OVERVIEW
Lassa fever is caused by Lassa virus. It was first described in the 1950s (5) and isolated in 1969. It was isolated from two missionary nurses in Nigeria, West Africa (1). The virus was traced back to the town of Lassa, for which it was named (1). The virus is endemic to Africa (2). It can be found in Guinea, Liberia, Mali, Senegal, Sierra Leone, Zaire and Zorzor (3). It is zoonotic (2), being carried by a multimammate rat called Mastomys natelensis (1).
TRANSMISSIONS AND RESEVOIRS
A human can contract this disease from the Mastomys by direct of indirect contact. These rodents are prolific and often live in the houses with the people. Coming into contact with urine or feces from the animal or eating food that has been contaminated by the urine or feces can spread the disease. Also, the Mastomys is sometimes used as a food source. Eating an infected animal will also spread the disease, as will a rodent bite. Less commonly, fecal dust particles can be inhaled (4).
Human to human infection occurs through contact with an infected person‚ blood, tissue, secretions, or excretions. The worst epidemics occur in hospitals (1).
ETIOLOGICAL AGENT
Lassa virus is of the arenaviridae family (1). It is spherical and pleomorphic in shape. The replication occurs in the cytoplasm and budding at the plasma membrane. On part of the virus neutralizes antibodies in the host an danother part promotes acid dependent membrane fusion for viral entry (3). The incubation period is 6 to 21 days (5).
TESTING
During the febrile phase until 14 days after onset, the virus can be isolated from the blood or plasma. In acute cases, it can be detected by ELISA but only when the antibodies are not detectable. The ELISA is the most common diagnostic test used. By the third day of hospitalization, the viral RNA can be detected using reverse transcriptase PCR. However, this is mainly a research tool (1).
CLINICAL DIAGNOSES
The symptoms of Lassa virus vary a lot and are non-specific, making it difficult to diagnose clinically (1). The symptoms include: fever, retrosternal pain, abdominal pain, capillary leakage leading to bleeding from the gums, nose, GI and vagina, pericarditis, encephalitis, hepatitis, and seizures (3). After recovery, one-third of the infected have hearing loss which is, in most cases, permanent (3).
MECHANISMS
The pathogenesis of Lassa virus is not completely known. The virus begins by replicating at the site of infection. The greatest amount of the virus are found in the spleen, lymph nodes, and bone marrow. When the patient dies, it is usually from organ failure or shock, however there is not much internal damage to the body (3).
TREATMENT
Lass virus is treated with an antiviral drug called ribavirin (1-Dribofuranosy I-1,2,4-triazole-3-carboxamide, virazole). It is an IMP dehydrogenase inhibitor. It is important to start the drug therapy early in the disease (by 6 days of onset). Also, the patient must be maintained through fluid and oxygenation support (3). The main side effect of this drug is anemia (3). There is no vaccine for Lassa available yet, though research is ongoing. The most promising is via vaccina vectored Lass genes (4).
CONTROL
The best way to control the outbreaks of Lassa is through rodent control. Trapping and hygienic house-keeping are the best way to do this. Also, education about the danger of these rats will help with control and to decrease the possibility of them being eaten (1).
Hospitals are the main site of severe outbreaks (1). It is important to isolate the patients according to Viral Hemorrhagic Fever precautions and use proper barrier nursing techniques when treating the patient. Also, all objects that have been in contact with the patient as well as all body secretions should be disinfected using .5% sodium hypochlorite or .5% phenol solutions. If possible they should be sterilized by heat disinfection methods (1).
SOURCES
(1)Anon. Lassa Fever. April 1, 2002. http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/lassaf.htm visited on May 1, 2003.
(2) Anon. Lassa Fever. Unknown date. http://www.merlin.org.uk/template7.asp?PageID=119 visited on May 3, 2003
(3) Andrecheck, E. and Yan, B. Lassa Virus. December 18,1996. http://www.science.mcmaster.ca/Biology/Virology/18/lassa.htm Visited on May 1, 2003
(4) Harper, Tara. Lassa Fever: Description, Vectors, Mechanisms, Symptoms, Mortality Rates, and so on.. 2002. http://www.tarakharper.com/v_lassa.htm Visited on May 3, 2003
(5) WHO spokesperson. Lassa Fever. April 2002. http://www.who.int/inf-fs/en/fact179.html Visited on May 1, 2003</HTML