Viral infections can occur at the cellular, individual, and community levels. At the cellular level, viral infection may cause cytocidal effects, cellular proliferation, or steady state infection through various mechanisms of cellular injury. Inclusion bodies are virus-specific intracellular masses that can be seen in infected cells under microscopy. Viral infections may be classified as inapparent, apparent acute, subacute, or chronic, and some viruses like herpes can cause latent infections. Viruses enter the body through routes like respiratory, alimentary, skin, genital, conjunctival, or congenital transmission. The host mounts non-specific responses like age, hormones, malnutrition, fever, and interferons as well as specific humoral

1. Virus-Host Interactions: Viral Infections
 Virus-host interactions may be considered at different levels;
the cell, the individual and the community
 At the cellular level, viral infection may cause a broad spectrum
of effects
a) Cytocidal effect
b) Cellular proliferation
c) Steady state infection

2. Cellular injury may be due to a number of causes
1) Early or nonstructural viral proteins often cause a shut-down
of host protein and DNA synthesis
2) Large amounts of viral macromolecules that accumulate in
the infected cell may distort the cellular architecture and exert
a toxic effect
3) The permeability of plasma membranes may be altered
4) Many viruses produce alterations in the cytoplasmic membrane
of infected cells
5) Certain viruses cause damage to the chromosomes of host cells
(eg: measles, mumps, adenovirus, varicella, cytomegaloviruses)

3. Inclusion bodies
 Virus-specific intracellular globular masses which are produced during
replication of virus in host cells
 They can be demonstrated in virus infected cells under light
-microscope after fixation and staining
 They may be present in the cytoplasm (rabies virus), nucleus
(herpes virus) or both (measles virus)
 Intracytoplasmic inclusions are found in cells infected with
rabies virus (Negri bodies), vaccinia (Guarnieri bodies), molluscum
contagiosum (molluscum bodies) and fowl pox (Bollinger bodies)
 Intranuclear inclusions were classified into two types by Cowdry (1934)
 Cowdry type A includes inclusion bodies of variable size and granular
appearance (eg: herpes virus, yellow fever virus) and type B inclusions
are more circumscribed and multiple (eg: adenovirus, poliovirus)

4. Demonstration of inclusion bodies helps in the diagnosis of some
viral infections (Negribodies: presumptive diagnosis of rabies)
Cytoplasmic inclusion body caused by rabies virus in
brain tissue

5. Pathogenesis of viral infections
Depending on the clinical outcome, viral infections can be classified as
1. Inapparent (subclinical)
2. Apparent (clinical or overt)
a) acute b) subacute c) chronic
Latent infections
 Herpes simplex and varicella-zoster viruses remain latent in nerve
root ganglia, to be reactivated periodically
 HBV and HIV infections
 Slow viral diseases

6. Viruses enter the body through the following routes
1. Respiratory tract
2. Alimentary tract
3. Skin
4. Genital tract
5. Conjunctiva
6. Congenital

7. Routes of transmission of viral infections
Route of
transmission
Viruses
Respiratory tract
Influenza, Parainfluenza, RSV, Measles,
Mumps, Rubella, Rhinovirus, Adenovirus
Coronavirus,Varicella-zoster, CMV, EBV
Alimentary tract
Poliovirus, Adenovirus, Hepatitis A, E
viruses, Rotavirus, Norwalk virus
Skin
Herpes simplex, Papilloma viruses
Molluscum contagiosum, Rabies virus
Arboviruses, HBV, HCV, HIV, HTLV
Genital tract
Herpes simplex viruses, HBV, HCV
HIV, Papillomaviruses
Conjunctiva
Some adenoviruses,
Few enteroviruses

8. Congenital infection
Many viruses are transmitted vertically from parent to progeny
 Congenital infection may occur at any stage from the development
of the ovum up to birth
 In acute systemic infections, congenital infection usually leads
to fetal death and abortion
 Rubella and cytomegalovirus produce maldevelopment or severe
neonatal disease
 HIV, HSV, many tumor viruses cause congenital infection

9. Spread of virus in the body

10. Schematic illustration of the pathogenesis of poliomyelitis

11. Incubation period
 It is the time taken for the virus to spread from the site of entry to the
target organs for the production of lesions
 Its duration is therefore influenced by the relation between the site of
entry, multiplication and lesion
 The incubation period in HBV may be 2-6 months and in slow viral
infections, many years
 Papillomas and molluscum contagiosum have long incubation periods,
probably because the viruses multiply slowly

13. 1.
Non-Specific responses
2.
Immunological
responses
Humoral immunity
Cell-mediated immunity
(CMI)
a)
b)
13

