Etiology

Electron microscopic examination of blood, liver, and spleen of guinea pigs infected with materials from patients show a long, viruslike agent, quite unlike any virus particle ever before visualized. The particles vary in length, up to several microns. The width of the particles is also variable, with limits of 720 to 1100 A (7,200 to 11,000 nanometers). There is an outer shell, an intermediate clear zone, and a nucleo-

capsid core of approximately 280 A width (Peters et al. 1971). Marburg virus was shown (Casals 1971) to be unrelated to the many arboviruses against which it was tested, and most closely resembles the Ebola virus agent discovered in 1976. In fact, both Marburg and Ebola viruses have since been placed in a new family, Filoviridae (Kiley et al. 1982), which includes but the two members thus far.

It was discovered that the Marburg virus could be inactivated by gamma irradiation (Elliot, McCor-mick, and Johnson 1982); this finding made the preparation of inactivated diagnostic reagents possible. Cultivation of Marburg in cell culture has allowed observation of growth and multiplication taking place in a number of mammalian and avian cell types. A cytopathic effect on cells was observed in Cercopithecus kidney cell lines, and human amnion cell lines (Hoffmann and Kunz 1971).

Immunoglobulin responses have been studied (Wulff and Johnson 1979), measured by immunofluorescence. Immunoglobulin M antibodies appear in sera of patients infected with Marburg virus 4 to 7 days after onset of illness. Titers peak 1 to 2 weeks later, and by 1 to 2 months have decreased considerably. Antiviral IgG antibodies appear at the same time as or a little later than IgM antibodies and persist much longer (Wulff and Johnson 1979). In autopsy material, antigenic material is present in large amounts, and can be demonstrated as late as 3

hours after death by use of immunofluorescent techniques (Wulff, Slenczka, and Gear 1978).

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