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INTRODUCTION TO VIROLOGY

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1 INTRODUCTION TO VIROLOGY
What is Virology: Virology is the study of viruses and virus-like agents: their structure, classification and evolution, their ways of infection and exploitation of cells for reproduction, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy

2 Introduction to virology cont..
What is virus: Is an invasive biological agent that reproduces inside the cells of living hosts In a host cell viruses produce many thousands of identical copies of the original viruses, at an extraordinary rate. Unlike most living things, viruses do not have cells that divide

3 Introduction to virology cont..
Viruses are not cells that are capable of independent replication They neither synthesize their own energy nor protein They are too small to be seen in the microscope New viruses assemble in the infected host cell

4 General Characteristics of Viruses:
Viruses non-cellular, non-cytoplasmic infectious agents. They are smaller than bacteria, and they can pass through bacteriological filter. Viruses are transmissible from diseased to healthy organisms. All viruses are obligate parasites and can multiply only within the living host cells.

5 Characteristics…. They are particles composed of internal core containing nucleic acid (DNA or RNA) but not both They are host specific that they infect only a single species and definite cells of the host organisms. They are effective in very small doses. They are highly resistant to germicides and extremes of physical conditions

6 Taxonomy of Viruses Family names end in -viridae.
Genus names end in -virus. Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species. Subspecies are designated by a number.

7 Taxonomy of Viruses cont…
Herpesviridae Herpesvirus Human herpes virus HHV-1, HHV-2, HHV-3 Retroviridae Lentivirus Human immunodeficiency virus HIV-1, HIV-2

8 Viral structure Shape and size The shapes vary.
They may be spherical or Golf ball-like Rod-shaped Tadpole-like Helical or Polyhedral.

9 Viral structure cont… Chemical structure and function:
Viruses have a very simple structure. The core of the viruses is made upon of nucleic acid, which is surrounded by a protein coat called capsid. The nucleic acid always contains only a single kind of nucleic acid i.e. either DNA or RNA. The infectious property of a virus is due to its nucleic acid.

10 Viral Structure cont……
Capsid or the protein coats: It is made up of many identical protein sub-units called capsomeres. The capsomeres are composed of either one or several type of proteins. Capsomeres give a specific shape to a particular virus. The host specificity of virus is due to proteins of the capsid.

11

12 Morphology of a Polyhedral Virus
Figure 13.2

13 Morphology of an Enveloped Virus
Figure 13.3

14 Enveloped Viruses Figure 13.16b

15 Morphology of a Helical Virus
Figure 13.4

16 Morphology of a Complex Virus
Figure 13.5

17

18 Biological properties of viruses
Viruses lack a cytoplasmic membrane and they do not have the basic component of a cell. They can only replicate inside the host cell. Outside the host cell, they are non-living. thus show characters of both living and non-living

19 Non Living characteristics
Can be crystallized. Behave like inert chemicals. Do not show growth, development, nutrition, reproduction, etc. Can be precipitated.

20 Characteristics cont…
Living characters of viruses: Multiply within host cells. Possess genetic material, either DNA or RNA. They have definitive races or strains. They exhibit mutations Due to the above reasons, viruses form unique bridge between living and non-living things

21 Classification of viruses
Classification of viruses is done on basis of : The host cell they infect eg: animal viruses, plant viruses, fungal viruses, and bacteriophages Geometrical shape of their capsid (often a helix or an icosahedron The type of nucleic acid they use as genetic material (RNA or DNA)

22 Common differences btn Non enveloped and Enveloped
Property Nonenveloped Viruses Enveloped Viruses Components Proteins Phospholipids,protein, glycoproteins Sensitivity to heat , acid, detergent,drying Resistant(stable) Sensitive(Labile) Release from host cell By cell lysis(host cell killed) By budding(host cell survives) and cell lysis Transmission(Mode of spread) Fomites,dust, fecal -oral Large droplets,secretions,organ or transplant Effect of drying Survival within GIT Retain infectivity Yes Lose infectivity No Host immune response(Protective) Antibody response Antibody and CM response