14. Non-specific responses
1. Age
 Most of the viral infections tend to
be more serious at extremes of life
 Rotaviruses cause severe disease
only in infants

15. 2. Hormones
 Corticosteroids administration enhances most viral infections
 The particularly severe course of many viral infections in pregnancy
may be related to the hormonal changes
3. Malnutrition
 Malnutrition interferes with the humoral and cell-mediated immune
responses, therefore it can exacerbate viral infections
(eg: measles – higher incidence of complications and a higher case
fatality rate in malnourished children)
4. Body temperature
 Fever may act as a natural defence mechanism against viral infections
as most viruses are inhibited by temperatures above 390C

16. 5. Phagocytosis
 Macrophages phagocytose viruses and are important in clearing
viruses from blood stream
 Polymorphonuclear leucocytes do not play any significant role
6. Interferons
 Interferons are a family of glycoproteins produced by cells on
induction by viral or nonviral inducers
 Interferon by itself has no direct action on viruses but acts on other
cells of the same species, rendering them refractory to viral infection
 On exposure to interferon, cells produce a protein (translation
inhibiting protein, TIP) which selectively inhibits translation of
viral mRNA

17. Types of interferons
They are classified into three types
1. IFN-α
 It is induced by virus infection and produced by leucocytes
 It has antiviral activity
2. IFN-β
 It is also induced by virus infection but produced by fibroblasts and
epithelial cells
 It also has antiviral activity

18. 3. IFN-γ
 Produced by T-lymphocytes and NK cells, on stimulation by antigens
or mitogens
 It is a lymphokine with immunoregulatory functions
 It enhances MHC antigens and activates cytotoxic T-lymphocytes,
macrophages and NK cells
 Modulates antibody formation and supresses DTH
Mechanism of action
 IFN-α and IFN-β induce the production of three enzymes namely
protein kinase, an oligonucletide synthetase and an RNaseL. This
leads to inhibition of viral protein synthesis but does not affect host
protein synthesis

19. Immunological responses
1. Antibody-mediated immunity
 IgG, IgM, IgA antibodies are produced in response to virus infection
 IgG and IgM play a major role in blood and tissue spaces while IgA
is more important in mucosal surfaces
 IgA is important in resistance to infection of the respiratory, intestinal
and urogenital tracts

20. Antibodies may act in the following ways
Neutralisation of virus which prevents attachment, penetration
or subsequent events
Antibody may attach to viral antigens on the surface of infected cells,
rendering these cells prone to lysis by complement or destruction by
phagocytes or killer lymphocytes
Immune opsonisation of virus for phagocytosis and destruction of
virus by macrophages

21. Cell-mediated immunity (CMI)
CMI prevents infection of target organs and promotes recovery from
disease by destroying virus and virus-infected cells. The different
mechanisms involved for virus destruction are as follows
1. Cytolysis by cytotoxic T-cells and Natural-killer (NK) cells
2. Antibody-dependent cell-mediated cytotoxicity (ADCC)
3. Antibody-complement-mediated cytotoxicity

22. Laboratory diagnosis of viral infections
Following are indications for laboratory diagnosis of viral infections
1. For proper management of certain diseases
2. Diagnosis of diseases caused by viruses for which antiviral
chemotherapy is available (herpes viruses)
3. Screening of blood donors for HIV and HBV
4. Early detection of epidemics like influenza, poliomyelitis,
encephalitis etc to initiate appropriate control measures
In the laboratory, the following methods are commonly employed
1. Direct demonstration of virus and its components
2. Isolation of virus
3. Detection of the specific antibodies

23. 1. Direct demonstration of virus and its components
a) Electron Microscopy
b) Immunoelectron microscopy
c) Fluorescent Microscopy
d) Light Microscopy
e) Viral antigens can be detected by ELISA,
RIA, latex agglutination
f) Nucleic acid probes
g) PCR

24. Detection of viruses in specimens by electron microscopy
Specimen
Viruses
Faeces
Rotavirus, hepatitis A virus,
adenovirus, Norwalk virus,
astrovirus
Vesicular fluid
Herpes simplex, Varicellazoster
CSF
Enterovirus, Varicella-zoster
Urine
Cytomegalovirus (CMV)

25. 2. Isolation of virus
 This is the commonest method used in the diagnosis of virus
infections
 The specimen should be collected properly and transported with
least delay to the laboratory
 Most viruses are heat labile, therefore, refrigeration is essential
during transport
 The methods used for isolation depend on the virus sought
 Since, many viruses (eg: adenoviruses, enteroviruses) are
frequently found in normal individuals, therefore the results of
isolation should always be correlated with clinical data

