Lecture 1 - Introduction to Viruses

Question 1

What are the Nature of Viruses?

Answer 1

1. Viruses are very small, about 100x smaller than bacteria
2. Particles consist of a genome (DNA or RNA), a protein coat (capsid), in some cases a lipid membrane (envelope) derived from the host cell membrane
3. Viruses are obligate intracellular parasites
   - They lack funtional organelles and ribosomes
   - They use cell machinery to synthesize proteins, nucleotides, carbohyrdates, and energy

Question 2

What are some of the characteristics of Viruses?

Answer 2

• Small (nm)
• Completely dependent on host cell machinery
• Contain one species of nucleic acid
• External viral-attachment proteins
• Genome may be infectious
• Some viruses can integrate their genome into the cellular chromosome

Question 3

Why are viruses the most abundant form of life on Earth?

Answer 3

• 10-50 million bacteriophages per mL of seawater
• 1031 bacteriophages in the world (10x more than bacteria)
• 20% of microbes in the Earth’s oceans are destroyed daily by virus infections
• Bacteria and plankton depend on nutrients released by bacteria killed by viruses

Question 4

How are viruses used as tools for understanding Biolody?

Answer 4

• Identification of promoters for eukaryotic RNA polymerases
• Understanding of enzymes involved in DNA replication
• Isolation and characterization of cellular oncogenes

Question 5

How are viruses and medicine related?

Answer 5

1. Vaccines

• Edward Jenner (1796): work with cowpox lead to 1st vaccine (vacca means cow)
• Anti-viral immunity
• Vaccines exist for a number of diseases today
• Vaccine programs have eradicated/almost eradicated a few viral diseases

2. Gene Therapy

• Viruses used to deliver “corrected” copies of genes to cells of the body.
• Successful, but safety concerns remain.

Question 6

How/Who discovered viruses?

Answer 6

• Adolf Mayer (1879): Sap from plants infected with tobacco mosaic disease transmitted disease to healthy plants
• Neither bacterial or fungal agent could be cultured from the sap (Koch’s postulates)
• Dimitri Ivanofsky (1892): Infected sap remained infectious even after passage through a Chamberlain filter
• Agent was unculturable bacterium small enough to pass through the filter
• Martinus Beijerinck (1898): Infectious agent was able to reproduce and multiply in living plant tissue
• Termed the term “virus”

Question 7

What are bacteriophages and how were they discovered?

Answer 7

Twort and d’Herelle: 1st description of bacteriophages in 1915 and 1917
Bacterial colonies became transparent (“glassy transformation)
Formation of “plaques” on lawns of dysentery bacillus
D’Herelle established concepts of adsorbtion, host specificity, cell lysis, and release of viral particles
Phage therapy
Bacteriophage poured onto open wounds, given orally, or injected
1924 Pulitzer Prize novel Arrowsmith (Sinclair Lewis)
Faded after discovery and increased use of antibiotics

Question 8

What are some of the theories for Viral Origin?

What is the Regressive Hypothesis?

What is Cellular Origin Hypothesis?

What is the coevolution hypothesis?

What is the Panspermia Hypothesis?

Answer 8

• Regressive hypothesis: viruses may have once been small cells that parasitized larger cells; genes not needed for parasitism were lost
• Cellular origin hypothesis: viruses may have evolved from bits of DNA or RNA that “escaped” from the genes of a larger organism
• Plasmids
• Transposons (“jumping genes”)
• Coevolution hypothesis: viruses may have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on earth and have been dependent on cellular life for billions of years
• Viroids and prions
• Panspermia hypothesis: viruses and other microorganisms are raining down from outer space upon the Earth

Question 9

What are animal viruses and cell culture?

Answer 9

• Advent of cell culture 1948-1955
• Development of cell lines (e.g. HeLa) and optimal culture media
• Lead to development of polio vaccine
• Vaccines grown in animals or embryonated eggs
• Modern era of molecular virology

Question 10

What is involved/ what is Polio Virus?

Answer 10

• Evidence of disease dates back to Ancient Egypt
• Epidemics peaked in the U.S. in 1916 and the mid-1940s to 1950s
• Seasonal incidence (July-October)
• Closure of parks, swimming pools and other public services for millions of people
• March of Dimes fundraising effort
• 2,680,000 dimes to President Roosevelt in a matter of weeks (1938)
• Mission focus shifted to premature births and birth defects
• Massive, global vaccination campaign over the past 20 years to eradicate disease
• polio exists only in a few countries in Africa and Asia
• Legacy of disability rights, vaccine development and campaigns

Question 11

Describe SmallPox

Answer 11

• Killed an estimated 300-500 million individuals in the 20th century
• Spanish conquest of Aztec and Peruvian empires aided by smallpox epidemics
• Settlement of the east coast accompanied by devastating outbreaks of smallpox among Native American populations
• Global vaccine efforts eradicated disease

Last naturally occurring case diagnosed in 1977

Mandatory vaccination stopped in 1982

Bioterrorism concerns

Question 12

Describe Foot and Mouth Disease

Answer 12

Significant epidemic disease threatening the cattle industry since the 16th century
Highly contagious among cloven-hoofed animals (cattle, pigs, sheep)
Blister-like lesions resulting in weight loss, loss of milk production, lameness
Methods of control
Slaughtering infected animals or animals in contact with infected animals
Public cleansing and disinfection efforts
Vaccination
Major economic and trading difficulties for infected countries
4 million animals slaughtered in the UK
cost the farming industry between £800m and £2.4b

Question 13

What was the SARS outbreak in 2002-2003?

Answer 13

Severe Acute Respiratory Syndrome; caused by a coronavirus
Mystery “flu-like” illness emerged in China
15% fatality rate
Healthcare workers and hospitals
Impacts on global travel and world economy
Modern technology allowed for careful tracking of cases and identification of viral agent
8,096 cases and 774 deaths worldwide
Live markets selling exotic animals (civets)

Question 14

Describe Hantavirus

Answer 14

Cause of hemorrhagic fever in several thousand soldiers during the Korean War
Outbreak in 1993 in Four Corners region of southwestern US
5 deaths of previously healthy adults
Respiratory failure following flu-like symptoms
~40-50% fatality rate
Carried by rodents (Deer mouse)

72 yr old NY professor bitten by an animal while camping near Mount Marcy
Developed flu-like symptoms and was hospitalized in ICU for 4 days; recovered
CDC lab tests confirmed hantavirus
9 people infected visiting Yosemite National Park last summer (3 died)

Question 15

What is the Influenza Virus?

Answer 15

1918 Spanish flu pandemic killed 20-50 million people (more than WWI)
Death rate for 15-34 yo 20x higher than past epidemics
Average lifespan in US lowered by 10 years
Schools, churches, theaters, stores, and restaurants closed
No public gatherings, not even funerals

H5N1 “avian” flu (1997-today)
Rare; human contact from infected poultry
High pathogenicity; 60% fatality rate
H1N1 “swine” flu outbreak of 2009
Novel strain spread rapidly throughout world
Impacts on freedoms, trade, and health regulations
Approx. 61 million cases; 12,470 deaths
Fears of a repeat of the 1918 global pandemic