23 Ways of Virus attacking the human/Virus Life cycle
Attachment is the binding of the virus to specific molecules on the surface of the cell and this is specific Penetration follows attachment; viruses penetrate the host cell by endocytosis or by fusion with the cell. Uncoating happens inside the cell when the viral capsid is removed and destroyed by viral enzymes or host enzymes

24 Ways…… Replication: is the stage where a cell uses viral messenger RNA in its protein synthesis systems to produce viral proteins Assembly: When the newly created viral proteins and nucleic acid combine to form hundreds of new virus particles Release: New viruses escape or are released from the cell , then cells burst, a process called lysis. Other viruses such as HIV are released more gently by a process called budding

25 Effect of Virus to the host cell
Due to bursting there will be death of host cell (lyisis) Also death cell can be caused by cessation of normal activity of the cell due to protein produced by virus. Alterations to the cell's surface membrane and apoptosis (cell "suicide) Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. 

26 Effect of Virus to the host cell cont..
This causes persistent infections and the virus is often dormant for many months or years. This is appreciated with herpes viruses infection Some viruses, such as Epstein-Barr virus, often cause cells to proliferate without causing malignancy.  But some other viruses, such as papillomavirus, are an established cause of cancer 

27 Effect cont…….. When a cell's DNA is damaged by a virus, and if the cell cannot repair itself, this often triggers apoptosis. One of the results of apoptosis is destruction of the damaged DNA by the cell itself. Some viruses have mechanisms to limit apoptosis so that the host cell does not die before progeny viruses have been produced; HIV, for example, does this.

28 Viruses cause diseases:
Ways of transmission: From bite of the vector usually blood-sucking insects. Direct transmission is more common. Some virus infections, (norovirus and rotavirus), Contaminated food and water, hands and communal objects and Intimate contact with another infected person, 

29 Ways of Transmission Airborne (influenza virus).
 Unprotected sex- Viruses such as HIV, hepatitis B and hepatitis C Contaminated hypodermic needles.  It is important to know how each virus spreads to prevent infections and epidemics

30 Classification of viruses
DNA Viruses Double strand Enveloped. Eg: hepadnaviruses, Herpseviruses &Poxvirus Undeveloped. Eg: Adenoviruses, Papovaviruses Single strand Eg: Parvoviruses

31 Classification of viruses cont
. RNA Viruses (+)RNA Unenveloped Eg: Calciviruses, Picornaviruses EnvelopedEg: Corona virus, Flavivirus, Togavirus. (_)RNA Enveloped Eg: Arenavirus, Bunyaviruses, Filoviruses, Orthoxoviruses, Paramyxovirus& Rhabdoviruses (+/_)RNA Double capsid. Eg: Reoviruses (+)RNA via DNA Eg: Retroviruses

32 Isolation, Cultivation, and Identification

33 Growing Viruses Viruses must be grown in living cells
Bacteriophages form plaques on a lawn of bacteria

34 Viruses Growing Animal viruses may be grown in living animals or in embryonated eggs Figure 13.7

35 Virus Identification Cytopathic effects(Structural changes in host cell that are caused by viral invasion) Serological tests Detection of antibodies against viruses in a patient Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot Nucleic acids Restriction Fragment Length Polymorphisms (RFLPs) PCR

36 Virus Identification USE OF EM Figure 13.9

37 Parvovirus

38 Parvovirus Family Parvoviridae
It includes human Parvovirus B19 (single serotype) and adeno-associated virus.

39 Parvovirus Structure Are non-enveloped, icosahedral particles 18 to 26 nm in diameter. Plus and minus DNA strands are packaged into separate virions in approximately equal proportion. They have two capsid proteins

40 Parvovirus Multiplication
Replication takes place in the nucleus of dividing cells. The single-stranded DNA genome forms an intermediate double-stranded form, which replicates to form progeny-positive and -negative single-stranded DNA. Positive and negative strands are packaged separately in viral capsids in equal numbers.

