Properties of Filoviridae

Virions of members of the family Filoviridae (filo, threadlike) are very long filamentous rods or more compact convoluted forms, each composed of a lipid bilayer envelope covered with peplomers surrounding a helically wound nucleocapsid (Fig. 30-1). The virions are 80 nm in diameter and have a unit nucleocapsid length of 800-1000 nm, but particles as long as 14,000 nm have been seen. The genome is a single molecule of minus sense ssRNA, 19 kb in size, with complementary end sequences. Virions contain seven proteins; the nucleocapsid contains proteins L, NP, VP35, and VP30 (see Fig 30-2).

Filoviruses replicate well in Vero (African green monkey) cells, as well as infecting guinea pigs, hamsters, and monkeys. Viral replication in the cytoplasm of host cells is marked by the formation of large inclusion bodies, and maturation occurs via budding from the plasma membrane. More detailed studies of the relationships between different filoviruses have been handicapped by the difficulty in developing a virus neutralization test. However, Marburg and Ebola viruses are distinguishable by small differences in genome size and protein profiles, as well as by the absence of antigenic cross-reactivity as determined by antibody-binding assays. There are also consistent differences in tryptic peptide and oligonucleotide fingerprints of the Sudan and Zaire strains of Ebola virus, but strains isolated in different places and in different years in each country are similar. The Ebola-Reston virus, which was initially identified with polyclonal Ebola antisera, reacts with most but not all Ebola virus monoclonal antibodies, but it differs in its pathogenicity for humans.

The sensitivity of filoviruses to lipid solvents and inactivating agents re-

Fig. 30-1 Filoviridae Negatively stained preparation of virions of Ebola virus. Bar, 100 nm (Courtesy Dr F A. Murphy)


Fig. 30-2 Gene organization of the single-stranded negative sense Marburg virus RNA (19 kb) Genes are indicated as stippled boxes, noncoding regions as open areas, and conserved înter-genic sequences as heavy vertical lines The arrow indicates the position of an mRNA overlap between VP30 and VP24. [Modified from H Feldmann, E Muhlberger, A. Randolph, C Will, M. P Kiley, A. Sanchez, and H.-D. Klenk, Virus Res. 24, 1 (1992).]

sembles that of other enveloped RNA viruses. They retain infectivity at room temperature for several days.

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