26. 3. Detection of specific antibodies
 The demonstration of a rise in titre of antiviral antibodies strongly
suggest the active infection
(Ist sample – early in the course of the disease and the convalescent
sample – 10-14 days later)
 Examination of a single sample of serum is meaningful when IgM
specific antibodies are detected
 The serological techniques employed would depend on the virus,
but those in general use are neutralisation, ELISA, haemagglutination
inhibition, complement fixation test, immunofluorescence and
latex agglutination tests

27. Serological diagnosis of viral infections

28. Microplate ELISA for HIV antibody: coloured wells
indicate reactivity

29. HIV-1 Western Blot





Lane1: Positive Control
Lane 2: Negative Control
Sample A: Negative
Sample B: Indeterminate
Sample C: Positive

30. Immunoprophylaxis
1. Active immunisation
2. Passive immunisation
Active immunisation
Viral vaccines
a) Live viral vaccines
b) Killed viral vaccines

31. Live viral vaccines
They are prepared from
 Attenuated strain (eg: yellow fever vaccines)
 Temperature sensitive (ts) mutants (eg: influenza)
 Live recombinant viruses (eg: influenza)
Advantages
1. A single dose of live vaccine is usually sufficient
2. They may be administered by the route of natural infection so that
local immunity is induced

32. 3. They induce a wide spectrum of immunoglobulins against the whole
range of viral antigens
4. They also induce cell mediated immunity
5. They can in general be prepared more economically, and
administered more convenientlly for mass immunisation
Disadvantages
1. There is a risk, however remote, of reversion of virulence
2. The vaccine may be contaminated with potentially dangerous viruses
(eg: oncogenic viruses)
3. Live viral vaccines are heat-labile and they have to be kept
under strict refrigeration

33. 4. Interference by preexisting viruses
5. The virus may spread from the vaccinees to contact
6. Some live viral vaccines may cause local but remote complications
Killed viral vaccines
Killed vaccines are prepared by inactivating viruses with heat, phenol,
beta-propiolactone and formaldehyde
Advantages
1. Safety and stability
2. They can be given in combination as polyvalent vaccines
3. There is no danger of spread of virus from the vaccinee

34. Disadvantages
1. Multiple injections are needed
2. These vaccines have to be given by injection, therefore,
local immunity (IgA immunoglobulins) immunity fails to develop
3. Cell mediated immunity is not induced
Passive immunisation
 Passive immunisation with human gammaglobulin, convalescent
serum or specific immune globulin gives temporary protection against
many viral diseases (eg: measles, mumps and infectious hepatitis)
 Indicated only when nonimmune individuals who are at special risk
are exposed to infection
 Combined active and passive immunisation is an established method
for the prevention of rabies

35. Commonly used viral vaccines
Type of vaccine
Mode of preparation
Live viral vaccines
Measles
Attenuated virus grown in cell culture
Mumps
Attenuated virus grown in chick embryo fibroblast culture
Rubella
Attenuated virus grown in cell culture
Poliomyelitis (Sabin)
Avirulent strain grown in monkey kidney cell culture
Inflenza
a)Live (attenuated)
b) Live (mutant)
c) Live (recombinant)
a)Virus attenuated by serial passage in eggs
b)Use of ts mutant which are avirulent
c)Recombinants with surface antigens of new strains
and growth characters of established strains
Yellow fever (17D)
Attenuated virus grown in chick embryo
Killed viral vaccines
Hepatitis B
HBs Ag from human carrier sera
Rabies
Influenza (subunit)
Virus disintegrated with sodium deoxycholate
Poliomyelitis (Salk)
Virulent strain grown in monkey kidney cell culture

36. Chemoprophylaxis and chemotherapy of viral diseases
Mode of action
Antiviral agents
Inhibits viral DNA polymerase
a)
b)
c)
Inhibits reverse transcriptase
a)
b)
c)
Inhibits proteases
a)
b)
c)
d)
Acyclovir
Ganciclovir
Ribavirin
Active against
a)
b)
c)
Herpes simplex, VZV
Cytomegalovirus
RSV, Lassa virus
Zidovudine
Dideoxycytidine
Dideoxyinosine
HIV
Indinavir
Nelfinavir
Ritonavir
Saquinavir
HIV
Blocks penetration of virus
into cells
Amantadine
Influenza virus
Inhibits protein synthesis
Interferons
Many viruses
Table: Antiviral agents and their modes of action