Lassa fever is a viral hemorrhagic fever transmitted by rats. It has been known since the 1950s, but the virus was not identified until 1969, when two missionary nurses died from it in the town of Lassa in Nigeria. Found predominantly in west Africa,1 it has the potential to cause tens of thousands of deaths. Even after recovery, the virus remains in body fluids, including semen. Increasing international travel and the possibility of use of the Lassa virus as a biological weapon escalate the potential for harm beyond the local level. Access to the country is improving, so renewed efforts to understand it are feasible.
Lassa fever is caused by a single stranded RNA virus and is a disseminated systemic primary viral infection. The main feature of fatal illness is impaired or delayed cellular immunity leading to fulminant viraemia. The prevalence of antibodies to the virus in the population is 21% in Nigeria.
Outbreaks have occurred in the Central African Republic, Guinea, Liberia, Nigeria, and Sierra Leone; serological evidence of human infection has been found in the Democratic Republic of the Congo, Mali, and …
SUMMARY POINTS
Lassa fever, a viral hemorrhagic fever transmitted by rats, is endemic in west Africa and may kill tens of thousands of people each year Peak incidence was thought to
be in the dry season (January to March), but data collected in Sierra Leone shows peaks in the overlap with the wet season (May to November) Many infections are subclinical; a high index of suspicion, given the difficulties of clinical diagnosis, is needed when travellers from west Africa present with a fever of unknown origin, with symptoms appearing up to 21 days after leaving the endemic area Th e virus is excreted in semen for three months after infection; we do not know how frequently it may be transmitted through sexual intercourse. Attempts are being made to produce a vaccine using the yellow fever virus as a vehicle.
The possibility that Lassa virus could be used as a biological weapon has raised the profile of the need for greater understanding of Lassa fever and for more effective control and treatment programmes.
Th e family Filoviridae consists of two genera, the Ebola and Marburg viruses, which are among the most virulent pathogens in humans. The Zaire species of Ebola virus
is the causative agent of the 2014-2015 epidemic in West Africa, in which the case fatality rate has been reported to be as high as 70 percent; rates in earlier outbreaks have reached 80 to 90 percent Marburg virus has caused a similar disease in a smaller number of outbreaks in Central Africa.
Ebola virus is a nonsegmented, negative-sense, single-stranded RNA virus that resembles rhabdoviruses (eg, rabies) and paramyxoviruses (eg, measles, mumps) in its genome organization and replication mechanisms. It is a member of the family Filoviridae, taken from the Latin “filum,” meaning thread-like, based upon their filamentous structure.
In the past, Ebola and Marburg viruses were classified as “hemorrhagic fever viruses”, based upon their clinical manifestations, which include coagulation defects, bleeding, and shock . However, the term “hemorrhagic fever” is no longer used to refer to Ebola virus disease since only a small percentage of Ebola patients actually develop significant hemorrhage, and it usually occurs in the terminal phase of fatal illness, when the individual is already in shock.
In addition to causing human infections, Ebola virus has also spread to wild nonhuman primates, apparently as a result of their contact with an unidentified reservoir host (possibly bats).
This has contributed to a marked reduction in chimpanzee and gorilla populations in Central Africa, and has also triggered some human epidemics due to handling of and/or consumption of sick or dead animals by local villagers as a source of food.
Measures that appear to have contributed to the control of the epidemic include the introduction of infection control precautions in hospitals and funerals, as well as the use of Ebola treatment units and community care centers to help isolate patients with suspected or confirmed Ebola virus disease.
