Chapter 13 Characterizing and Classifying Viruses, Viroids, and Prions.

Chapter 13

Characterizing and Classifying

Viruses, Viroids, and

Prions

Characteristics of Viruses

• Virus– Minuscule, acellular infectious agent having

either DNA or RNA– Causes many infections of humans, animals,

plants, and bacteria– Causes most of the diseases that plague the

industrialized world

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• Cannot carry out any metabolic pathway• Neither grow nor respond to the

environment• Cannot reproduce independently• Recruit the cell’s metabolic pathways to

increase their numbers• No cytoplasmic membrane, cytosol,

organelles (with one exception)• Have extracellular and intracellular state

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• Extracellular State– Called virion– Protein coat (capsid) surrounding nucleic acid– Nucleic acid and capsid also called

nucleocapsid– Some have phospholipid envelope– Outermost layer provides protection and

recognition sites for host cells• Intracellular State

– Capsid removed– Virus exists as nucleic acid

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Figure 13.1 Virions-overview

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• Genetic Material of Viruses– Show more variety in nature of their genomes

than do cells– Primary way scientists categorize and classify

viruses– May be DNA or RNA, but never both

–dsDNA, ssDNA, dsRNA, ssRNA– Linear and segmented or single and circular– Much smaller than genomes of cells

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Figure 13.2 The relative sizes of genomes

Partial genomeof E. coli

Viralgenome

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• Hosts of Viruses– Most viruses infect only particular host’s cells

– Affinity of viral surface proteins for proteins on host cell

– May be so specific they infect only particular kind of cell in a particular host

– Generalists – infect many kinds of cells in many different hosts

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Figure 13.3 Hosts of viral infections-overview

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Figure 13.4 Sizes of selected virions

Red blood cell(10,000 nm in diameter)

E. coli (bacterium)(1000 nm 3000 nm)

Poliovirus(30 nm)

Bacteriophage MS2(24 nm)

Bacteriophage T4(50 nm 225 nm)

Smallpox virus(200 nm 300 nm)

Tobacco mosaic virus(15 nm 300 nm)

Bacterialribosomes(25 nm)

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• Capsid Morphology– Capsids

– Provide protection for viral nucleic acid– Means of attachment to host’s cells– Composed of proteinaceous subunits called

capsomeres– Capsomere made of single or multiple types of

proteins

– Three basic shapes - helical,polyhedral,complex

Figure 13.5 The shapes of virions-overview

Figure 13.6 Bacteriophage T4-overview

• The Viral Envelope– Acquired from host cell during viral replication

or release– Envelope is portion of membrane system of host

– Composed of phospholipid bilayer and proteins– Some proteins are virally coded glycoproteins

(spikes)

– Envelope’s proteins and glycoproteins often play role in host recognition

Figure 13.7 Enveloped virion-overview

Table 13.2 Families of Human Viruses

Viral Replication

• Lysogeny– Modified replication cycle

– Infected host cells grow and reproduce normally for generations before they lyse

– Temperate phages– Prophages – inactive phages

– Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium

Viral Replication

• Replication of Animal Viruses– Same basic replication pathway as

bacteriophages

– Differences result from– Presence of envelope around some viruses– Eukaryotic nature of animal cells– Lack of cell wall in animal cells

Viral Replication

• Replication of Animal Viruses– Attachment of animal viruses

– Chemical attraction– Animal viruses do not have tails or

tail fibers– Have glycoprotein spikes or other

attachment molecules that mediate attachment

– Uncoating– Direct penetration– Membrane fusion

Viral Replication

• Replication of Animal Viruses– Synthesis of animal viruses

– Requires different strategy depending on its nucleic acid

– DNA viruses often enter the nucleus– RNA viruses often replicate in the cytoplasm– Must consider

– How mRNA is synthesized

– What serves as template for nucleic acid replication

Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses-overview

Viral Replication

• Replication of Animal Viruses– Assembly and release of animal viruses

– Most DNA viruses assemble in nucleus– Most RNA viruses develop solely in cytoplasm– Number of viruses produced depends on type of

virus and size and initial health of host cell– Enveloped viruses cause persistent infections– Naked viruses are released by exocytosis or lysis

