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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Chapter 18 Eucaryotic Viruses and Other Acellular Infectious Agents 1 Viruses of Fungi and Protists • Fungal Viruses: • Most mycoviruses, viruses that infect fungi, have been isolated from higher fungi such as Penecillium and Aspergillus. • They are infected with dsRNA viruses – most cause latent infections. Some mycoviruses include disease symptoms in host such as the mushroom Agricus bisporous, but cytopathic effects and toxic virus products have not been observed – Only few lower fungi infected by dsRNA or ssRNA viruses – Destruction and cell lysis 2 Protists: A protist is any organism that is not a plant, animal or fungus Protista = the very first Algal viruses the 4 genera recognized by ICTV have linear dsDNA genomes protozoan viruses viruses of only 3 genera of protozoa have been studied giant dsDNA virus (tentative named a Mimivirus) found in the amoeba Acanthamoeba polyphaga 3 Insect Viruses • Members of many virus families are known to infect insects – Eg. Baculoviridae, Reoviridae, Iridoviridae, and Polydnaviridae • Some use insects as agents for spreading to populations of susceptible animals and plants • Yellow fever, West Nile disease, and several types of encephalitis are the examples of human viral diseases. • infection often accompanied by formation of granular or polyhedral inclusion bodies 4 Inclusion Bodies Figure 18.18 5 Insect larvae are infected when they feed on leaves contaminated with inclusion bodies. Insect viruses can persist in a latent state within the host for generations while producing no disease symptoms. Baculoviruses have potential as biological control agents for insect pests Received attention due to they attack only invertebrates and have considerable host specificity They are encased in protective inclusion bodies, these viruses have a good shelf life and better viability when dispersed in the environment They are well suited for commercial productions because they often reach extremely high concentrations in larval tissue. 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Viroids and Virusoids • viroids – Smallest known infectious agents composed only of RNA • virusoids – formerly called satellite RNAs – infectious RNAs that encode one or more gene products – require a helper virus for replication 7 VIROIDS: The Plant Invaders… • • • • 8 • A VIROID is a… • VIR(virus)OID(like) particle. viroid, an infectious particle smaller than any of the known viruses cause plant diseases They are an extremely small circular single-standed RNAs, ~250-450 nucleotides long. lacking the protein coat of a virus. They do not code for any specific protein but are able to replicate themselves in the nuclei of infected cells. Symptoms: Stunted or distorted growth and sometimes death.: Transmission: It can be transmitted by pollen, seed, or farm implements. Mechanically transmitted; often seed transmitted More than 40 viroid species and many variants have been characterized 9 Few Important Viroid Disease Potato spindle tuber viroid Chrsanthemum stunt viroid Eggplant latent viroid Tomato chlorotic dwarf viroid Rice yellow mottle virus satellite NOTE: Potato spindle tuber viroid was the first to be identified. 10 11 12 13 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display More about Viroids… • unable to replicate itself because its RNA does not encode gene products – may be replicated by a host DNAdependent RNA polymerase – RNA synthesis may use a rolling circlelike mechanism • may cause disease by triggering RNA silencing 15 Virusoids – Virusoids are similar to viroids in that they are also covalently closed circular, ssRNA molecules. • Virusoids are circular single-stranded RNAs dependent on plant viruses for replication and encapsidation. • The genome of virusoids consist of several hundred nucleotides and only encodes structural proteins. • Virusoids are similar to viroids in size, structure and means of replication (rollingcircle replication) 16 Virusoids, while being studied in virology, are not considered as viruses but as subviral particles. Since they depend on helper viruses, they are classified as satellites. In the virological taxonomy they appear as Satellites/Satellite nucleic acids/Subgroup 3: Circular satellite RNAs. The term virusoid is also sometimes used more generally to refer to all satellites. 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Prions Proteinaceous Infectious Particle They contain no nucleic acid. • examples of neurodegenerative diseases in humans and animals caused by prions – Scrapie disease in sheep – bovine spongiform encephalopathy (BSE) or mad cow disease – Creutzfeldt-Jakob disease (CJD) and varient CJD (vCJD) – kuru (Human Disease) 18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display What about Mad Cow Disease? • prions cause bovine spongiform encephalopathy (BSE or mad cow disease) • epidemic proportions in England in 1990s • initially spread because cows were fed meal made from all parts (including brain tissue) of infected cattle 19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Transmission Spread of the disease is via horizontal transmission, i.e., transmission from one person to another, either directly or by fomites or by ingestion of contaminated meat. 20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Variant Creutzfeldt-Jakob (vCJD) v. CJD • difference in diseases is origin – eating meat from BSE infected cattle can cause variant Creutzfeldt-Jakob (vCJD) in humans – CJD is caused by spontaneous mutation of the gene that codes the prion protein • all prion caused diseases – have no effective treatment – result in progressive degeneration of the brain and eventual death 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The End 22