Understanding Viruses, Viroids, Virusoids, and Prions

1. Viruses are sub-microscopic, obligate intracellular parasites

2. Viroidsare small (200-400nt), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope which are associated with certain plant diseases. Their replication strategy like that of viruses - they are obligate intracellular parasites. • Virusoidsare satellite, viroid-like molecules, somewhat larger than viroids (e.g. approximately 1000nt) which are dependent on the presence of virus replication for multiplication (hence 'satellite'), they are packaged into virus capsids as passengers. • Prionsare rather ill-defined infectious agents believed to consist of a single type of protein molecule with no nucleic acid component. Confusion arises from the fact that the prion protein & the gene which encodes it are also found in normal 'uninfected' cells. These agents are associated with diseases such as Creutzfeldt-Jakob disease in humans, scrapie in sheep & bovine spongiform encephalopathy (BSE) in cattle.

3. The first written record of a virus infection consists of a heiroglyph from Memphis, the capital of ancient Egypt, drawn in approximately 3700 BC, which depicts a temple priest called Ruma showing typical clinical signs of paralytic poliomyelitis .

4. The mummy shows that his left leg was withered and his foot was rigidly extended like a horse's hoof - classic paralytic poliomyelitis . The Year 1193 BC The Pharaoh Siptah rules Egypt from 1200-1193 BC when he dies suddenly at the age of about 20. His mummified body lays undisturbed in his tomb in the Valley of the Kings until 1905 when the tomb was excavated. FOOT

5. Ramesses V's preserved mummy shows that he died of smallpox at about the age of 35 in 1143 BC. The pustular lesions on the face of the mummy are very similar to those of more recent patients

6. Mesopotanian laws concerning rabiddogs date from 1000 BC. People wholet rabid dogs run free were fined.

7. Smallpox is endemic in China by 1000BC. In response, the practice of variolation is developed. Recognizing that survivors of smallpox outbreaks are protected from subsequent infection, variolation involves inhalation of the dried crusts from smallpox lesions like snuff, or in later modifications, inoculation of the pus from a lesion into a scratch on the forearm

8. The Year 1520 Smallpox , which had reached Europe from the East in 710 A.D., was transferred to the Americas by Hernando Cortez. 3,500,000 Aztecs died in the next 2 years - effectively the end of the Aztec empire .

9. Lady Mary Wortley Montagu introduced Variolation to England

10. The Year 1796 On 14th May 1796, Edward Jenner vaccinated an 8 year old boy, James Phipps, with material from a cowpox lesion on the hand of a milkmaid, Sarah Nelmes.James, who had never had smallpox, developed a small lesion at the site of vaccination which healed in 2 weeks.On 1st July 1796, Jenner challenged the boy by deliberately inoculating him with material from a real case of smallpox !

11. The Year 1886 LouisPasteur tested a rabies vaccine. This depended on deliberate, experimental production of the vaccine by serial passage of infectious virus in rabbit spinal cords. The next vaccines for yellow fever and influenza did not appear until the 1930s.

12. The Year 1892 On 12th February, Dmitri Iwanowski, a Russian botanist, presents a paper to the St. Petersburg Academy of Science which shows that extracts from diseased tobacco plants can transmit disease to other plants after passage through ceramic filters fine enough to retain the smallest known bacteria. This is generally recognised as the beginning of Virology. Unfortunately, neither Iwanowski nor the scientific community fully realize the significance of these results.

13. The Birth of Virology *Adolph Mayer-1883. Inoculated plants with agent that he named Tobacco mosaic virus. Showed that the agent was soluble and could not be grown in culture. Thought that the pathogen might be a small bacterium. *Martinus Beijerinck-1897 Found that the agent could reproduce only in the host and was not inactivated by alcohol. Called the agent a “contagium vivum fluidum” or a “contageous living liquid”. Clearly stated that the agent was not a bacterium, fungus or other culturable pathogen. He is known as the “Father of Virology”.

14. Walter Reed (1851-1902) Through experimental transmission to mice, in 1900 Walter Reed demonstrated that yellow fever was caused by a virus, spread by mosquitoes.

15. Karl Landsteiner (1868-1943) Karl Landsteiner (1868-1943) and Erwin Popper proved that poliomyelitis was caused by a virus. Landsteiner and Popper were the first to prove that viruses could infect humans as well as animals.

