WHAT IS A VIRUS? Viruses may be defined as acellular organisms whose genomes consist of nucleic acid, and which obligately replicate inside host cells.

WHAT IS A VIRUS?

Viruses may be defined as acellular organisms whose genomes consist of nucleic acid, and which obligately replicate inside host cells using host metabolic machinery to different extents, to form a pool of components which assemble into particles called virions.

A virus differs from a cell in three fundamental ways:i A virus usually has only a single type of nucleic acid serving as its genetic material. This can be single or double stranded DNA or RNA;ii Viruses contain no enzymes of energy metabolism, thus cannot make ATP;iii Viruses do not encode sufficient enzymatic machinery to synthesize their component macromolecules, specifically, no protein synthesis machinery.

Viruses cannot be grown on sterile media, but require the presence of specific host cells.

Fundamentally then, a virus is:Fundamentally then, a virus is:

A package of genetic information protected by a protein shell for delivery into a host cell

to be expressed and replicated

Viruses are distinguished from other obligate parasites, some of which are even simpler than viruses:MYCOPLASMA: Small bacterium that grows only in complex medium or attached to eucaryotic cells.

CHLAMYDIA: Obligate intracellular bacterial parasite which depends on eucaryotic cell for energy.

PROTOZOA: Obligate intracellular parasite that replicate within eucaryotic cells.

VIROID: Infectious agents of plants that exist as naked nucleic acid (circular single stranded (ss) ssRNA).

HEPATITIS DELTA VIRUS (HDV): Viroid-like agent whose replication is dependent upon HBV.

PRION (proteinacious infectious agent): Hypothesized identity of the unconventional slow viruses (such as the Kuru, Scrapie and “Mad cow” disease agents). No nucleic acid is known to be required for prion function. They are thought by many to consist solely of protein and perhaps lipids. Study of these agents has resulted in 2 nobel prize awards.

Kuru

Scrapie BSE vCJD

BSEin

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1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 20000

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CJD, nvCJD

inBritian

Recognition of viruses

How long viruses have been within our midst?

1500 BC: Leg deformities indicative of poliomyelitis, pock marks indicative of smallpox and.

During the 1800's, all infectious agents were considered to be viruses until Koch developed pure culture techniques which allowed the separation and growth of bacteria. In the late 1800's: Bacteria were purified and established as disease causing agents. It then became possible to distinguish them from the "filterable agents", those able to pass through special filters designed to prevent the passage of bacteria. The first viruses described were foot and mouth disease (a picornavirus), 1898, Yellow fever (a flavivirus), 1900, Rous sarcoma virus (an oncogenic retrovirus), 1906.

"Virus" is from the Greek meaning for "poison" and was initially described by Edward Jenner in 1798.

Viral diseases have played a major role in human history over whatever time scale we choose to explore:

Over the past 1000 years: Smallpox and measles were brought to North and South America by early European explorers/conquerers. These diseases, for which the native American populations had no acquired partial immunity, killed large fractions of the populations, and were a major factor in the decimation of these societies.

Over the past 100 years: A newly emerged strain of influenza killed 20 million people in 1918-1919 in the immediate aftermath of World War I. A decade later, polio became one of the most feared infections of children and young adults (including Franklin D. Roosevelt, the U.S. President throughout the Depression and World War II).

As the century entered its final 20 years, a new ~100% lethal virus, HIV, spread rapidly around the world via body fluid transmission.

Over the past 10 years: As the global HIV epidemic continues, sporadic cases and outbreaks in humans of some non-human host viruses such as Ebola and Hanta raise the concern about future epidemics by other viruses in the new century.

FourCorners Virus (Hanta)

THE DISCIPLINE OF VIROLOGY

The study of virology inherently involves a merging together of what has traditionally been thought of as two separate "kinds" of science: basic and applied science. We want to figure out how viruses are transmitted, how they replicate, and how the host organism responds. We also want to figure out how to prevent transmission, how to interfere with virus replicaton, and how to confer immunity on the host. The "applied" follows from, and is dependent upon, the "basic" in a quite direct way. Virology as it is studied today, is therefore an outgrowth of both:

Mouse primary spleen cells transduced with a GFP (green fluorescent protein)-retrovirus vector

Infectious diseases - because of the recognition of viral pathogens.

