1. By Prof. Abeer Abd El Fattah Elsayed
BASIC VIROLOGY By
Prof. Abeer Abd El Fattah Elsayed

2. PATHOGENESIS OF VIRAL DISEASES

3. By the end of this course, the student is expected to:
Explain how the viruses can cause disease.
Describe different methods of laboratory diagnosis of viral infection

4. PATHOGENESIS OF VIRAL DISEASES
 It is the interaction of viral and host factors that lead to disease production.

5. Steps of Viral Pathogenesis
 1-Entry into the host.
2-Local or primary replication.
3-Viral spread to target organs and cell tropism .
4-Effect of the virus on the host cell.
5-Host immune response and recovery from infection.
6-Fate of the virus after clinical recovery.
7-Virus shedding into the environment.

6. Steps of Viral Pathogenesis
1-Entry into the host:
Through :
inhalation, ingestion, sexual contact or vertical transmission( from a mother to her baby through the placenta, birth canal or breast feeding).

7. Some viruses can enter the body through :
skin abrasions
injections
bite of an arthropod vector or an animal.

8. 2-Local or primary replication:
Viruses usually replicate at the primary site of entry.
Some viruses produce disease at the site of entry:
They spread locally over the epithelial surfaces
With no invasion of the underlying tissues or spread to distant sites
Examples: Influenza virus causes respiratory tract infection
Rotavirus causes gastrointestinal tract (GIT) infection.

9. 3-Viral spread to target organs and cell tropism:
Many viruses produce disease at sites distant from site of entry,
e.g. enteroviruses enter via the GIT and after primary replication,
They spread within the host to produce central nervous system (CNS) disease.

10. Through blood (viremia). Or lymphatics.
Routes of spread:
Through blood (viremia).
Or lymphatics.
Or in few cases neuronal spread occurs, as in rabies.

11. Mechanisms of cell tropism (viral affinity for specific body tissues)
I.Presence of certain proteolytic enzymes:
Certain paramyxoviruses are not infectious until their envelope glycoprotein undergoes proteolytic cleavage.

12. II.Local temperature, pH and oxygen tension,
Rhinoviruses multiply exclusively in the upper respiratory tract, as they are adapted to multiply at low temperature and high oxygen tension.

13. 4-Effect of the virus on the host cell:
a. Cell death:
i.The infection is lethal and kills the cell causing a cytopathic effect (CPE).
ii.The virally infected cells may also be killed by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells.

14. Cell injury produced by the virus causes lesions and dysfunction (symptoms and signs) in organs concerned.
Some tissues rapidly regenerate e.g. intestinal epithelium, while others do not e.g. brain.

15. b. Cell transformation:
The cell is not killed but is changed from a normal cell to a cell with the properties of malignant or cancerous cell.

16. Involves 5-Host immune response and recovery from infection:
Humoral immune response
Cell mediated immune response
Interferon
other host defense factors.

17. It is either one of two outcomes
6-Fate of the virus after clinical recovery:
It is either one of two outcomes
A.Complete resolution:
Most viruses do not remain in the host after clinical recovery being completely eliminated from the host body (viral clearance).

18. B-Persistent viral infections:
Sometimes the virus persists in the host producing either chronic infection or latent infection.
i.Chronic infection: Virus is continuously detected with mild or no clinical symptoms, e.g. HBV.

19. ii.Latent infection:
Virus persists in an occult form inside the infected cell.
During latency, no viral markers could be detected.
Intermittent flare up (reactivation) may occur where the virus can be recovered, e.g. Herpes viruses become latent in sensory ganglia.

20. iii.Slow infection:
virus cause disease with long incubation period, usually years
the disease progression is prolonged, e.g. progressive multifocal leukoencephalopathy caused by JC virus.

21. Shedding usually occurs from body surfaces involved in viral entry.
7-Virus shedding into the environment:
It represents the time at which the infected person becomes infectious to contacts.
Shedding usually occurs from body surfaces involved in viral entry.
In some viral infections, the human represents dead-end host and shedding does not occur i.e. human is not infectious to others e.g. Rabies.

22. Types of Viral Diseases
Systemic Diseases:
The virus spreads widely and invades many tissues and organs because of viremia.
Usually associated with long incubation period.
In most cases, it produces long lasting immunity.
Localized Diseases:
The virus invades only tissues adjacent to the site of entry.
Associated with short incubation period.
It produces transient immunity.

23. LABORATORY DIAGNOSIS OF VIRAL INFECTION

24. I-Direct Demonstration of the Virus or Its constituents in Clinical Specimens
A .Detection of viral particles by Electron Microscopy (E/M):
It is required for detection of viruses of special morphology e.g. Herpes simplex viruses and Rota viruses.

