Lassa fever, a recently recognized infectious disease of man, is caused by a pantropic virus, isolated and identified in l969.
Lassa virus is an arenavirus, Arenaviridae family. It is 30-300 nm in diameter, spherical or pleomorphic in form, envelopped, and conrtains sandlike granules (arenosus: latin for "sandy").
Lassa virus has a natural rodent host, Praomys (Mastomys) natalensis, in which it induces a persistent infection characterised by pantropism, viremia and viruria. Man is an unusual host, in whom the spectrum of manifestations varies from a subclinical infection to a severe, at times fatal, infection, accompanied by high fever, viremia, viruria, hemodynamic alterations, and cardiovascular collapse.
In man, the gross pathological changes include edema, congestion of the heart, lungs, kidneys, and intestines, pleurisy, and ascites, gastointestinal haemorrhage. The histopathological changes include focal intersticial pneumonitis, myocarditis, focal tubular necrosis in the kidney, scattered and focal eosinophilic necrosis of hepatocytes, atrophy of Malpighian corpuscles in the spleen, and lymphocytic infiltration about splenic veines. Findings, compiled from four complete autopsies, are essentially similar to those previouly described in Argentinian and Bolivian hemorhagic fever. A direct viral pathogenic action is suggested in the human disease.
Virological and pathological findings in Praomys (Mastomys) natalensis, Squirrel monkeys (Saimiri sciureus) and guinea pigs experimentally infected with Lassa virus substentiate the existence of viral pantropism without clinical disease or histopathological lesions in the natural host. In one infected monkey, necrotic changes (myocarditis, arteritis, renal tubulure necrosis, splenic lymphoid necrosis) and hepatocytic regeneration have been note at the time of death and were associated with high organ virus titers. Dying guinea pigs showed respiratory insufficiency with pulmatory edema, alveolar hyaline membrane, myocarditis and myocardial fibers and hepatocytes, paralleled by high organ virus titers.
So far as is known, the ecology of Lassa fever involves enzootic transmission of virus in semi commensal population of Praomys (Mastomys) natalensis, possibly also Rattus rattus and Mus minutoides. Virus shed in the urine may leak from the rodents to man by direct route and possibly also by aerosol spread; Accidental transfers between humans being may occur within families or in hospitals, e.g., following admission of an index case. Man has been the indicator for Lassa virus activity at the rodent level.
As yet, a vaccine is not available. Side by side with control of rodents in urban and rural tropical areas in Africa, the basic health needs must be met.