Viruses

1. Viruses Presented by: Dr. Christopher Miller

2. Viruses are microscopic pathogens that are basically composed of genetic material (RNA or DNA) wrapped in a protein or lipid shell

3. VirusesIntroduction • Viruses are unique pathogens often described as walking the line between life and death • Are obligate intracellular parasites that use a host cell’s nuclear material to replicate • Basically, viruses hijack the cell’s genetic material and take over the cell’s functions • Viruses have single or double stranded DNA or RNA, and once penetrated into the host cell, is translated inside the nucleus to make viral proteins

4. Viruses Introduction (cont) • Viruses are not cells • When free in the environment, they do not show any characteristics of living organisms • Because they can enter cells and replicate, they are often called “infectious agents” • People often confuse viruses and bacteria, however the two pathogens are very different

5. Structure and Classification of Viruses • A single virus particle is called a virion • A virion contains a core of either DNA or RNA • The core is surrounded by a shell made of proteins (in some cases lipids) called a capsid • Viruses are very small and even the largest ones are barley visible with the light microscope • Viruses range in structure from relatively simple shaped to very complex

6. Capsid Surrounding Genetic Material

7. Structure and Classification of Viruses (cont) • Capsids are single or double layer • Together, the nucleic acid and the capsids are called nucleocapsids • The capsids are composed of smaller subunits called capsomers that are arranged in systemic patterns • Capsomers are arranged in two fundamental patterns of capsid symmetry, either icosahedral or helical

8. Structure and Classification of Viruses (cont) • Viruses with icosahedral symmetry contain a defined number of capsomers (20 triangular faces and 12 vertical) • Helical symmetry viruses can have a varied number and combination of capsomers • Some viruses have mixed symmetry where the capsid is icosahedral or helical but the nucleic core is the opposite known as complex symmetry

9. Icosahedral Symmetry

10. Complex Symmetry

11. Structure and Classification of Viruses (cont) • Many viruses have an envelope that surrounds the nucleocapsid called enveloped viruses • Nonenveloped viruses are those without the envelope around the nucleocapsid • Many viruses carry out virion-associated enzymatic activities, depending on the strategy for replication of their nucleic acid

12. In order for a virus to be successful, it must replicate in side its host

13. Viral Replication • The steps in viral replication includes: • Infection of a susceptible host cell • Reproduction of nucleic acid and proteins • Assembly of release of infectious progeny

14. Viruses have a variety of replication strategies due to their wide genetic and structural diversity

15. Viral Host Entry • The first step in viral attachment is called absorption • Absorption is a reversible step resulting from the collisions between targets and virions • In order to enter a cell, a virus must first attach to the cell membrane • A virus attaches to the membrane at one of the normal membrane proteins

16. Attachment • Viral attachment requires appropriate ionic and pH conditions • Viral attachment is temperature independent and does not require any energy • Viral attachment involves the specific binding of viral proteins to receptors on the host cell’s membrane

17. Attachment and Penetration • Some viruses have “spikes” protruding from the capsid, which allows them to penetrate by piercing their way through the host cell membrane • Other viruses undergo a process of receptor mediated endocytosis (cellular eating) and will appear in the cytoplasm inside endocyotic vesicles

18. Spikes aid with penetration

19. DNA vs. RNA Viruses • DNA and RNA viruses enter the cell differently despite the fact that various entry and replication strategies exist for all viruses • DNA viruses replication strategies are less complex then RNA viruses

20. Factors Which Affect Host Range • Whether the virus can get into the host cell • If the virus can enter the cell then is the appropriate cellular machinery available for the virus to replicate? • If the virus can replicate, can infectious virus get out of the cell and spread the infection?

21. Viruses range in size from less than 100 nanometers in diameter to several hundred nanometers in length

22. DNA Replication Strategies • Viral genomes contain information which: • ensures replication of viral genomes • ensures packaging of genomes into virions • alters the structure and/or functioning of the host cell to a greater or lesser degree

23. DNA Replication Strategies (cont) • The virus needs to make mRNAs (messenger genetic material) that can be translated into protein by the host cell translation machinery. • The virus needs to replicate its genome. • Host enzymes for mRNA synthesis and DNA replication are nuclear • If a virus is to replicate itself via these enzymes, it needs to enter the nucleus.

