unrelenting slide. Survivors remain symptomatic for approximately 2-3 weeks following the onset of symptoms, whereupon the fever dissipates and the virus becomes undetectable in the blood. (Note that virus has still been found in the semen of surviving patients up to six weeks following infection.)


Treatment for Lassa fever is largely symptomatic. Management of bleeding and hydration is critical, especially in hemorrhagic cases. Pain management is prescribed through the use of opiates. Due to the permeability of blood vessels, pulmonary edema is a concern, and fluid infusion must therefore be carefully monitored. Ribavirin has been employed principally in Lassa fever patients with poor prognoses, and is usually reserved for patients with an AST value above 150. Though a harsh drug with significant side-effects, ribavirin is the drug of choice in cases of Lassa fever.


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Jamie Dyal and Ben Fohner Stanford University Humans and Viruses Class of 2005