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Medical VirologyIntroduction to Basics

Dr.T.V.Rao MD

Dr.T.V.Rao MD 1


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History Virology

Smallpox was endemic inChina by 1000BC. In response,the practice ofvariolation wasdeveloped. Recognizing that

survivors of smallpoxoutbreaks were protected fromsubsequent infection,variolation involved inhalationof the dried crusts fromsmallpox lesions like snuff, orin later modifications,inoculation of the pus from alesion into a scratch on theforearm of a child.

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Virus infections are Universal .


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Introduction to Virology

A virus is an obligate intracellular

parasite containing genetic material

surrounded by protein Virus particles can only be

observed by an electronmicroscope

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Introduction to Virology

Recognizing the shape, size, and

structure of different viruses is critical to

the study of disease Viruses have an inner core of nucleic acid

surrounded by protein coat known as an

envelope

Most viruses range in sizes from 20 250

nanometers

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Viral Properties

Viruses are inert (nucleoprotein ) filterable Agents

Viruses are obligate intracellular parasites

Viruses cannot make energy or proteins independent

of a host cell Viral genome are RNA or DNA but not both.

Viruses have a naked capsid or envelope withattached proteins

Viruses do not have the genetic capability to multiplyby division.

Viruses are non-living entities

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Viruses are Ultramicroscopic

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Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997

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The size of viruses

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VIRAL STRUCTURESOME

TERMINOLOGY

virus particle = virion

protein which coats the genome =

capsid

capsid usually symmetrical

capsid + genome = nucleocapsid

may have an envelope

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Virion The complete

infectious unit

of virus particle Structurally

mature,

extracellularvirus particles.

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Virion

Capsid

Viral core

envelope

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Virion Structure

Nucleic Acid

Spike

Projections

Protein

Capsid

Lipid Envelope

Virion

Associated

Polymerase

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Distinguishing characteristics of viruses

Obligate intracellular parasites

Extreme genetic simplicity

Contain DNA or RNA

Replication involves disassembly

and reassembly Replicate by "one-step growth

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How are viruses named?

Based on:

- the disease they causepoliovirus, rabies virus

- the type of diseasemurine leukemia virus

- geographic locationsSendai virus, Coxsackie virus

- their discoversEpstein-Barr virus

- how they were originally thought to be contracteddengue virus (evil spirit), influenza virus (the influence of bad air)

- combinations of the aboveRous Sarcoma virus

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White, DO and Fenner, FJ.Medical Virology, 4th Ed. 1994

Virus particle = virion

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5 BASIC TYPES OF VIRAL STRUCTURE

HELICAL ENVELOPED HELICAL

ENVELOPED ICOSAHEDRAL

COMPLEX

ICOSAHEDRAL

Adaptedfrom Schaechter et al., Mechanisms of Microbial Disease

nucleocapsidicosahedralnucleocapsid

nucleocapsid

helicalnucleocapsid

lipid bilayer

lipid bilayer

glycoprotein spikes= peplomers

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Icosahedral

Adeno-associated Virus (AAV)AdenovirusB19Coxsackievirus - ACoxsackievirus - BCytomegalovirus (CMV)Eastern Equine EncephalitisVirus (EEEV)Echovirus

Epstein-Barr Virus (EBV)Hepatitis A Virus (HAV)Hepatitis B Virus (HBV)Hepatitis C Virus (HCV)Hepatitis Delta Virus (HDV)

Hepatitis E Virus (HEV)

Herpes Simplex Virus 1 (HHV1)Herpes Simplex Virus 2 (HHV2)Human Immunodeficiency Virus (HIV)Human T-lymphotrophic Virus (HTLV)Norwalk Virus

Papilloma Virus (HPV)Polio virusRhinovirusRubella VirusSaint Louis Encephalitis VirusVaricella-Zoster Virus (HHV3)

Western Equine Encephalitis Virus(WEEV)Yellow Fever Virus

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Viral Structure

Varies in size, shape and symmetry

VIP for classification

3 types of capsid symmetry:

Cubic (icosahedral)

Has 20 faces, each an equilateral triangle. Eg. adenovirus

Helical

Protein binds around DNA/RNA in a helical fashion eg. Coronavirus

Complex

Is neither cubic nor helical eg. poxvirus

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The Baltimore classification systemBased on genetic contents and replication strategies of

viruses. According to the Baltimore classification, viruses aredivided into the following seven classes:

1. dsDNA viruses

2. ssDNA viruses

3. dsRNA viruses4. (+) sense ssRNA viruses (codes directly for protein)

5. (-) sense ssRNA viruses

6. RNA reverse transcribing viruses

7. DNA reverse transcribing viruses

where "ds" represents "double strand" and "ss" denotes "single

strand".