24. DNA Virus Family • Parvovirus • very small single stranded DNA viruses. • icosahedral capsid, and are non-enveloped • primarily in erythrocyte progenitors in the bone marrow • causes fifth disease, decreased production of red blood cells

25. DNA Virus Family • Papillomavirus • similar structure to papovavirus family • however, the two families have a very different replication strategy • Polyomaviruses • icosahedral, non-enveloped viruses that replicate in the nucleus • can either transform the cell or replicate the virus and lyze the cell, all depending on the host cell

26. Polyomaviruses

27. RNA Replication Strategies • Viruses that replicate via RNA intermediates need an RNA-dependent RNA-polymerase to replicate their RNA, this is a special enzyme found in the host cell. Most animal cells do not have this enzyme • No viral proteins can be made until viral messenger RNA is available. • the nature of the RNA in the virion affects the strategy of the virus

28. RNA Replication Strategies • Three types of RNA viruses include • Plus-stranded RNA viruses • RNA is the same sense as mRNA and so functions as mRNA • Negative-stranded RNA viruses • they can make mRNAs upon infecting the cell • must be copied into the complementary plus-sense mRNA before proteins can be made • Double-stranded RNA viruses • need to package an RNA polymerase to make their mRNA after infection of the host cell

29. RNA Virus Family • Is very vast and includes some of these commonly known viruses • Paramyxovirus: HPIV 1 and HPIV 3 • Rubulavirus: HPIV 2, HPIV 4 and the mumps virus • Morbillivirus: measles virus • Pneumovirus, respiratory syncytial virus, pneumonia

30. Encounter and Entry • Sources of human-to-human transmission include acutely ill people, chronic carriers, and pregnant women can also transmit viruses to fetuses • Transmission may be accomplished by direct contact such as sexual contact or via the environment (fecel-oral, aerosols, or direct inoculation) • Animal-human contact may take place via a bite from a diseased animal or via the bite of an infected insect vector

31. How Our Body Responds to Viral Infections • Infections by a virus often gives rise to morphological changes in the host cell known as cytopathic effect • Cytopathic effects may consist of cell rounding, disorientation, swelling or shrinking, death and/or detachment from the surface • Many viruses induce apoptosis (programmed cell death) in infected cells

32. Possible Outcomes of Infection • Lytic Infection • Virus undergoes multiple rounds of replication • Results in the death of the host cells • Polio viruses and influenza are examples • Latent Infection • Opposite of lytic infections • Often Transforms the cell but does not kill it • Can be reactivated and dormant

33. Possible Outcomes of Infection (cont) • Chronic Infection • Virus particles continue to shed after the period of acute illness has passed • Marked by a slow release of particles with out death of the host or any other injury • Usually associated with RNA viruses • Chronic infection is usually associated with a defective host immunity that is too weak to rid the host from infection

34. Diagnosis of Viral Disease • Only a few viral diseases can by accurately diagnosed on clinical grounds such as measles • In other cases only the group of likely pathogens can be identified clinically • Definitive clinical identification requires isolation of the virus in animals or tissue culture • Isolation is done in cell cultures, embryonated eggs, and animals such as mice

35. Diagnosis of Viral Disease • Difficulty occurs as appropriate specimens must be obtained during a suitable phase of the illness • Viruses can not be grown on artificial medians • Viruses can be diagnosed by detection of virus-specific antigens or viral nucleic acids in tissues or body fluids • Demonstration of specific serological responses may also assist in identification

36. Treatment of Viruses • Both physical and chemical approaches are possible • The most obvious approach is to block the infectivity of the offending agent • Prevention techniques including disinfection and physical or chemical barriers are abundant and effective but not therapeutic once infected • Simple techniques like hand washing has been proven effective

37. Treatment Continued • Antibiotics have no effect against viruses • Immunoglobulins pooled from human sera are commercially available for prophylaxis and therapeutic management • Antiviral drugs target the steps of viral replication to disrupt the penetration and replication