41 Pathogenesis The disease is transmitted by the respiratory route. The virus replicates in committed erythroid precursor cells in the bone marrow, leading to erythroid aplasia. Aplastic anemia develops in patients with underlying hemolytic anemia, and rash and arthralgia develop at the time specific antibody appears.

42 Host defenses: Specific IgM and IgG antibodies develop in response to infection.

43 Disease: B19 parvovirus causes a mild disease, erythema infections in children with a slapped cheek” rash. . Clinical symptoms develop in a biphasic fashion 7 to 8 days after infection, a prodromal influenza-like illness develops, characterized by headache, malaise, chills, and pyrexia. Aplastic crisis may occur following B19 virus infection in individuals with underlying hemolytic anemias

44 Disease B19 virus infection cause intense viremia develops. In a pregnant woman the virus may cross the placenta and establish infection in the fetus. The fetus is unable to control and eradicate B19 virus, and viral replication continues for several weeks. In the first and second trimesters of pregnancy, B19 virus infection is associated with an increased risk of fetal abortion and nonimmune hydrops fetalis.

45 Parvovirus Disease: Severe fetal anemia (the aplastic anemia seen in patients with hemolytic anemia) and edema occur up to 12 weeks after maternal infection and may cause the fetal hydrops. Infection in the third trimester of pregnancy has been less intensively studied, but a macerated stillbirth fetus has been reported at 39 weeks of gestation.

46 Transmission Via respiratory droplets
Pathogenesis: Virus spreads from respiratory tract and can infect hemopoietic cells in bone marrow

47 Laboratory diagnosis Detection of parvovirus – specific IgM antibody or viral DNA-PCR

48 Treatment and Prevention
There is no specific antiviral treatment and no vaccine. Supportive measure includes: fetal exchange transfusion, in hydrops fetal and using human intravenous immunoglobulin which contains b19 IgG to damp down viral replication in infected immunosuppressed patient with recurrent episodes of anemia

49 Adenoviruses

50 Adenoviruses Adenoviruses are medium-sized ( nm), nonenveloped icosohedral viruses containing double-stranded DNA. cause human infections. unusually stable to chemical or physical agents and adverse pH conditions, allowing for prolonged survival outside of the body.

51 Adenoviruses Cont…. Pathogenesis :
Adenoviruses infect epithelium of respiratory tract, and probably intestine. Spread to lymphoid tissue to tonsil and viral interfere the immune system by blocking action of interferon and Tc cells.

52 Adenoviruses Cont…. Transmission :
Via respiratory droplets, feces,and sometimes from eye to eye via contaminated hands, towels, or eye drops.

53 Adenoviruses Cont…. Diseases caused:
Pharyngoconjuctival fever, epidemics of acute respiratory diseases including pneumonia, intestine illnes, keratoconjuctivitis, hepatitis and disseminate diseases to immunosuppressed individuals. Common cold ( Read on it symptoms, treatment, prevention)

54 Adenoviruses Cont…. Common cold symptoms: Symptoms of the common cold are generally mild and include runny nose, sneezing, and nasal congestion. Symptoms can vary depending on the specific type of virus causing the common cold. Some people can also experience fever, cough, body aches, headache, fatigue, post- nasal drip, and sore throat

55 Adenoviruses Cont…. Fluids Cold-and-flu medications Treatment:
Aspirin - but not for babies, children or teenagers because of the risk of Reye's syndrome. Acetaminophen Treatment:

56 Adenoviruses Cont…. Laboratory identification:
Viral isolation in cell culture in samples includes throat swab, feces, and urine to detect antigen in nasopharyngeal aspirate by immunofluorescence DNA detection in various sample by PCR Serology by detecting a rise in complement fixing antibody.

57 Wishing U a Happy and Prosperous
. Wishing U a Happy and Prosperous New Year(2017)


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