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Virus Classification I- the Baltimore classification

All viruses must produce mRNA, or (+) sense RNA

A complementary strand of nucleic acid is () sense

The Baltimore classification has + RNA as its central point

Its principles are fundamental to an understanding of virus

classification and genome replication, but it is rarely usedas a classification system in its own right

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From Principles of Virology Flint et al ASM PressDr.T.V.Rao MD 23


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Virus classification II -the Classical system

This is a based on three principles -

1) that we are classifying the virus itself,not the host

2) the nucleic acid genome

3) the shared physical properties of the infectious agent(e.g capsid symmetry, dimensions, lipid envelope)

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Virus classification III -the genomic system

More recently a precise ordering of viruses

within and between families is possible based

on DNA/RNA sequence

By the year 2000 there were over 4000 viruses

of plants, animals and bacteria - in 71 families,

9 subfamilies and 164 genera

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Viral Structure - Overview

Fig 1. Schematic overview of the structure of animal viruses

** does not exist in all viruses

Nucleic acid

Capsid

Nucleocapsid

Envelope protein

Membrane proteinViral envelope**

Spike protein

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a) Crystallographic structure of a

simple icosahedral virus.b) The axes of symmetry

Icosahedral capsids

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C


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Cubic or icosahedral symmetry

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ICOSAHEDRAL SYMMETRY

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Adenovirus

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Adenovirus

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Helical symmetry

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Helical

California Encephalitis VirusCoronavirusHantavirus

Influenza Virus (Flu Virus)Measles Virus ( Rubeola)Mumps VirusPara influenza VirusRabies VirusRespiratory Syncytial Virus(RSV)

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Helical symmetry

How to

assemble

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In 1955, Fraenkel,

Conrat, and Williams

demonstrated that

tobacco mosaic virus

(TMV) spontaneously

formed when mixtures

of purified coatprotein and its

genomic RNA were

incubated together.

Helical symmetry

TMV, a filamentous virus

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Enveloped helical virus Enveloped icosahedral virus

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Properties of naked viruses

Stable in hostile environment

Not damaged by drying, acid, detergent, and heat

Released by lysis of host cells

Can sustain in dry environment

Can infect the GI tract and survive the acid and bile

Can spread easily via hands, dust, fomites, etc

Can stay dry and still retain infectivity Neutralizing mucosal and systemic antibodies are

needed to control the establishment of infection


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Naked viruses( Non Enveloped )

Adeno-associated Virus (AAV)AdenovirusB19

Coxsackievirus - ACoxsackievirus - BEchovirusHepatitis A Virus (HAV)

Hepatitis E Virus (HEV)Norwalk Virus


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COMPLEX SYMMETRY

POXVIRUS FAMILY

surface view cross section

White, DO and Fenner, FJ.

Medical Virology, 4th

Ed. 1994 Dr.T.V.Rao MD


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ENVELOPE

OBTAINED BY BUDDING THROUGH ACELLULAR MEMBRANE (except poxviruses)

POSSIBILITY OF EXITING CELL WITHOUT

KILLING IT CONTAINS AT LEAST ONE VIRALLY CODED

PROTEIN

ATTACHMENT PROTEIN

LOSS OF ENVELOPE RESULTS IN LOSS OFINFECTIVITY

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RNA or DNA

segmented or non-segmented

linear or circular

single-stranded or double-stranded

if single-stranded RNA

is genome mRNA (+) sense or

complementary to mRNA (-) sense

CLASSIFICATION

NUCLEIC ACID

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Genome

The genome of a virus can be either DNA orRNA

DNA-double stranded (ds): linear or circular

Single stranded (ss) : linear or circular

RNA- ss:segmented or non-segmented

ss:polarity+(sense) or polarity(non-sense)

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DNA RNA

double-stranded single-strandeddouble-

strandedsingle-stranded

linear

circular linear

circular linear linear (circular)*

singl

e

sing

le

multip

le

singl

e

sing

le

multip

le

singl

e

multipl

e

(+)sense (-)sense

sing

le

multip

le

sing

le

multip

le

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Viral genome strategies

dsDNA (herpes, papova, adeno, pox)

ssDNA (parvo)

dsRNA (reo, rota)

ssRNA (+) (picorna, toga, flavi, corona)

ssRNA (-) (rhabdo, paramyxo, orthomyxo,

bunya, filo)

ssRNA (+/-) (arena, bunya)

ssRNA (+RTase) (retro, lenti)