25. B.Immunoelectron microscopy:
Virus specific antibodies bind the virus forming aggregates of virus-antibody complexes that can be seen by EM.

26. B-Detection of viral inclusion bodies (IB) by light microscopy (L/M):
These are eosinophilic or basophilic bodies stained with hematoxylin and eosin (H&E).

27. They may be intracytoplasmic (e. g. rabies virus), intranuclear (e. g
They may be intracytoplasmic (e.g. rabies virus), intranuclear (e.g. herpes viruses), or both (e.g. measles virus).
Inclusion bodies represent:
*aggregation of mature virions
or *areas of altered staining at sites of viral synthesis
or *degenerative changes in the host cell.

28. C.Detection of viral antigens:
Viral antigens are detected by:
* enzyme linked immunosorbant assay (ELISA).
Or * immunofluorescence (IF) tests
Or * radioimmunoassay (RIA) e.g. hepatitis B virus.

29. D.Detection of viral genome by molecular techniques:
 i.Polymerase chain reaction (PCR): It is a highly specific test.
It detects minimal amounts of viral nucleic acid "DNA or RNA" by amplification of the specific nucleic acid sequences of the virus.

30. ii.Hybridization reaction: Labeled nucleic acid probes are used to detect virus genome in tissues.

31. II-Virus Isolation Virus isolation needs living cells:
A. Tissue culture:
On inoculation of the virus on a living susceptible cell culture, the virus infects the cell and replicates.
Viral growth in infected cells is recognized by:

32. CPE is a manifestation of virus-infected cells that are dying or dead:
1. Cytopathic effect (CPE) :
CPE is a manifestation of virus-infected cells that are dying or dead:
i. The virus kills the cells, which detach from the wall of the tissue culture flasks.
ii. The virus causes fusion of tissue culture cells producing syncytia (multinucleated giant cell), e.g. respiratory syncytial virus.
iii. The virus causes rounding of cells, e.g. herpes viruses,
Or iv. grape like cells e.g. adenoviruses.

33. Cytopathic effects induced by virus growth in tissue culture

34. It is the adsorption of erythrocytes to the infected cells
2-Hemadsorption:
It is the adsorption of erythrocytes to the infected cells
due to the presence of viral-encoded hemagglutinins
As in(influenza, parainfluenza) in cellular membrane.

35. 3-Transformation:
This is produced by oncogenic viruses that may lead to:
uncontrolled cell growth
loss of contact inhibition between infected cells
piling up of cells with formation of discrete foci.

36. 4-Inclusion body formation:
Detection of inclusion bodies is performed by L/M examination of virus infected cells stained with H&E, e.g. Negri bodies in rabies virus infection.

37. 5-Interference:
Non-cytopathic viruses (e.g. rubella) interfere with replication and appearance of CPE produced by cytopathic-viruses, e.g. (echovirus), which is added to the cell culture as an indicator.

38. A definitive identification
The virus grown in cell culture is identified by:
i-Neutralization of CPE with standard antiviral serum .
ii-Hemagglutination or hemadsorption inhibition with standard antiviral serum.
iii-Fluorescent antibody assay and ELISA using specific viral antibody
Iv-Immunoelectron microscopy.

39. Neutralization of CPE by antiviral serum

40. B-Chick embryo:
Rarely used for diagnosis.
It is used mainly for multiplication of viruses for production of vaccines.
The virus is inoculated on :
- yolk sac
-chorioallantoic membrane
or - the amniotic sac.

41. Identification:
Viral growth causes:
* death of the embryo,
*production of hemagglutinin
or*formation pocks.

42. C-Laboratory animals:
Some viruses can be only detected by inoculation in laboratory animals (e.g. mice, rabbits or monkeys).
Virus growth results either in disease development or death of the laboratory animal.

43. Identification: Neutralization of viral pathogenicity by standard antisera.

44. III.Serological Detection of Specific Antibodies
Detection of antibody in patient's serum is the most widely used method of diagnosis.
The following tests can be used:
1- ELISA.
2-Indirect immunofluorescence (IIF) test.
3-Complement fixation test (CFT).
4-Hemagglutination inhibition (HI) test.
5-Neutralization test (Nt).

45. Types of specific antibodies detected by serology
Detection of IgM:
It appears early in infection and is therefore used for diagnosis of recent or acute infection.
If paired sera are not available, demonstration of IgM antibodies to the virus even in the first sample taken is diagnostic.

46. Detection of IgG:
A four-fold rise in IgG titer in paired serum samples with an interval of days between the first and second sample, is diagnostic of acute infection.

47. Seroconversion:
the finding of antibody to a virus (or any microbe) in a patient’s serum when the patient previously had no antibody.

48. 9/18/2018

49. Thank You