16. Francis Peyton Rous (1879-1970) Francis Peyton Rous (1879-1970) demonstrated that a virus (Rous sarcoma virus) can cause cancer in chickens. (For this work, he was eventually awarded the Nobel Prize, in 1966. Rous is the first person to show that a virus could cause cancer in animals (see also 1981).

17. Frederick Twort (1915) • First to isolate viruses that infect bacteria (bacteriophages)

18. Felix d'Herelle (1873-1949) Following Frederick Twort's work, Felix d'Herelle independently recognizes viruses which infect bacteria, which he calls bacteriophages (eaters of bacteria). The discovery of bacteriophages provids an invaluable opportunity to study virus replication at a time prior to the development of cell culture when the only way to study viruses was by infecting whole organisms.

19. Wendell Stanley (1887-1955) Wendell Stanley (1887-1955) crystallizes tobacco mosaic virus (TMV) and shows that it remains infectious (Nobel Prize, 1946). Stanley's work is the first step towards describing the molecular structure of any virus and helps to further illuminate the nature of viruses.

20. Max Theiler (1899-1972) Max Theiler was the first to propagate yellow fever virus in chick embryos and successfully produced an attenuated vaccine - the 17D strain. Theiler's vaccine was so safe and effective that it is still in use today! This work saved millions of lives and set the model for the production of many subsequent vaccines. For this work, Theiler was awarded the Nobel Prize in 1951.

21. The Year 1939 Emory Ellis (1906-) and Max Delbrück (1906-1981) Established the concept of the "one step virus growth cycle" essential to the understanding of virus replication. This work laid the basis for the understanding of virus replication - that virus particles do not "grow" but are instead assembled from preformed components.

22. The Year 1941 George Hirst demonstrated that influenza virus agglutinates red blood cells. This was the first rapid, quantitative method of measuring eukaryotic viruses. Now viruses could be counted!

23. The Year 1945 Salvador Luria (1912-1991) and Alfred Hershey (1908-1997) demonstrated that bacteriophages mutate. (Nobel Prize, 1969) This work proves that similar genetic mechanisms operate in viruses as in cellular organisms and lays the basis for the understanding of antigenic variation in viruses. Salvador Luria (1912-1991) Alfred Hershey (1908-1997)

24. The Year 1949 Thomas Weller (1915–) Frederick Robbins (1916–) John Enders (1897-1985) John Enders, Thomas Weller (1915–) and Frederick Robbins (1916–) were able to grow poliovirus in vitro using human tissue culture. (Nobel Prize, 1954) This development led to the isolation of many new viruses in tissue culture.

25. The Year 1950 André Lwoff (1902-1994) Louis Siminovitch and Niels Kjeldgaard discovered lysogenic bacteriophages in Bacillus megaterium irradiated with ultra-violet light and coined the term prophage. (Nobel Prize, 1965). Although the concept of lysogeny had been around since the 1920s, this work clarified the existence of temperate and virulent bacteriophages and led to subsequent studies concerning the control of gene expression in prokaryotes, resulting ultimately in the operon hypothesis of Jacob and Monod. Also in 1950, the World Health Organization proposed a programme to eradicate smallpox from the Americas. This was acheived in 8 years.

26. The Year 1952 Renato Dulbecco (1914-) showed that animal viruses can form plaques in a similar way to bacteriophages. (Nobel Prize, 1975) Dulbecco's work allowed rapid quantitation of animal viruses using assays which had only previously been possible with bacteriophages. Alfred Hershey (1908-1997) and Martha Chase demonstrated that DNA was the genetic material of a bacteriophage. Although the initial evidence for DNA as the molecular basis of genetic inheritance was discovered using a bacteriophage, this principle of course applies to all cellular organisms (though not all viruses!).

27. The Year 1957 Heinz Fraenkel-Conrat (1910-1999) and R.C. Williams showed that when mixtures of purified tobacco mosaic virus (TMV) RNA and coat protein were incubated together, virus particles formed spontaneously. Alick Isaacs and Jean Lindemann discovered interferon. Although the initial hopes for interferons as broad spectrum antiviral agents equivalent to antibiotics have faded, interferons were the first cytokines to be studied in detail.

