Chapter 25- Pathogenic RNA Viruses

Question 1

What are general characteristics of RNA viruses?

Answer 1

Only infective agents that store genetic information in RNA molecules

Broken into 15 families: genomic structure, presence of an envelope, and size and shape of capsid

Many RNA viruses have more than only molecule of RNA- genomes are segmented

Question 2

What is the first class of naked, +ssRNA viruses?

Answer 2

Picornaviridae

Smallest of the animal viruses (22-30nm)

Picornaviruses that cause disease are in several genera

Question 3

What are rhinoviruses?

Answer 3

Class of Picornaviridae

Cause most cases of the common cold - infections limited to the upper respiratory tract

Single virus is sufficient to cause a cold

Transmitted by aerosols, bomites, or hand-to-hand contact (most common)

Some immunity can be acquire against serotypes infected with in the pact

Number of infections decrease with age

Question 4

How is a rhinovirus diagnosed, treated, and prevented?

Answer 4

Diagnosis- manifestations are characteristic

Treatment- medication for the management of symptoms, will not reduce duraction of disease

Prevention- handwashing

Question 5

What are characteristics of enteroviruses?

Answer 5

Group of picornaviridae family

Transmitted via the fecal- oral route- by ingestion of contaminated substance, fomites, oral contact

Infect the pharynx and intestine- spread via the blood infecting various targets in the body, are cytolytic

3 main genera: Poliovirus, coxsackievirus, and echovirus

Question 6

What is poliomyelitis?

Answer 6

Polio is caused by one of the three serotypes of poliovirus

Last case of wild-type poliomyelities in the Americas was in 1979

Question 7

What are the 4 conditions caused by poliovirus?

Answer 7

Asymptomatic

Minor polio- fever, headache, and sore throat

Nonparalytic polio- virus invades the meninges and CNS producing muscle spasms and back pain

Paralytic polio- virus invades the cells of the spinal cord and motor cortex of the brain, producing paralysis by limiting nerve impulse conduction, may results in bulbar poliomyelitis (brain stem and medulla are infected in the paralysis of muscles in limbs)

Question 8

What is postpolio syndrome and what are the vaccines against polio?

Answer 8

Postpolio syndrome- crippling deterioration in the function of polio-affected msucles

Near elimination of polio due to development of two vaccines

Inactivated polio vaccine (IPV)- Joan Salk

Oral Polio vaccine (OPV)- Albert Sabin

Question 9

What are characteristics of Hepatitis A?

Answer 9

Hepatitis A virus- noncytolytic picornavirus is the causative agent

Survives on various surfaces and is resistant to common household disinfectants

Transmitted through the fecal-oral route

Fever, nausea, and jaundice are due to patient’s immune response- infection does not cause chronic liver disease

Complete recovery occurs most of the time

Two doses of Hepatitis A vaccine are recommended

Question 10

What are some characteristics of Hepeviridae?

Answer 10

Hepatitis E- enteric hepatitis- hepatitis E virus

Formerly classified as calicivirus

Fatal in 20% of pregnant women

No treatment

Prevent by interrupting fecal-oral route

Question 11

What are characteristics of Togaviridae and Flaviviridae?

Answer 11

Enveloped, +ssRNa virus

Designated as Arboviruses
Mosquitoes and ticks transmit arboviruses among animal hosts causing zoonotic diseases
Arthropod vectors remain infected
Are a continual source of new infections
Most infections result in mild, flu-like symptoms
Arboviruses can occasionally result in second-stage infections
Encephalitis (caused by togavirus), dengue fever and yellow fever (caused by flavivirus)
Diagnosis by serological tests like ELISA
No treatment available
Prevention by vaccination for EE, JE, West Nile encephalitis, Yellow fever; no vaccine available for dengue

Question 12

What are characteristics of Togaviridae?

Answer 12

Enveloped, +ssRNA viruses

Rubella- also called “German measles”

Rubella virus/ rubivirus is the causative agent

One of the five childhood diseases that causes skin lesions- rash of falt, pink red spots

Infection begins in respiratory system but spreads throughout the body

Infections in children are not usually serious- may result in arthritis or encephalitis in adults

Infection in pregnant women can cause congenital defects

Vaccination is effective

Question 13

What are characteristics of Hepatitis C? What type of virus causes Hepatitis C?

Answer 13

Flaviviridae- enveloped, +ssRNA virus

Hepatitis C caused by Hepatitis C virus

Accounts for most non-A, non-B hepatitis infections in US

Spread through use of needles, organ transplants, sexual activity, but not arthropod vectors

Chronic infection with few or any symptoms- results in severe liver damage and failure

No treatment or vaccine

Question 14

What are the characteristics and the disease caused by coronaviridae?

Answer 14

Named due to corona-like halo formed by their envelopes

Transmitted via large droplets from upper respiratory tract

2nd most common cause of colds

Severe Acute Respiratory Syndrome- SARS, emerging disease caused by previously unknown coronavirus, rapid progressive respiratory illness, symptoms include fever, dypnea, cough, may lead to respiratory failure or death

No treatment or vaccine

Question 15

What are the physical characteristics of retroviridae?

