Chair of Medical Biology, Microbiology, Virology, and Immunology

STRUCTURE, CLASSIFICATION AND

PHYSIOLOGY OF VIRUSES

RNA-VIRUSES

Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.

Viruses are unable to generate energy.

As obligate intracellular parasites, during replication, they fully depend on the complicated biochemical machinery of eukaryotic or prokaryotic cells.

The main purpose of a virus is to deliver its genome into the host cell to allow its expression (transcription and translation) by the host cell.

A complete virus particle is called a virion.

Classification

DNA-Containing Viruses

Parvoviridae

Papovaviridae

Adenoviridae

Herpesviridae

Poxviridae

Hepadnaviridae

Iridovoridae

Classification

RNA-Containing Viruses

Picornaviridae

Reoviridae

Togaviruses

Arenaviridae

Coronaviridae

Retroviridae

Bunyaviridae

Orthomyxoviruses

Paramyxoviruses

Rhabdoviridae

Caliciviridae

Flaviviridae

Filoviridae

Birnaviridae

Some Useful Definitions in Virology

Capsid: The symmetric protein shell that encloses the nucleic acid genome. Often, empty capsids are by-products of the viral replicative cycle.

Nucleocapsid: The capsid together with the enclosed nucleic acid.

Structural units: The basic protein building blocks of the capsid.

Capsomeres: Morphologic units seen in the electron microscope on the surface of virus particles. Capsomeres represent clusters of polypeptides, which when completely assembled form the capsid.

Structure of viruses

A – naked, not containing an envelope aroud capsid

B – enveloped, containing an envelope around the capsid

The helical structure of the rigid tobacco mosaic virus rod

In the replication of viruses with helical symmetry, identical protein subunits (protomers) self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path. Such nucleocapsids form rigid, highly elongated rods or flexible filaments;

Icosahedral Symmetry

An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices

Lines through opposite vertices define axes of fivefold rotational symmetry: all structural features of the polyhedron repeat five times within each 360° of rotation about any of the fivefold axes.

Combined symmetry

Replication of poliovirus, which containing an RNA genome

Cultivation of Viruses A. Chick Embryos: Virus growth in an embryonated chick egg may result in the death of the embryo (eg, encephalitis virus), the production of pocks or plaques on the chorioallantoic membrane (eg, herpes, smallpox, vaccinia), the development of hemagglutinins in the embryonic fluids or tissues (eg, influenza), or the development of infective virus (eg, polio virus type 2).

Cultivation of Viruses B. Tissue Cultures:

Primary cultures are made by dispersing cells (usually with trypsin) from host tissues. In general, they are unable to grow for more than a few passages in culture, as secondary cultures.

- Diploid cell strains are secondary cultures which have undergone a change that allows their limited culture (up to 50 passages) but which retain their normal chromosome pattern.

- Continuous cell lines are cultures capable of more prolonged (perhaps indefinite) culture which have been derived from cell strains or from malignant tissues They invariably have altered and irregular numbers of chromosomes.

HeLA,

Hep-2,

Detroit-6,

KB,

Vero,

Fibroblasts of human embryou,

Kidney of rhesus monkey,

WI-38,

RD,

Primary cultures of chiken fibroblasts

Cell Cultures

Morphologic and Structural Effects

1 The cytopathic effect, or necrosis of cells in the tissue culture (polio-, herpes-, measles-, adenovirus, cytomegalovirus, etc).

2 The inhibition of cellular metabolism, or failure of virus-infected cells to produce acid (eg, enteroviruses).

3 The appearance of a hemagglutinin (eg, mumps, influenza) or complement-fixing antigen (eg, poliomyelitis, varicella, measles).

4 The adsorption of erythrocytes to infected cells, called hemadsorption (paramfluenza, influenza). This reaction becomes positive before cytopathic changes are visible, and in some cases it is the only means of detecting the presence of the virus

5 Interference by a noncytopathogenic virus (eg, rubella) with replication and cytopathic effect of a second, indicator virus (eg, echovirus).

Morphologic and Structural Effects

6 Morphologic transformation by an oncogenic virus (eg, SV40, Rous sarcoma virus), usually accompanied by the loss of contact inhibition and the piling up of cells into discrete foci Such alterations are a heritable property of the transformed cells.

7 Formation of syncytia, or polykaryocytes, which are large cytoplasmic masses that contain many nuclei (poly, many; karyon, nucleus).

