Arenaviruses

The arenaviruses are “ambisense” viruses, meaning they possess genomic elements with minus (antisense) as well as plus (sense) polarity. It is quite possible for both coding orientations to occur on one and the same segment (of the segmented genome). Rodents are the natural reservoir of these viruses, from which they can infect humans. An infection with the LCM (lym­phocytic choriomeningitis) virus is normally harmless. By contrast, infections by the African Lassa and the South American Junin and Machupo viruses show high lethality rates (hemorrhagic fevers).

Diagnosis: Lassa: virus isolation in special laboratories (biosafety level 4). LCM: serology.

Pathogen. Most members of the Arenaviridae family were first identified in the 1960s. The protoype arenavirus, the pathogen that causes lymphocytic choriomeningitis (LCM), was identified 30 years earlier. Studies involving this virus have contributed a great deal to our understanding of cellular im­munity in general and infection-related immunopathology in particular.

The human pathogens among the arenaviruses are the LCM virus (Europe, America), the Lassa virus (Africa), and the Junin and Machupo viruses (South America). All arenaviruses are spherical to pleomorphic and 50-300 nm in size (on average 110-130 nm). They consist of a “spiked” envelope derived from the plasma membrane, with an inner structure that appears to be granulated when viewed in ultrathin sections. It is to these granula the viral family owes its name (arenosus = sandy). They are considered to be host-cell ribosomes. The virion contains at least three strands of host RNA in addition to two viral RNA segments.

Pathogenesis and clinical picture. The source of nearly all human arenavirus infections is to be found in rodents. The virus enters the body per os, aerogenically or possibly also by skin contact. A pronounced viremia develops at first, followed by organ manifestations. In the case of LCM these are normally harmless and flulike, although they can also develop into meningitis or en­cephalitis, in rare cases with a lethal outcome. The Lassa virus is pantropic. It causes a hemorrhagic fever affecting nearly all inner organs and has a high rate of lethality. Death results from shock and anoxia. The clinical picture re­sulting from Junin and Machupo virus infections is similar. Compared to Lassa infections, CNS involvement is more frequent and the lethality rate is some­what lower with these two viruses.

Diagnosis. In the acute stage, arenaviruses can be isolated from the patient's blood. Postmortem isolation is best done from liver tissue. In the hemorrhagic fevers, especially Lassa fever, the blood is highly infectious and handling it requires proper precautions and utmost care (aerosol formation!). Isolation of the virus is relatively easy in cell cultures. For reasons of safety, only special high-security laboratories are qualified to handle these organisms (e.g., at the Centers for Disease Control and Prevention in Atlanta, GA, USA).

Serodiagnosis is also feasible using standard serological techniques.

Epidemiology and prevention. All arenaviruses are endemic to rodents and are transmitted to humans by these animals.

No specific immunoprophylactic tools have been developed for any of these viruses. As far as exposure prophylaxis is concerned, it must be remem­bered that the LCM, Junin, and Machupo viruses are not transmitted among humans, but that the Lassa virus is transmitted by this route. The most strin­gent precautions are therefore called for when treating Lassa patients. Healthcare staff must wear special clothing and facemasks and special reduced-pressure plastic tents are recommended as patient cubicles. The therapeutic tools available for treatment are ribavirin and human immuno­globulin.