Other Viruses

There are a number of known human viruses that are as yel not allocated to families, including hepatitis D virus and the unusual agents that cause the subacute spongiform encephalopathies.

Hepatitis D Virus

Hepatitis D virus, recently assigned to the genus Dcltavirus, is a satellite virus, the replication of which is dependent on simultaneous infection of cells with a hepadnavirus. The virion is about 32 nm in diameter and consists of the 24 kDa delta (8) antigen encapsidaled by the surface antigen of the helper hepatitis 13 virus. I he genome of onlv I 7 kb of covalenlly closed circular minus sense ssRNA contains regions of extensive base pairing and probably exisls as a partially double-stranded rodlike structure with self-cleaving ribonucleic (ribozyme) activity, similar to the plant virus "satellites." Known only as a virus of humans who aie simultaneously infected \vith hepatitis B virus, hepatitis D virus causes severe disease which often progresses to chronic hepatitis and/or cirrhosis.

Prions {Causing Subacute Spongiform Encephalopathies)

The causative agents of the subacute spongiform encephalopathies, prions (proleinaceous infectious particles), are composed largely of a protein designated as the scrapie isoform of the prion protein, PrPSc. They are highly resistant to inactivaiion by physical and chemical agents, they are nonim-munogenic, and they are devoid of nucleic acid. All produce slow infections with incubation periods measured in years, followed by progressive disease, which leads inexorably to death from a degenerative condition of the brain characterized by a spongiform appearance. The prototype is scrapie, a disease of sheep, but there are several prion diseases in humans (e.g., kuru, Creutzfeldt-Jakob syndrome). Recently, cattle in several European countries have been infected by feeding on material containing scrapie-infected sheep offal, and there has been a very large outbreak of bovine spongiform encephalopathy (popularly known as "mad cow disease") in the United Kingdom.

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