Figure 13.14 The process of budding in enveloped viruses

Envelopedvirion

Budding ofenveloped virus

Viral capsid

Viral glycoproteins

Cytoplasmicmembraneof host

Viral Replication

• Replication of Animal Viruses– Latency of animal viruses

– When animal viruses remain dormant in host cells– May be prolonged for years with no viral activity– Some latent viruses do not become incorporated

into host chromosome– Incorporation of provirus into host DNA is

permanent

The Role of Viruses in Cancer

• Animal’s genes dictate that some cells can no longer divide or are prevented from unlimited division

• Genes for cell division “turned off” or genes inhibiting division “turned on”

• Neoplasia – Uncontrolled cell division in multicellular animal – Mass of neoplastic cells is tumor

• Benign vs. malignant tumors– Metastasis– Cancers

Figure 13.16 The oncogene theory of the induction of cancer in humansNormal state:

DNAProtooncogene

Represses

Gene for repressor

Repressor Result: No cancer

First “hit”:

Virus inserts promoter

DNAOncogene Gene for repressor

Represses mRNA

Repressor Result: Still no cancer

Second “hit”:

DNAOncogene

Virus inserts into represssor gene

Protein Causes cell division Result: Cancer

No repressorprotein because

gene is segmented

• Environmental factors that contribute to the activation of oncogenes– Ultraviolet light

– Radiation

– Carcinogens

– Viruses

• Viruses cause 20–25% of human cancers– Some carry copies of oncogenes as part of their

genomes– Some promote oncogenes already present in host– Some interfere with tumor repression– Specific viruses are known to cause ~15% of

human cancers– Burkitt’s lymphoma– Hodgkin’s disease– Kaposi’s sarcoma– Cervical cancer

Are Viruses Alive?

• Some consider them complex pathogenic chemicals

• Others consider them the least complex living entities– Use sophisticated methods to invade cells

– Have the ability to take control of their host cell

– Are able to replicate themselves

Other Parasitic Particles: Viroids and Prions

• Characteristics of Viroids– Extremely small, circular pieces of RNA that

are infectious and pathogenic in plants

– Similar to RNA viruses, but lack capsid

– May appear linear due to H bonding

Figure 13.20 The RNA strand of the small potato spindle tuber viriod (PSTV)

Genome of bacteriophage T7 PSTV

Figure 13.21 One effect of viroids on plants

• Characteristics of Prions– Proteinaceous infectious agents– Cellular PrP protein

– Made by all mammals– Normal structure with -helices called cellular PrP

– Prion PrP– Disease-causing form with -pleated sheets called

prion PrP– Prion PrP changes shape of cellular PrP so it

becomes prion PrP

ANIMATION Prions: Overview

• Characteristics of Prions– Normally, nearby proteins and polysaccharides

force PrP into cellular shape

– PrP mutations result in formation of prion Pr

ANIMATION Prions: Characteristics

• Characteristics of Prions– Prion diseases

– Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter

– Large vacuoles form in brain – Characteristic spongy appearance

– Spongiform encephalopathies

– Prions only destroyed by incineration or autoclaving in 1 N NaOH

ANIMATION Prions: Disease

Viral Diseases of the Skin and Wounds

• Herpes Infections– Signs and symptoms

– Slow spreading skin lesions– Recurrence of lesions is common

– Pathogen and virulence factors– Caused by human herpesviruses 1 and 2– Produce various proteins that act as virulence factors

– Pathogenesis– Painful lesions caused by inflammation and cell death– Cause fusion of cells to form syncytia

Figure 19.11 Oral herpes lesions

Viral STDs

• Genital Herpes– Signs and symptoms

– Small blisters on or around the genitals or rectum

– Pathogen and virulence factors– Human herpesvirus 2 causes most cases– Virus can become latent in nerve cells

– Pathogenesis– Herpesvirus kills epithelial cells at infection site– Blisters may form at sites far from initial infection

site– Babies can become infected during birth10/15/11 MDufilho 41

Figure 24.12 Herpes lesions of the eyes and skin-overview

• Herpes Infections– Epidemiology

– Spread between mucous membranes of mouth and genitals

– Herpes infections in adults are not life threatening

– Diagnosis, treatment, and prevention– Diagnosis made by presence of characteristic

lesions– Immunoassay reveals presence of viral antigens– Chemotherapeutic drugs help control the disease

but do not cure it10/15/11 MDufilho 43

• Warts– Benign epithelial growths on the skin or mucous

membranes– Can form on many body surfaces

– Various papillomaviruses cause warts

– Most warts are harmless

– Transmitted via direct contact and fomites

– Diagnosed by observation

– Various techniques to remove warts – New warts can develop due to latent viruses

