Arenaviruses(causative agent of Lassa hemorrhagic fever): An overview
▶The History of Arenavirus Discovery
The first arenavirus – a causative agent of lymphocytic choriomeningitis – was isolated in 1933 by R. Lilly and C. Armstrong in USA.
Further it has been discovered that most of arenaviruses are the severest zoonotic pathogens that in some situations may cause acute hemorrhagic fevers in humans with high fatality rate. These infections are endemic in various regions of Africa and Latin America.
Arenavirus known as Junin virus, a causative agent of Argentine hemorrhagic fever was discovered in 1958 by A. Parodi; Machupo virus, the agent of Bolivian hemorrhagic fever was found by K. Johnson in 1963; Lassa virus, the agent of Lassa hemorrhagic fever was isolated in Nigeria by J. Frame in 1969; Guanarito virus of Venezuelan hemorrhagic fever – by R. Salas in 1991; and Lujo virus of hemorrhagic fever – by W.I. Lipkin in Africa in 2008.
▶Classification of Arenaviruses
The viral members of family Arenaviridae, pathogenic for mammals, are assigned to genus Mammarenavirus.
The most dangerous for humans are numerous species – the agents of viral hemorrhagic fevers like Lassa virus or Junin, Machupo, Guanarito, Lujo, and some other less common viruses.
According to their geografical location these pathogens are divided into the Old World viruses and the New World viruses.
Lassa virus is responsible for most of the endemic cases of arenaviral hemorrhagic fevers.
Besides these agents, the virus of lymphocytic choriomeningitis may also cause human disorders (e.g., fever or meningoencephalitis). Also it demonstrates evident teratogenic potential.
The name of arenaviruses originated from Lat. arena – sand, as these agents hold in their envelope sand-like granular inclusions – captured cellular ribosoms – visible by electron microscopy.
▶Structure of Arenaviruses
Arenaviruses are polymorphic enveloped viral particles 50-300 nm in size.
Genome of arenaviruses comprises 2 segments (small S and large L) of linear single-stranded ambisense (–) RNA.
The ambisense nature of genomic RNA indicates that synthesis of viral proteins is based on either genomic or its complementary antigenomic RNA templates.
Every segment of viral genomic RNA codes for 2 viral proteins.
L segment is responsible for synthesis of L protein (RNA polymerase) and matrix Z protein.
S segment encodes NP nucleoprotein and GP glycoprotein of viral spikes. The molecule of GP is further proteolyzed into GP1 (receptor protein) and GP2 (fusion protein).
Nucleoprotein NP and matrix Z protein inhibit the synthesis of interferons by infected cells, thus promoting viral dissemination.
Outside the host arenaviruses stay alive 15-30 minutes to several hours; their survival increases at low humidity conditions.
The viruses are sensitive to UV radiation and sunlight.
Heating at 60oC for 30-60 min irreversibly destroys arenaviruses.
They are readily inactivated by all conventional disinfectants.
▶Viral Replication Cycle
Viral attachment to host cells is promoted by binding of receptor GP1 protein to various membrane molecules (e.g., transferrin receptor) present on many types of human cells.
Specific binding induces viral endocytosis. Acidification of endosome activates GP2 protein that stimulates uncoating and viral entry into cytoplasm with genomic RNA release.
The replication of arenaviruses is performed in the cytoplasm of infected cells.
After synthesis of viral genomic RNAs by viral RNA polymerase and translation of viral proteins capsid assembly is performed at the inner part of cytoplasmic membrane. Viral self-assemblage is mediated by matrix Z protein. Occasionally viral particles may capture cellular ribosomes.
Virions are released through the cell membrane, being covered with lipid envelope. Viral egress occurs by budding.
Culture of arenaviruses similar to isolation of other causative agents of acute hemorrhagic fevers (e.g., filoviruses) is performed only inreference centers (laboratories) that operate at highest biosafety level 4 (BSL-4).
In BSL-4 conditions arenaviruses can be isolated in various cell lines (e.g., Vero cells), or by inoculation into laboratory animals (guinea pigs, or primates).