Answer 15

Studied more than any other virus

Have polyhedral capsules with spiked envelopes- 2 glycoprotein spikes, gp120 and gp41

Genomes contain 2 identical molecules of +ssRNA, have enzymes reverse transcriptase, protease, and integrase, tRNA (primer to make dsDNA)

Do not conform to central dogma

Transcribe dsDNA from ssRNA using reverse transcriptase

Question 16

What are the two types of retroviruses?

Answer 16

Primarily oncogenic- Human T-lymphotrophic viruses- cancer of lymphocytes

Primarily immunosuppressive- Lentivirus human immunodeficiency virus (HIV 1 & 2)

Question 17

What is AIDS?

Answer 17

Acquired Immunodeficiency Syndrome- not a disease

Certain opportunistic or rare infections that occur in the presence of antibodies against HIV and a CD4 white blood cell count below 200 cells per microliter of blood

Question 18

What is HIV?

Answer 18

Human Immunodeficiency Syndrome

Likely arose from a mutation of simian immunodeficiency virus, emerged in African populations around 1930

Only replicates in humans and destroys immune system

Two major types of HIV: 1- US and Europe, 2- West Africa

Question 19

How do the two glycoproteins serve in the attachment of the HIV virus?

Answer 19

Two antigenic glycoproteins

1. gp 120- primary attachment molecule, responsible for antigenic variability, binds to CD4 receptor molecules on a host cell
2. gp 41- promotes fusion of viral envelope with the target cell, form syncyticum

Both glycoproteins impede immune clearance of HIV

Question 20

How does the reverse transcriptase enzyme function?

Answer 20

+ssRNA taken up by enzyme to form a -ssDNA strand, use of tRNA molecules

+ssRNA is degraded

+ssDNA strand is made as complementary strand to form dsDNA

Question 21

What is the replication cycle of HIV?

Answer 21

1. Attachment by gp120
2. Uncoating - gp 41
3. Synthesis of dsDNA by reverse transcriptase, etnry into nucleus
4. Integration by integrase- latency
5. Synthesis of viral proteins
6. Assembly and budding
7. Maturation of virion (protease)

Question 22

What is the pathogenesis of AIDS?

Answer 22

1. Initial burst of virion production and release from infected cells
2. Immune system produces antibodies and number of free virions falls, no symptoms, loss of T cells
3. Virions are replenished in blood, body cannot replace helper T cells, over 5-10 years helper T cells decline to a level that impairs immune response
4. Rate of antibody formation falls
5. HIV production climbs, and patient dies from infections

Question 23

What are the opportunistic infections associated with AIDS?

Answer 23

Tuberculosis, Kaposi’s Sarcoma, Herpes, Pneumonia, Fungal infections

Question 24

What is the epidemiology of AIDS?

Answer 24

AIDS first recognized in young male homosexuals in US

AIDS is now found throughout the world, approximately 33 million infected individuals

HIV found in blood, semen, saliva, vaginal secretions, and breast milk

Blood and semen are more infective than other secretions

Infected fluids must contact a tear or lesion in the skin or mucous membranes

Infected fluids can be injected into the body

Question 25

What are the most common modes of tranmission of HIV?

Answer 25

Females- high risk heterosexual contact followed by intravenous drug use

Males- homosexual contact followed by intravenous drug use

Question 26

How is AIDS diagnosed?

Answer 26

Patient has one or more rare disease, antibodies against HIV, fewer than 200 CD4 lymphocytes per microliter of blood, unexplained weight loss, fatigue

Serological diagnosis- test for antibodies through ELISA, Western blot, agglutination, PCR gives definitive answer

Signs and symptoms vary according to the disease present

Long term non-progressors appear not to develop AIDS- may be due to defective virions or lack of effective coreceptors of virus

Question 27

How is an HIV infection treated?

Answer 27

HAART (highly active antiretroviral therapy)- used to reduce viral replication

Cocktail of antiviral drugs including nucleoside analogs, protease inhibitors, reverse transcriptase inhibitors

Various problems must be overcome to develop a vaccine

Question 28

How is an HIV infection prevented?

Answer 28

Behavioral changes can slow progression of AIDS epidemic

Abstinence and safe sex

Use of clean needles

Screening of blood products

Administer AZT to infected pregnant women

Question 29

What are characteristics of paramyxoviridae?

Answer 29

Enveloped, unsegemnted, -ssRNA

Causes cells to fuse and form giant, multinucleate syncytia, syncytia enable virus to evade immune system

4 Genera infect humans:

Morbillivirus (measles virus)

Paramyxovirus- causes croup

Rubulavirus (mumps virus)

Pneumovirus (respiratory syncytia virus)

Question 30

What are some characteristics of measles?