28. The Year 1963 Baruch Blumberg discovers hepatitis B virus (HBV). (Nobel Prize, 1976) Blumberg went on to develop the first vaccine against the HBV, considered by some to be the first vaccine against cancer because of the strong association of hepatitis B with liver cancer.

29. The Year 1970 Howard Temin (1934-1994) and David Baltimore independently discovered reverse transcriptase in retroviruses. (Nobel Prize, 1975). The discovery of reverse transcription established a pathway for genetic information flow from RNA to DNA, refuting the so-call "central dogma" of molecular biology.

30. Year 1973 Peter Doherty and Rolf Zinkernagl demonstrate the basis of antigenic recognition by the cellular immune system. (Nobel Prize, 1996) The demonstration that lymphocytes recognize both virus antigens and major histocompatibility antigens in order to kill virus-infected cells established the specificity of the cellular immune system.

31. 1978 Smallpox Eradication Program Ali Marlin Last Natural Smallpox Case - October, 1997. • Only 10 years after the eradication program began in 1977, the last smallpox case occurred in Somali. • No natural cases have occurred in the past 25 years. • Lab outbreak of Smallpox occurred in Britain in 1978. • Smallpox was declared eradicated throughout world in 1980. • Only remaining viral stocks are stored at CDC in Atlanta GA & in Novosibirsk, Russia. There are proposals for their destruction. • Major worry is bioterrorism.

32. Year 1983 Luc Montaigner and Robert Gallo announced the discovery of human immunodeficiency virus (HIV), the causative agent of AIDS. In only two years since the start of the AIDS epidemic the agent responsible has been identified.

33. Year 2001 The complete nucleotide sequence of the human genome is published. About 11% of the human genome is composed of retrovirus-like retrotransposons: "transposable elements in which transposition involves a process of reverse transcription with an RNA intermediate similar to that of a retrovirus".

34. Viruses • Small size • Filtrable agents • Obligate intracellular parasites: Can not make energy or proteins independently of a host cell

35. Viral genome • RNA or DNA • Never both !!!!!! • A naked capsid or +envelope • Do not replicate by division=binary fusion • Viral components are produced in the host cell and assembled.

36. Consequences of viral properties • Viruses are not living • Viruses must be infectious to endure in nature. • Viruses must be able to use host cell processes to produce their components (viral messenger RNA, protein and identical copies of the genome)

37. Consequences of viral properties • Viruses must encode any required processes not provided by the cell. • Viral components must self-assemble.

38. Knowledge of • the structural (size and morphology) • Genetic (type and structure of nucleic acid) Provides insight how the virus replicates, spreads and causes disease.

39. Viruses • Very small • Nanometers (nm) • Clinically important viruses range from 18nm (parvoviruses) to 300 nm (poxviruses) Range from small and simple (parvoviruses and picornaviruses) to Large and complex viruses (pox viruses and herpesviruses)

41. Naming of viruses • Structure: size, morphology and nucleic acid (picornavirus (small RNA) togavirus (cloak) • Members of its family : papovavirus (papilloma, polyoma, vacuolating viruses) • The disease they cause:smallpox(poxviruses)

42. Naming of viruses • Tissue or organ tropism: adenovirus, enterovirus, reovirus(respiratory,enteric, orphan) • Place of isolation: Norwalk, Coxsackie and many toga,arena and bunyaviruses are named after African places where they were first isolated.

43. Means of classification • Biochemical characteristics: structure and mode of replication: current means • Host cell (host range): animal (human, mouse, bird), plant, bacteria • Means of transmission: arboviruses by insects • Disease: ensephalites, hepatitis viruses

44. Viruses • DNA viruses: 6 families • Poxviridae • Herpesviridae • Adenoviridae • Hepadnaviridae • Papovaviridae • Parvoviridae

45. RNA viruses • Paramyxoviridae • Orthomyxoviridae • Coronaviridae • Arenaviridae • Rhabdoviridae • Picornaviridae • Filoviridae • Togaviridae • Bunyaviridae • Flaviviridae • Retroviridae • Caliciviridae • Reoviridae • Delta

46. Naked viruses • DNA or RNA +structural proteins= Nucleocapsid

47. Enveloped viruses Nucleocapsid+envelope

49. Virus capsid • Helical (rod) • İcosahedral (spherical)