Figure 19.13 Various kinds of warts--lesions caused by papillomaviruses-overview

Figure 24.13 Genital warts

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Viral STDs

• Genital Warts– Signs and symptoms

– Warts on the genitalia and surrounding areas– Large growths called condylomata acuminata

may form

– Pathogen– Caused by human papillomaviruses (HPV)– HPV can cause various cancers

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Viral STDs

• Genital Warts– Pathogenesis and epidemiology

– HPVs invade skin or mucous membranes during sex

– Most common STD in the U.S.

– Diagnosis, treatment, and prevention– Diagnosis made by presence of warts– Variety of methods available to remove warts– Vaccine available against HPV strain associated

with cervical cancer

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• Chickenpox and Shingles– Signs and symptoms

– Chickenpox characterized by lesions on the back and trunk that spread across body

– Shingles lesions localized to skin along an infected nerve

– Pathogen– Varicella-zoster virus (VZV) causes both diseases

– Pathogenesis– Infected dermal cells cause rash characteristic of

chickenpox– Virus becomes latent in nerve ganglia

– Reactivated VZV causes shingles

• Chickenpox and Shingles– Epidemiology

– Chickenpox occurs mostly in children– Disease is more severe in adults

– Risk of shingles increases with age

– Diagnosis, treatment, and prevention– Diagnosis based on characteristic lesions– Treatment based on relief of symptoms– Vaccine available against chickenpox

• Other Viral Rashes– Erythema infectiosum

– Caused by an erythrovirus of family Parvoviridae– Respiratory disease that manifests as a rash – Also referred to as fifth disease

– Roseola– Caused by human herpesvirus 6 (HHV-6)– Characterized by a rose-colored rash

– Coxsackievirus infection– Caused by coxsackie A viruses– Produces lesions like those from herpes infections – Also causes hand-foot-and-mouth disease

Figure 19.16 A case of erythema infectiosum (fifth disease)

Viral Diseases of the Nervous System

• Viruses more readily cross the blood-brain barrier

• Occur more frequently than bacterial and fungal infections

• Include meningitis, polio, rabies, and encephalitis

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• Viral Meningitis– Signs and symptoms

– Similar to those of bacterial meningitis– Usually milder than those of bacterial or fungal

meningitis

– Pathogens and virulence factors– 90% of cases caused by viruses in the genus

Enterovirus

– Pathogenesis– Damage to cells in the meninges triggers

meningitis10/15/11 54MDufilho

• Viral Meningitis– Epidemiology

– More common than bacterial and fungal meningitis

– Spread via respiratory droplets and feces

– Diagnosis, treatment, and prevention– Diagnosed by characteristic signs and

symptoms in the absence of bacteria in the CSF– No specific treatment exists

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• Arboviral Encephalitis– Arboviruses are arthropod-borne viruses

– Transmitted via blood-sucking arthropods (e.g., mosquitoes)

– Mosquito-borne arboviruses can cause arboviral encephalitis

– As zoonotic diseases, they rarely affect humans

– Arboviruses usually cause mild, coldlike symptoms

– Can cause if cross the blood-brain barrier

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Table 20.2 Characteristics of Arboviral Encephalitis Diseases and Viruses in the United States

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Figure 20.15 Transmission of six encephalitis arboviruses

Domestic fowls

Wild birds

Mosquitoesare vectors.

Humanscan be infected

via mosquito bites.

Small mammalsare hosts for VEE and

California viruses only.

Horses,and rarely other

domestic mammalsare hosts for equine viruses. Birds are hosts for

all six encephalitisarboviruses.

Encephalitis arbovirusescan overwinter insidemosquito eggs.

