Background
Though first described in the 1950s, the virus causing Lassa
disease was not identified until 1969. The virus is a single-stranded
RNA virus belonging to the virus family Arenaviridae.
About 80% of people who become infected with Lassa virus have
no symptoms. One in five infections result in severe disease, where the
virus affects several organs such as the liver, spleen and kidneys.
Lassa fever is a zoonotic disease, meaning that humans become
infected from contact with infected animals. The animal reservoir, or
host, of Lassa virus is a rodent of the genus Mastomys, commonly known
as the “multimammate rat.” Mastomys rats infected with Lassa virus do
not become ill, but they can shed the virus in their urine and faeces.
Because the clinical course of the disease is so variable,
detection of the disease in affected patients has been difficult.
However, when presence of the disease is confirmed in a community,
prompt isolation of affected patients, good infection protection and
control practices and rigorous contact tracing can stop outbreaks.
Symptoms of Lassa fever
The incubation period of Lassa fever ranges from 6-21 days.
The onset of the disease, when it is symptomatic, is usually gradual,
starting with fever, general weakness, and malaise. After a few days,
headache, sore throat, muscle pain, chest pain, nausea, vomiting,
diarrhoea, cough, and abdominal pain may follow. In severe cases facial
swelling, fluid in the lung cavity, bleeding from the mouth, nose,
vagina or gastrointestinal tract and low blood pressure may develop.
Protein may be noted in the urine. Shock, seizures, tremor,
disorientation, and coma may be seen in the later stages. Deafness
occurs in 25% of patients who survive the disease. In half of these
cases, hearing returns partially after 1-3 months. Transient hair loss
and gait disturbance may occur during recovery.
Death usually occurs within 14 days of onset in fatal cases.
The disease is especially severe late in pregnancy, with maternal death
and/or fetal loss occurring in greater than 80% of cases during the
third trimester.
Transmission
Humans usually become infected with Lassa virus from exposure
to urine or faeces of infected Mastomys rats. Lassa virus may also be
spread between humans through direct contact with the blood, urine,
faeces, or other bodily secretions of a person infected with Lassa
fever. There is no epidemiological evidence supporting airborne spread
between humans. Person-to-person transmission occurs in both community
and health-care settings, where the virus may be spread by contaminated
medical equipment, such as re-used needles. Sexual transmission of Lassa
virus has been reported.
Lassa fever occurs in all age groups and both sexes. Persons
at greatest risk are those living in rural areas where Mastomys are
usually found, especially in communities with poor sanitation or crowded
living conditions. Health workers are at risk if caring for Lassa fever
patients in the absence of proper barrier nursing and infection control
practices.
Diagnosis
Because the symptoms of Lassa fever are so varied and
non-specific, clinical diagnosis is often difficult, especially early in
the course of the disease. Lassa fever is difficult to distinguish from
other viral haemorrhagic fevers such as Ebola virus disease; and many
other diseases that cause fever, including malaria, shigellosis, typhoid
fever and yellow fever.
Definitive diagnosis requires testing that is available only
in specialized laboratories. Laboratory specimens may be hazardous and
must be handled with extreme care. Lassa virus infections can only be
diagnosed definitively in the laboratory using the following tests:
- antibody enzyme-linked immunosorbent assay (ELISA)
- antigen detection tests
- reverse transcriptase polymerase chain reaction (RT-PCR) assay
- virus isolation by cell culture.
Treatment and vaccines
The antiviral drug ribavirin seems to be an effective
treatment for Lassa fever if given early on in the course of clinical
illness. There is no evidence to support the role of ribavirin as
post-exposure prophylactic treatment for Lassa fever.
There is currently no vaccine that protects against Lassa fever.
Prevention and control
Prevention of Lassa fever relies on promoting good “community
hygiene” to discourage rodents from entering homes. Effective measures
include storing grain and other foodstuffs in rodent-proof containers,
disposing of garbage far from the home, maintaining clean households and
keeping cats. Because Mastomys are so abundant in endemic areas, it is
not possible to completely eliminate them from the environment. Family
members should always be careful to avoid contact with blood and body
fluids while caring for sick persons.
In health-care settings, staff should always apply standard
infection prevention and control precautions when caring for patients,
regardless of their presumed diagnosis. These include basic hand
hygiene, respiratory hygiene, use of personal protective equipment (to
block splashes or other contact with infected materials), safe injection
practices and safe burial practices.
Health workers caring for patients with suspected or confirmed
Lassa fever should apply extra infection control measures to prevent
contact with the patient’s blood and body fluids and contaminated
surfaces or materials such as clothing and bedding. When in close
contact (within 1 metre) of patients with Lassa fever, health-care
workers should wear face protection (a face shield or a medical mask and
goggles), a clean, non-sterile long-sleeved gown, and gloves (sterile
gloves for some procedures).
Laboratory workers are also at risk. Samples taken from humans
and animals for investigation of Lassa virus infection should be
handled by trained staff and processed in suitably equipped
laboratories.
On rare occasions, travellers from areas where Lassa fever is
endemic export the disease to other countries. Although malaria, typhoid
fever, and many other tropical infections are much more common, the
diagnosis of Lassa fever should be considered in febrile patients
returning from West Africa, especially if they have had exposures in
rural areas or hospitals in countries where Lassa fever is known to be
endemic. Health-care workers seeing a patient suspected to have Lassa
fever should immediately contact local and national experts for advice
and to arrange for laboratory testing.
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