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HERPESVIRIDAE

HEPADNAVIRIDAE

ENVELOPED

PAPILLOMAVIRIDAE

POLYOMAVIRIDAE(formerly grouped together as the

PAPOVAVIRIDAE)

CIRCULAR

ADENOVIRIDAE

LINEAR

NON-ENVELOPED

DOUBLE STRANDED

PARVOVIRIDAE

SINGLE STRANDED

NON-ENVELOPED

POXVIRIDAE

COMPLEX

ENVELOPED

DNA VIRUSES

Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

All families shown areicosahedral except for

poxviruses

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DNA i


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DNA viruses

From Principles ofVirology Flint et alASM Press

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FLAVIVIRIDAETOGAVIRIDAE

RETROVIRIDAE

ICOSAHEDRAL

CORONAVIRIDAE

HELICAL

ENVELOPED

ICOSAHEDRAL

PICORNAVIRIDAECALICIVIRIDAE

ASTROVIRIDAE

NONENVELOPED

SINGLE STRANDED

positive sense

BUNYAVIRIDAEARENAVIRIDAE

ORTHOMYXOVIRIDAEPARAMYXOVIRIDAE

RHABDOVIRIDAEFILOVIRIDAE

SINGLE STRANDED

negative sense

REOVIRIDAE

DOUBLE STRANDED

RNA VIRUSES

ENVELOPED

HELICAL ICOSAHEDRAL

NONENVELOPED

Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991Dr.T.V.Rao MD


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BASIC STEPS IN VIRAL LIFE CYCLE

ADSORPTION

PENETRATION

UNCOATING AND ECLIPSE

SYNTHESIS OF VIRAL NUCLEIC ACID AND

PROTEIN

ASSEMBLY (maturation)

RELEASE

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RECEPTOR VIRUS


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RECEPTOR VIRUS

ICAM-1 polio

CD4 HIV

acetylcholine rabies

EGF vaccinia

CR2/CD21 Epstein-

Barr

HVEM herpes

Sialic acid Influenza,

reo, corona


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Virus Replication1 Virus attachment

and entry

1 2 Uncoating of virion

2

3 Migration ofgenome nucleic

acid to nucleus

3

4 Transcription5 Genome replication

4

5

6 Translation of virusmRNAs

6

7 Virion assembly7

8 Release of newvirus particles

8

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ADSORPTION

TEMPERATURE INDEPENDENT

REQUIRES VIRAL ATTACHMENTPROTEIN

CELLULAR RECEPTORS

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PENETRATION

- ENVELOPED VIRUSES

FUSION WITH PLASMA MEMBRANE

ENTRY VIA ENDOSOMES

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PENETRATION


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PENETRATION

herpesviruses, paramyxoviruses, HIV

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PENETRATION

- ENVELOPED VIRUSES

FUSION WITH PLASMA MEMBRANEENTRY VIA ENDOSOMES, FUSION WITH

ACIDIC ENDOSOME MEMBRANE

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VIRUS UPTAKE VIA ENDOSOMES

CALLEDVIROPEXIS / ENDOCYTOSIS /

PINOCYTOSIS

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PENETRATION


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PENETRATION

NON-ENVELOPED VIRUSES

entry directly across

plasma membrane:

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Replicative cycle As obligate intracellular parasites, Virus must

enter and replicate in living cells in order to

reproduce themselves. This growth cycle

involves specific attachment of virus,penetration and uncoating, nucleic acid

transcription, protein synthesis, maturation

and assembly of the virions and theirsubsequent release from the cell by budding

or lysis

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http://com.net.okstate.edu/schmidt/VirologyLectures/Vi2Rep/replicn.htm


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http://com.net.okstate.edu/schmidt/VirologyLectures/Vi2Rep/replicn.htmhttp://com.net.okstate.edu/schmidt/VirologyLectures/Vi2Rep/replicn.htm


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UNCOATING

NEED TO MAKE GENOME AVAILABLE

ONCE UNCOATING OCCURS, ENTER ECLIPSEPHASE

ECLIPSE PHASE LASTS UNTIL FIRST NEWVIRUS PARTICLE FORMED

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SYNTHESIS OF VIRAL NUCLEIC ACID


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SYNTHESIS OF VIRAL NUCLEIC ACID

