Ambisense Genome

The genome of the arenaviruses contains genomic components with sense (plus) polarity and others with antisense (minus) polarity (ambisense viruses, see p. 387) and is structured as follows: the smaller S part (S = small) codes in the 3' part as an antisense-strand RNA for the nucleocapsid protein (NP) and in the 5' part as sensestrand RNA for a viral glycoprotein. Each protein is translated separately from the subgenomic RNA; the NP, coded with the antisense orientation, is first transcribed into a sense-strand RNA. The L part (l = large) codes at the 3' end in antisense-strand orientation for the viral polymerase and at the 5' end in sense-strand orientation for a regulatory RNA-binding protein.

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, aero-genically 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 immuno-globulin.

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Essentials of Human Physiology

Essentials of Human Physiology

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