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Figure 20.16 Human West Nile virus encephalitis in the United States

Time (months/years)

Reported cases

Annual deaths

(84)(86) (43)

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• Arboviral Encephalitis– Diagnosis based on signs and symptoms

– Confirmed by presence of arbovirus-specific antibodies in CSF

– Treatment is supportive

– Prevention involves limiting contact with mosquitoes– Use netting and insect repellents– Eliminate stagnant water

– Vaccines for horses available against EEE, WEE, VEE, and WNV

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Viral Cardiovascular and Systemic Diseases

• Infectious Mononucleosis– Signs and symptoms

– Severe sore throat and fever occur initially – Followed by swollen lymph nodes, fatigue,

appetite loss

– Pathogen and virulence factors– Epstein-Barr virus (EBV or HHV-4) is the

causative agent– EBV establishes latent infection in host– EBV implicated in number of other diseases

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Figure 21.13 Diseases associated with Epstein-Barr virus

Diseasesof EBV Oral hairy leukoplakia* Burkitt’s lymphoma (shown)

Chronic fatigue syndrome*Nasopharyngeal cancer

Asymptomatic Infectiousmononucleosis

State ofcellularimmunity

*EBV implicated, not proven

Lacking Poor Normal Vigorous

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• Infectious Mononucleosis– Pathogenesis and epidemiology

– Transmission occurs via saliva– EBV infects B lymphocytes– Majority of adults have antibodies against EBV

– Diagnosis, treatment, and prevention– Diagnosed by presence of large, lobed B

lymphocytes and neutropenia– Treatment focuses on relieving symptoms– Prevention is difficult since EBV occurrence is

widespread10/15/11 63MDufilho

• Cytomegalovirus Disease– Signs and symptoms

– Asymptomatic in most cases– Complications in neonates and immunodeficient

individuals– Pathogen and virulence factors

– Caused by Cytomegalovirus– Pathogenesis and epidemiology

– Transmit by direct contact with body fluids or transplacentally

– One of the most common infections of humans– Diagnosis, treatment, and prevention

– Fomivirsen is administered for eye infections– No vaccine is available

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Figure 21.14 An abnormally enlarged “owl’s eye” cell indicates Cytomegalovirus (CMV) infection

“Owls eye” cell

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• African Viral Hemorrhagic Fever– Signs and symptoms

– Fever and fatigue – Minor petechiae progress to severe internal

hemorrhaging– Pathogens and virulence factors

– Caused by Ebolavirus or Marburgvirus– Pathogenesis and epidemiology

– Occurs primarily in Africa– Transmitted via contact with bodily fluids of

infected individual

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Figure 21.17 Filamentous Ebolavirus

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Figure 21.18 Sites in which locally acquired cases of Marburg and Ebola viruses have occurred

Marburg

Ebola10/15/11 68MDufilho

• African Viral Hemorrhagic Fever– Diagnosis, treatment, and prevention

– Diagnosis based on characteristic symptoms and presence of virus in the blood

– Treatment involves fluid and electrolyte replacement

– Vaccines are being studied for their effectiveness in humans

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Viral Diseases of the Upper Respiratory System

• Common Cold– Signs and symptoms

– Sneezing, runny nose, congestion, – sore throat, malaise, and cough

– Pathogens and virulence factors– Enteroviruses (rhinoviruses) are the most

common cause– Numerous other viruses cause colds

– Pathogenesis– Cold viruses replicate in and then kill infected

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Viral Diseases of the Upper Respiratory System

• Common Cold– Epidemiology

– Rhinoviruses are highly infective– Spread by coughing/sneezing, fomites, or

person-to-person contact

– Diagnosis, treatment, and prevention– Signs and symptoms are usually diagnostic– Pleconaril can reduce duration of symptoms– Hand antisepsis is important preventive measure

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Viral Diseases of the Lower Respiratory System

• Influenza– Signs and symptoms

– Sudden fever, pharyngitis, congestion, cough, myalgia

– Pathogens and virulence factors– Influenza virus types A and B are the causative

agents– Mutations in hemagglutinin and neuraminidase

produce new strains– Occurs via antigenic drift and antigenic shift

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Figure 22.12 Influenzavirus budding from a cell

HemagglutininNeuraminidase

Envelope

ssRNAmoleculein helicalcapsid

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• Influenza– Mutations in hemagglutinin and neuraminidase

produce new strains– Occurs via antigenic drift and antigenic shift– Named by type (A or B), location and date or

original identification– Example – A/Singapore/1/80 (H1N2)