AND PROTEIN

MANY STRATEGIES

NUCLEIC ACID MAY BE MADE INNUCLEUS OR CYTOPLASM

PROTEIN SYNTHESIS IS ALWAYS IN

THE CYTOPLASM

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ASSEMBLY AND MATURATION

NUCLEUS

CYTOPLASM

AT MEMBRANE

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RELEASE

LYSIS

BUDDING THROUGH PLASMA MEMBRANE

NOT EVERY RELEASED VIRION IS INFECTIOUS

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Transmission of Viruses


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Transmission of Viruses Respiratory transmission

Influenza A virus

Faecal-oral transmission

Enterovirus

Blood-borne transmission

Hepatitis B virus

Sexual Transmission

HIV

Animal or insect vectors

Rabies virus

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Viruses enter the body of the host


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in a variety of ways, for example...

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The commonest forms of


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transmission are via...

INHALED DROPLETSin sneezing of coughing

for example the COMMON COLD

or INFLUENZA VIRUSES.

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or by


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or by...

drinking water or

eating raw food, for example,

HEPATITIS A and POLIOVIRUS.

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The commonest forms of

transmission are also via...

sexual intercourse for exampleHIV and HEPATITIS B and...

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also...

vertical transmission -

from mother to baby for example

HIV, HEPATITIS B and RUBELLA...

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also...

bites of vector arthropods such as

mosquitoes for example YELLOW FEVER,

RIFT VALLEY FEVER and DENGUE.

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Most viral infections...

do not lead to such seriouscomplications and the host...

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get well after a period of sickness

to be immune for the rest of their lives.

Examples are MEASLES INFECTION,

RUBELLA or German measles,

MUMPS and man others...Dr.T.V.Rao MD 79

A b i h


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A bacteriophage

A bacteriophage is any one of a number of

viruses that infect bacteria. They do this by

injecting genetic material, which they carry

enclosed in an outer protein capsid. Thegenetic material can be ssRNA, dsRNA, ssDNA,

or dsDNA ('ss-' or 'ds-' prefix denotes single-

strand or double-strand) along with eithercircular or linear arrangement.

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S f B i h


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Structure of Bacteriophage

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Cl ifi i f B i h


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Classification of Bacteriophages

The dsDNA tailed phages, or Caudovirales,account for 95% of all the phages reported inthe scientific literature, and possibly make upthe majority of phages on the planet.However, other phages occur abundantly inthe biosphere, with different virions, genomesand lifestyles. Phages are classified by the

International Committee on Taxonomy ofViruses (ICTV) according to morphology andnucleic acid.

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Sub-viral agents


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Sub-viral agents

Satellites Contain nucleic acid

Depend on co-infection with a helper virus

May be encapsidated (satellite virus)

Mostly in plants, can be human e.g. hepatitis delta virus

If nucleic acid only = virusoid

Viroids Unencapsidated, small circular ssRNA molecules that replicate

autonomously

Only in plants, e.g. potato spindle tuber viroid

Depend on host cell polII for replication, no protein or mRNA

Prions No nucleic acid

Infectious protein e.g. BSE

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Vi id & P i


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Viroids & Prions

Viroids ss RNA genome and the smallest known pathogens. Affects plants

Prions

Infectious particles that are entirely protein. No nucleic acid Highly heat resistant Animal disease that affects nervous tissue Affects nervous tissue and results in

Bovine spongiform encepahltits (BSE) mad cow disease, scrapie in sheep kuru & Creutzfeld-Jakob Disease (CJD) in humans

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Vi id


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Viroids

Viroids are small (200-400nt),circular RNA molecules with a rod-like secondary structure whichpossess no capsid or envelope whichare associated with certain plantdiseases. Their replication strategylike that of viruses - they are obligateintracellular parasites.


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Dependovirus /Virusoids

Viroids are small (200-400nt), circular

RNA molecules with a rod-like secondary

structure which possess no capsid orenvelope which are associated with

certain plant diseases. Their replication

strategy like that of viruses - they areobligate intracellular parasites.

(Prions)


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(Prions)

Prions are rather ill-defined infectiousagents believed to consist of a single type ofprotein molecule with no nucleic acidcomponent. Confusion arises from the fact

that the prion protein & the gene whichencodes it are also found in normal'uninfected' cells. These agents areassociated with diseases such asCreutzfeldt-Jakob disease in humans,scrapie in sheep & bovine spongiformencephalopathy (BSE) in cattle.


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Programme Created by Dr.T.V.Rao MD for

Medical and Paramedical Students in the

Developing World

Email

[email protected]

medical virology, introductionmedical virology

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