8 Alteration of cytoskeleton organization by virus infection.

9 Genotoxic Effects (Chromosome damage may be caused directly by the virus particle or indirectly by events occurring during synthesis of new viral macromolecules (RNA, DNA, protein).

10 Inclusion Body Formation (They may be situated in the nucleus (herpesvirus), in the cytoplasm (pox virus), or in both (measles virus)

Orthomyxovirus Family

The name myxovirus was originally applied to influenza viruses. It meant virus with an affinity for mucins.

A model of the influenza A model of the influenza virionvirion

TypesTypes: A, B, C

Influenza A: In Birds

–16 H variants –9 N variants In Humans–3 H variants

(H1, H2, and H3)

–2 N variants(N1 and N2)

SubtypesSubtypes: H1N1, H2N2,H2N3

Influenza Viruses:Influenza Viruses:Antigenic Shift Antigenic Shift

Influenza Viruses:Influenza Viruses:Antigenic Shift Antigenic Shift

Avian virus

Avian virusAvian

ReservoirAvian Reservoir

New Reassorted

virus

New Reassorted

virus

Human virus

Human virus

SwineSwine

Other mammals?Other mammals?

Influenza Viruses:Influenza Viruses:Antigenic DriftAntigenic Drift

Gradual accumulation of mutations that allow the hemagglutinin to escape neutralizing antibodies (Point mutation in HA gene)

Epidemic strains thought to have changes in three or more antigenic sites

Influenza VaccinesInfluenza Vaccines Whole virus vaccine: inactivated virus

vaccine grown in embryonated eggs; 70-90% effective in healthy persons <65 years of age, 30-70% in persons ≥65 years

Split virus vaccine: previously associated with fewer systemic reactions among the elderly and children <12 years

Subunit vaccine: composed of H and N Live, attenutated influenza virus

vaccines under development

Genes:

Morbillivirus – measles virus,

Respirovirus – parainfluenza virus (serotypes 1 and 3)

Rubulavirus - mumps virus, parainfluenza virus (serotypes 2, 4а, 4b),

Pneumovirus – RS-virus

Family Paramyxoviridae

M protein

helical nucleocapsid (RNA minusNP protein)

HN/H/G glycoprotein SPIKES

polymerase complex

lipid bilayer membrane

F glycoprotein SPIKES

PARAMYXOVIRUSESpleomorphic

STRUCTURE-PARAMYXOVIRUSESSTRUCTURE-PARAMYXOVIRUSES

Cell fusion. In the course of infection, paramyxo-viruses cause cell fusion, long recognized as giant cell formation.

Mumps is an acute contagious disease characterized by a nonsuppurative enlargement of one or both of the parotid glands, although other organs may also be involved.

Properties of the Virus: The mumps virus particle has the typical paramyxovirus morphology. Typical also are the biologic properties of hemagglutination, neuraminidase, and hemolysin.

MUMPS (Epidemic Parotitis)

The disease reaches its highest incidence in children age 5-15 years, but epidemics occur in army camps.

Humans are the only known reservoir of virus.

The virus is transmitted by direct contact, airborne droplets, or fomites contaminated with saliva and, perhaps, urine. The period of communicability is from about 4 days before to about a week after the onset of symptoms.

Epidemiology

Pathogenesis and Pathology

The virus travels from the mouth to the parotid gland, where it undergoes primary multiplication. This is followed by a generalized viremia and localization in testes. ovaries, pancreas, thyroid, or brain.

The ducts of the parotid glands show desquamation of the epithelium, and polymorphonuclear cells are present in the lumens. There are interstitial edema and lymphocytic infiltration.

Orchitis

PARAINFLUENZA VIRUS

The parainfluenza viruses are paramyxoviruses with morphologic and biologic properties typical of the genus.

They grow welt in primary monkey or human epithelial cell culture but poorly or not at all in the embryonated egg. They produce a minimal cytopathic effect in cell culture but are recognized by the hemadsorption method. Laboratory diagnosis may be made by the HI, CF, and Nt tests.

PIV STRUCTUREPIV STRUCTURE

Measles is an acute, highly infectious disease characterized by a maculopapular rash, fever, and respiratory symptoms.

Properties of the Virus: Measles virus is a typical paramyxovirus. It lacks neuraminidase activity.

MEASLES (Rubeola)

Measles virusMeasles virus

Koplik's spots

RESPIRATORY SYNCYTIAL (RS) VIRUS

This labile paramyxovirus produces a characteristic syncytial effect, the fusion of cells in human cell culture. It is the single most serious cause of bronchiolitis and pneumonitis in infants.

