INTRODUCTION
Lassa fever is an acute viral
haemorrhagic illness caused by Lassa virus, a member of the arenavirus family
of viruses. It is transmitted to humans from contacts with food or household
items contaminated with rodent excreta. The disease is endemic in the rodent
population in parts of West Africa. Person-to-person infections and laboratory
transmission can also occur, particularly in the hospital environment in the
absence of adequate infection control measures. Diagnosis and prompt treatment
are essential.
The primary animal host of the Lassa
virus is the Natal multimammate mouse (Mastomys
natalensis), an animal found in most of sub-Saharan Africa.[3]
The virus is probably transmitted by contact with the feces or urine of animals
accessing grain
stores in residences.[3]
Given its high rate of incidence, Lassa fever is a major problem in affected
countries.
Lassa fever occurs commonly in West Africa. It results in 300,000 to 500,000
cases annually and causes about 5,000 deaths each year. Outbreaks of the
disease have been observed in Nigeria, Liberia, Sierra
Leone, Guinea, and
the Central African Republic.
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.
Clinically, Lassa
fever infections are difficult to distinguish from other viral hemorrhagic
fevers such as Ebola
and Marburg,
and from more common febrile
illnesses such as malaria.
The virus is excreted in urine for 3-9
weeks and in semen for three months.
Lassa virus
is zoonotic
(transmitted from animals), in that it spreads to humans from rodents,
specifically multimammate mice (Mastomys natalensis).[8]
This is probably the most common mouse in equatorial Africa, ubiquitous in
human households and eaten as a delicacy in some areas.[8]
In these rodents,
infection is in a persistent asymptomatic
state. The virus is shed in their excreta (urine and feces), which can be
aerosolized. In fatal cases, Lassa fever is characterized by impaired or
delayed cellular immunity leading to fulminant
viremia.
Infection in humans typically occurs by
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 also been
established, presenting a disease risk for healthcare workers. Frequency of
transmission by sexual contact has not been established.
A range of laboratory investigations
are performed to diagnose the disease and assess its course and complications.
An ELISA test
for antigen and IgM
antibodies give 88% sensitivity and 90% specificity for the presence of the
infection. Other laboratory findings in Lassa fever include lymphopenia
(low white blood cell count), thrombocytopenia
(low platelets), and elevated aspartate aminotransferase levels in the
blood. Lassa fever virus can also be found in cerebrospinal fluid.[9]
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.[10]
In cases with abdominal pain, diagnoses in countries where Lassa
is endemic are often made for other illnesses, such as appendicitis
and intussusception, delaying
treatment with ribavirin.
Lassa fever is endemic in Nigeria. It
was first discovered in a town called Lassa in Borno state in 1969. It was from
the name of the town that the disease got its name.
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