Pathogenesis and Clinical Findings of Lassa Hemorrhagic Fever
From all arenaviral diseases Lassa hemorrhagic fever seems to be the most life-threatening infection. Being localized in endemic regions of West Africa (e.g., in Nigeria, Liberia, Guinea and some other countries) it affects up to 300,000 people annually resulting in a number of human deaths about 5,000-10,000.
The incidence of other arenaviral hemorrhagic fevers (e.g., Junin, Machupo, Guanarito, or Lujo diseases) is substantially lower.
Lassa virus belongs to the arenaviruses of the Old World.
Lassa hemorrhagic fever is a typical zoonotic endemic disease. It pertains to ecological group of rodent-borne infections.
The main source of infection is a certain species of endemic African rodent (Natal multimammate rate or Mastomys natalensis). These animals continuously excrete the viruses with feces and urine.
The mechanisms of development of Lassa hemorrhagic fever share a number of common traits with other hemorrhagic fever diseases (e.g., caused by filoviruses).
The infection is transmitted predominantly by contact route; foodborne and dust aerosol transmission are also possible.
Infectious dose for Lassa hemorrhagic fever is extremely low – it is estimated as 1-10 viral particles.
Incubation period lasts 1-2 weeks.
Despite the high virulence of the infectious agent, more than 80% of infection cases are mild or subclinical, indicating the natural resistance of local human communities.
Lassa virus usually enters the host through the mucosal or skin lesions. The virus propagates in regional lymph nodes. Hematogenous dissemination results in viremia. The levels of viremia correspond to disease severety.
The main targets for Lassa viruses are the cells of innate immunity (primarily, monocytes, macrophages, and dendritic cells).
The basic mechanism of the development of systemic viral infection is the prominent ability of arenaviruses to inhibit cellular and humoral reactions of antiviral immunity.
Viral nucleoprotein NP and matrix Z protein inhibit the synthesis of interferons and activation of macrophages and dendritic cells. This blocks the presentation of viral antigens to immune lymphocytes and sharply diminishes cytokine secretion.
In conditions of immune insufficiency arenaviruses rapidly spread throughout the body, affecting parenchymatous organs and other tissues.
The viruses affect endothelial cells with deep microcirculation disorders.
Lassa disease manifests with fever and ulcerative pharingitis, followed by cough and vomiting. It progresses towards generalized hemorrhagic syndrome resulting in multiple organ dysfunctions with severe hepatitis, myocarditis, viral pneumonia, encephalitis, and renal failure.
The lethality in hospitalized patients is about 15-20%; amongst pregnant women – more than 80%. Hearing loss is a common neurologic sequela.
Convalescence is slow; the virus sheds for 1-3 months after the recovery.
▶Laboratory Diagnosis of Arenaviral Infections
All manipulations with Lassa viruses and other arenaviral agents are elaborated in laboratories of biosafety level 4 (BSL-4).
The specimens are taken from patient’s blood, urine, sputum, cerebrospinal fluid, nasopharyngeal washes, and autopsy materials.
The virus is most rapidly identified by RT-PCR or other molecular genetic tests.
For detection of viral antigens immunofluorescence or ELISA tests are elaborated.
Isolation of arenaviruses is conducted in various cell lines or by inoculation into laboratory animals followed by viral identification with ELISA or PCR.
For serological diagnosis ELISA test is elaborated detecting specific antiviral antibodies of IgM class.
▶Principles of Prophylaxis and Treatment of Arenaviral Infections
Antiviral drug ribavirin is commonly used for treatment of arenaviral hemorrhagic fevers. If administrated on the 1st week of the disease, it substantially reduces lethality.
Administration of the sera of convalescent patients with high titers of specific antiviral antibodies facilitates the recovery.
Other treatments presume the support of vital body functions of respiratory and cardiovascular system.
Common prophylaxis measures are non-specific and similar to other cases of hemorrhagic fevers.
Various kinds of vaccines for prevention of Lassa fever are actively developed now. A first example of efficient live arenaviral vaccine against Argentine hemorrhagic fever caused by Junin virus has been introduced already into clinical practice.