Properties of the Virus: RS virus does not hemagglutinate.

RSV- StructureRSV- Structure

immunofluoresent stain

RSV- syncytium formationRSV- syncytium formation

This disease is transmitted by coughing, sneezing, sharing wash cloths towels and other things with someone with RSV.

Pathogenesis and PathologyPathogenesis and Pathology

This disease is extremely serious when it comes to children and infants under the age of 3 and elders.This disease can result in death.

Symptoms for this disease are: sneezing, runny nose, sore throat, low fever, common cold symptoms just more severe.

RSV Bronchiolitis- clinical featuresRSV Bronchiolitis- clinical features

Treatment:Treatment:SupportiveFluids, oxygen, respiratory support, bronchodilators

Antiviral AgentsRibavirin (Virazole), a synthetic guanosine analogue, given as an aerosol

ProphylaxisProphylaxis

Combination live virus vaccines (measles-mumps-rubella)

Live attenuated measles virus vaccine effectively prevents measles.

  Genera of PicornavirusesEnterovirus 

Polio, types 1-3 Coxsackie A , types 1-24Coxsackie B, types 1-6Echo, types 1-34Other enteroviruses, types 68-71

Diseases of the human (and other) alimentary tract (e.g. polio virus)

Rhinovirus,          types 1-115

Disease of the nasopharyngeal region (e.g. common cold virus)

Cardiovirus Murine encephalomyocarditis, Theiler's murine encephalomyelitis virus

Aphthovirus Foot and mouth disease in cloven footed animals

Hepatovirus Human hepatitis virus A

Others Drosophila C virus, equine rhinoviruses, cricket paralysis virus

Properties of enteroviruses

Property Enteroviruses

Size (nm)Capsid form PolypeptideRNA typeAcidOptimal temperature for growth(oC)

22-30Icosahedral VP1, VP2, VP3, VP4‘+’, single stainStable37

RNA

POLIOMYELITIS

“Picornavirus” 3 types: Poliovirus

1,2,3 Ingested, spread by

alimentary route: Commoner in areas of poor sanitation

Infants protected by maternal antibodies

Poliomielitis virus

Poliomyelitis is an acute infectious disease that in its serious form affects the central nervous system. The destruction of motor neurons in the spinal cord results in flaccid paralysis (less than 0.1%). However, most poliovirus infections are subclinical.

The virus is 30 nm in size and forms intranuclear inclusions. The virion is icosahedral and consists of a single sense-strand RNA and a protein capsid containing 32 spherical subunits (capsomeres).

Morphology.

The poliomyelitis virus is cultivated on kidney cells of green African monkeys and on diploid human cells devoid of latent SV40 viruses.

The cytopathic effect is attended by destruction and the formation of granules in the infected cells.

Cultivation.

Transmission

Fecal – oral route: poor hygiene, dirty diapers (especially in day-care settings)

Ingestion via contaminated food and water

Contact with infected hands Inhalation of infectious aerosols

Nonparalytic Poliomyelitis (Aseptic Meningitis). In addition to the above symptoms and signs, the patient with the nonparalytic form presents stiffness and pain in the back and neck. The disease lasts 2-10 days, and recovery is rapid and complete. In a small percentage of cases, the disease advances to paralysis.

Paralytic Poliomyelitis. The major illness usually follows the minor illness described above, but it may occur without the antecedent first phase. The predominating complaint is flaccid paralysis resulting from lower motor neuron damage. The maximal recovery usually occurs within 6 months, with residual paralysis lasting much longer.

Child with polio sequelae

Prevention

Both oral polio vaccine (OPV live, attenuated ， Sabin, 1957) and inactivated poliovirus vaccine (IPV, Salk, 1954) are avilable

IPV is used for adult immunization and Immunocopromised patients

Advantages and disadvantages of OPV

Advantages Effectiveness Lifelong immunity Induction of secretory antibody response

similar to that of natural infection Possibility of attenuated virus circulating

in community by spread to contacts (indirect immunization)(herd immunity)

Ease of administration Lack of need for repeated boosters

Advantages and disadvantages of OPV

DisadvantagesRisk of vaccine-associated

poliomyelites in vaccine recipients or contacts

Spread of vaccine to contacts without their consent

Unsafe administration for immunodeficient patients

Advantages and disadvantages of IPV

AdvantagesAdvantagesEffectivenessGood stability during transport and

in storage Safe administration in

immunodeficient patientsNo risk of vaccine-related disease