Molecular Biology- because of the usefulness of viruses as probes of cell and molecular biology and metabolism, and as vectors with strong potential for gene therapy.

WHERE WE STAND IN 2001: PREVENTING • CONTROLING • CURING VIRAL DISEASES

Smallpox: effective vaccine; this is the only viral disease that has been wiped out worldwide

Measles: effective vaccine since 1963; this disease could be eliminated with a world-wide effort

Influenza: effective strain-specific vaccine, but new variant strains emerge periodically

Polio: effective vaccine; will soon be the second viral disease wiped out

HIV: no vaccine; effective drugs, but they are costly and toxic, plus resistant strains appear. World-wide spread continues via intimate contact. 50 million infected thus far

Ebola: no vaccine; important host species unknown (found recently in chimps and rodents); outbreaks controllable because people die quickly and human-human transmission is via blood

Hanta: no vaccine; rodent host; easy transmission to humans, but outbreaks controllable

We also share the world, and our bodies, with viruses that cause hepatitis, respiratory disease, mononucleosis, diarrhea, genital warts, genital herpes, and some forms of cancer

How are viruses classified ?

Hierarchical virus classification: (order) family - subfamily - genus - species - strain/type

All families have the suffix viridae, e.g.:

* Poxviridae * Herpesviridae * Parvoviridae * Retroviridae

Genera have the suffix virus. Within the Picornaviridae there are 5 genera:

* enterovirus (alimentary tract), species e.g. poliovirus 1, 2, 3 * cardiovirus (neurotropic), species e.g. mengovirus * rhinovirus (nasopharyngeal region), species e.g. Rhinovirus 1a * apthovirus (cloven footed animals ), species e.g. FMDV-C * hepatovirus (liver), species e.g. Hepatitis A virus

Virus naming and classificationUsually based on data available at the time of discovery:

i Disease they are associated with, e.g.:Poxvirus, Hepatitis virus, HIV, measles virus

ii Cytopathology they cause, e.g.:Respiratory Syncytial virus, Cytomegalovirus

iii Site of isolation, e.g.:Adenovirus, Enterovirus, Rhinovirus

iv Places discovered or people that discovered them, e.g.:Epstein-Barr virus, Rift Valley Fever

v Biochemical features, e.g.:Retrovirus, Picornavirus, Hepadnavirus RSV

These naming conventions can lead to confusion later, e.g., viral hepatitis is caused by at least 6 different viruses

DD

““Infectious”Infectious”

““Serum”Serum”

Viral Viral hepatitishepatitis

AA

NANBNANB

BB

EntericallyEntericallytransmittedtransmitted

ParenterallyParenterallytransmittedtransmitted

EE

CC

F, G,F, G,? Other *? Other *

* 10-20% of cases of presumed viral hepatitis are still not accounted for

Thus,Thus,

Different viruses can cause (nearly) the same symptoms. e.g., the hepatitis viruses

However, different members of the same group can cause different symptoms. e.g., the herpes viruses

HerpesvirusesHerpesviruses

HSV Herpes Simplex Virus Cold sores (type 1), Genital lesions (type 2)

VZV Varicella Zoster Virus Chicken pox CMV Cytomegalovirus Mononucleosis EBV Epstein-Barr Virus Mononucleosis,

Burkitt’s lymphoma, Nasopharyngeal carcinoma

and HHV-6, HHV-7, HHV-8…..(Human HerpesVirus-#)

Virus Classification is now based principally on analysis of the particle:

Morphology:by electron microscopy

Serology: antigenic cross-reactivity

Genetic material: form of nucleic acid

ssDNA (+ or - strand) dsDNA

ssRNA (+ or - strand) dsRNA

segmented RNA genetic organization sequence homology

DNA sequence Hybridization

Rotavirus

Family Pox Herpes Adeno Papova Parvo Hepadna

Genome <---------------------------------------dsDNA--------------------------------------> ssDNA Partial dsDNA

Capsidsymmetry

Complex <---------------------------------------------Icosahedral-------------------------------------------------->

Envelope <-----------------Yes------------------> <-----------------------------No------------------------------> Yes

e.g. Vaccinia virus Herpes simplexvirus 2

Humanadenovirus

Papilloma Hepatitis BAdeno-Associated

MolluscumContagiosum

Animal virus classification: DNA Viruses

Plus Sense RNA Viruses

Minus Sense RNA Viruses