– If isolated from an animal, that is included – Hong Kong flu or swine flu

– Asia is major site of antigentic shift

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Figure 22.13 The development of new strains of flu viruses-overview

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Figure 22.11 A scene from the flu pandemic of 1918-19

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• Influenza– Pathogenesis

– Symptoms produced by the immune response to the virus

– Flu patients are susceptible to secondary bacterial infections

– Virus causes damage to the lung epithelium

– Epidemiology– Transmitted via inhalation of viruses or by self-

inoculation– Complications occur most often in the elderly,

children, and individuals with chronic diseases10/15/11 77MDufilho

• Influenza– Diagnosis, treatment, and prevention

– Signs and symptoms during a community-wide outbreak are often diagnostic

– Treatment involves supportive care to relieve symptoms

– Oseltamivir and zanamivir can be administered early in infection

– Prevent by immunization with a multivalent vaccine

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• Severe Acute Respiratory Syndrome (SARS)– Signs and symptoms

– High fever, shortness of breath, and difficulty breathing

– Later develop dry cough and pneumonia– Pathogen and virulence factors

– Caused by a coronavirus called SARS virus– Pathogenesis and epidemiology

– SARS virus spreads via respiratory droplets– Diagnosis, treatment, and prevention

– Diagnosis based on signs and symptoms of SARS– Treatment is supportive10/15/11 79MDufilho

Figure 22.14 The face of SARS

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• Respiratory Syncytial Virus Infection– Most common childhood respiratory disease

– Signs and symptoms– Fever, runny nose, and coughing in babies or

immunocompromised individuals– Mild coldlike symptoms in older children and adults

– Pathogen– Respiratory syncytial virus (RSV)

– Pathogenesis– Virus causes syncytia to form in the lungs– Immune response to RSV further damages the lungs10/15/11 81MDufilho

Figure 22.15 A syncytium forms when RSV triggers infected cells to fuse with uninfected cells

Multinucleatedsyncytium

Newlyinfected cell

Nucleus

Infectedhost cell

UninfectedcellNewly

infected cell

Infectedhost cell

Multinucleatedsyncytium

10/15/11 82MDufilho

• Respiratory Syncytial Virus Infection– Epidemiology

– Transmission occurs via close contact with infected persons

– Diagnosis, treatment, and prevention– Diagnosis made by immunoassay– Supportive treatment for young children– Prevention includes aseptic technique of health

care and day care employees

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Viral Diseases of the Digestive System

• Viral Gastroenteritis– Signs and symptoms

– Similar to bacterial gastroenteritis– Pathogens and pathogenesis

– Caused by caliciviruses, astroviruses, and rotaviruses– Epidemiology

– More cases occur in winter– Diagnosis, treatment, and prevention

– Serological test distinguishes among viruses– Treatment is based on fluid and electrolyte

replacement– Vaccine for rotavirus exists10/15/11 84MDufilho

Figure 23.13 Some viruses causing gastroenteritis-overview

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Figure 23.14 Deaths from rotaviral diarrhea are most common in developing countries

1000 deaths

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• Viral Hepatitis– Signs and symptoms

– Jaundice, abdominal pain, fatigue, vomiting, appetite loss

– Symptoms may occur years after initial infection – Host immune response causes much of the liver

damage– Pathogen and pathogenesis

– Hepatitis A virus (HAV) – Hepatitis B virus (HBV)– Hepatitis C virus (HCV)– Hepatitis delta virus (HDV)– Hepatitis E virus (HEV)

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Table 23.2 Comparison of Hepatitis Viruses

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• Viral Hepatitis– Diagnosis, treatment, and prevention

– Initial diagnosis made by observation of jaundice, enlarged liver, or fluid in the abdomen

– Serological testing can identify viral antigens– HBV diagnosed by presence of viral proteins in

body fluids– Supportive care for symptoms– Prevent with good hygiene and protected sex or

abstinence– Vaccines are available against HAV and HBV

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Figure 23.15 The three types of viral particles produced by hepatitis B viruses-overview

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Chapter 13 Characterizing and Classifying Viruses, Viroids, and Prions.

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nm poliovirus

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nm smallpox virus

nm bacteriophage ms2

viral nucleic acid

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