Answer 30

1 of 5 clasical childhood diseases caused by paramyxovirus

Spread in the air via respiratory droplets

Viruses infect the respiratory tract and spread throughout the body

Koplik’s spots appear on mucous membrane of the mouth

Lesions then appear on the head and spread over body

Rare complications- pneumonia, encephalities, and subacutre sclerosing panencephalitis (SSPE)

Question 31

How is measles diagnosed, treated, and prevented?

Answer 31

Signs of measles, particuarly Koplik’s spots, are diagnostic

No antiviral treatment available

Vaccine (MMR) has eliminated endemic measles in USA

Measles remain a frequent cause of death in other countries

Question 32

What are some characteristics of mumps?

Answer 32

Causative agent: Mumps virus, paramyxovirus

Transmission occurs via respiratory secretions

Viruses infect respiratory system and spreads to other organs

Results in parotitis- painful enlargement of the parotid salivary glands

Infections are often asymptomatic

No specific treatment is available

Vacccine has almost eradicated mumps in industrialized world

Question 33

What are some characteristics of Rhabdoviridae?

Answer 33

Enveloped, unsegmented, -ssRNA virus

Rabies virus - a rhabdovirus (bullet-shaped)

Classic zoonotic disease of animals

Primary reservoir of urban areas is a dog, bats are the source of most cases of rabies in humans

Transmission occurs via a bite- virus sometimes introduced through break in skin or inhalation

Virus attaches to nerve cells and travels to CNS - results in rabies specific neurological symptoms (hydrophobia, seizures, hallucinations, paralysis, and death)

Death results from respiratory paralysis and other complications

Question 34

How is rabies diagnosed, treated, and prevented?

Answer 34

Diagnosis- neurological symptoms, characteristic negri bodies in the brain

Treatment- too late to intervene by the time symptoms and antibodies occur, treat at site of infection with injection of human rabies immune globulin, vaccination with human diploid cell vaccine at days 0,3,7, and 14

Prevention- vaccination of domestic cats and dogs

Question 35

What are some characteristics of Respiratory Syncytial Virus?

Answer 35

Disease is limited to respiratory tract

Transmission occurs via fomites, hands, and respiratory droplets

Virus causes formation of syncytia in lungs

Result in difficult breathing known as dyspnea

Leading cause of fatal respiratory disease in infants and children

Infection is asymptomatic or a mild cold in older children and adults

Diagnosis based on signs of respiratory distresss - verified by immunoassay

Treatment is supportive

Question 36

What are some characteristics of Filoviridae?

Answer 36

Hemorraghic fevers- caused by Marburg Virus and Ebola Virus

Found in Germany, Zimbabwe, Kenya, Uganda, Angola, etc.

Natural reservoir and mode of transmission to humans unknown

Spread person to person by contaminated body fluids and syringes

Virions attack many cells of the body, especially the macrophages and liver cells

Uncontrolled bleeding under the skin from every body opening due to jelly like consistency of organs (viral glycoprotein prevents the neighboring cells from adhering- allows blood to leak out of vessels)

Only treatment is fluid replacemtn

90% of human victims die

Question 37

What are some characteristics of Orthomyxoviridae? What are some characteristics of the disease it causes?

Answer 37

Influenza- Orthomyxovirus Type A and B

Infection occurs primarily through inhalation of airborne viruses

Rarely attack cells outside the lungs

Death of infected epithelial cells eliminates the lungs epithelial linings

Patients more susceptible to secondary bacterial infections

Symptoms include fever, malaise, headache, and myalgia

Induced by cytokines released as part of immune response

Question 38

What are the characteristics of the genome and envelope of influenza virus?

Answer 38

Genome consists of 8 different -ssRNA molecules

Envelope components- highly variable with respect to glycoprotein spikes, contains:

Hemagglutinin- entry, attachment to pulmonary epithelial cells or trigger endocytosis

Neuraminadase- exit, provide virus access to cell surfaces by hydrolyzing mucus

Question 39

How are new strains of the influenza virus produced?

Answer 39

Mutations result in production of new strains of influenza

Antigenic drift- minor variations

Antigenic shift- major antigenic changes

Question 40

How is influenza diagnosed, treated, and prevented?

Answer 40

Diagnosis- signs and symptoms, community wide outbreak

Treatment- drugs prevent viral uncoating or block virion release from infected cells

Oseltamivir pills or inhaled zanamivir mist are neuramindase inhibitors, must be administered within 48h of infection

Prevention- multi valent vaccine

Question 41

What are some characteristics of Hepatitis D? What virus is it caused by?

Answer 41

Arenavirus- Hepatitis D is caused by Hepatitis D virus

Transmitted through bodily fluids via sexual contact and contaminated needles

Hepatitis D requires Hepatitis B to be virulent- cannot attach to liver cells

Hepatitis D plays a role in triggering liver cancer

Vaccination against Hepatitis B limits Hepatitis D

Question 42

What are some characteristics of reovirdiae?

Answer 42

Only microbes with genomes composed of dsRNA

Originally considered respiratory enteric orphans (REO)

Not initially associated with any diseases

Includes Rotavirus- most common cause of infantile gastroenteritis, unique wheel-like appearance, significant cause of death in developing countries