Family Filoviridae

Table 4.5 lists the known filoviruses, which are classified into two genera, the Marburg-like viruses and the Ebola-like viruses. The filovirus genome is 19 kb in size and contains seven genes, which result in the production of seven or eight proteins following infection (Fig. 4.1). The molecular biology of filoviruses is not well understood, in part because most known filoviruses are severe human pathogens that must be handled under biosafety level 4 conditions. The genomes of

Ebola virus and Marburg virus have both been sequenced, and they have a genome organization similar to that of other members of the Mononegavirales. Their sequences suggest that they are most closely related to the pneumoviruses, and they are assumed to replicate in a manner similar to that for the rhabdoviruses and paramyxoviruses.

The filovirus virion is enveloped, as is the case for all minus-strand viruses, but rather than being spherical, the virion is long and thread-like (whence the name filo as in filament). The infectious virion is thought to be 800-1000 nm in length and 80 nm in diameter (Fig. 2.19E), but preparations examined in the electron microscope are pleomorphic and oddly shaped, often appearing as circles or the number 6 but sometimes branched (Figs. 2.19F and G). There is one glyco-protein (called GP) in the envelope, present as homotrimers, that is both N- and O-glycosylated and has a molecular weight of 120-170 kDa. GP mRNA is edited in Ebola virus, but not in Marburg virus, to produce an mRNA for a second protein called secreted glycoprotein. Its function is unknown but one speculation is that it interferes with the host immune system.

The filoviruses first came to the attention of science in 1967 when outbreaks of hemorrhagic fever occurred in Marburg and Frankfurt, Germany, and in Belgrade, Yugoslavia. The cause was a virus subsequently named Marburg that was present in African green monkeys whose

TABLE 4.4 Viruses Causing Respiratory Disease

Family

Virus"

Nucleic acid

Host range

Disease(s)

Orthomyxoviridae Paramyxoviridae

Picornaviridae

Caliciviridae Coronaviridae

Adenoviridae Herpesviridae

Influenza ss(-)RNA

Canine distemper NDV

Human parainfluenza Measles

Rhinoviruses ss(+)RNA

Coxsackie A

Feline calicivirus ss(+)RNA

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