Virulence and Pathogenicity-Virology

Question 1

What is pathogenicity

Answer 1

The ability of a virus to cause disease/harm in host

Question 2

What is pathogenesis?

Answer 2

The manner/mechanism of development of a disease.

Question 3

What is virulence?

Answer 3

Used as a quantitative or relative measure of the degree of

pathogenicity of the infecting virus.

Question 4

Factors related to the virus (in relation of the “tug of war”)

Answer 4

• Genetic Variation of Virus
• Route of entry of Virus inHost
• Affinity of Virus to Host Organs
• Dose of Infection
• Immuno evasion

Question 5

Factors related to the host (in relation of the “tug of war”)

Answer 5

•Host Species
•Host Immunity
•Host Physiological factors (Nutrition status, Age,
Hormonal Factors, Stage of Cell Differentiation)
• Fever

Question 6

Other factors (in relation to the “tug of war”)

Answer 6

• Environment

* Dual infections

Question 7

What is the LD50 or lethal dose 50?

Answer 7

• The dose of the virus required to cause death in 50% of animals, such as mice.
• Measures the virulence of a pathogen.
• Lower LD50 means the virus is more virulent since it takes less to kill half the population

Question 8

What are the sequential steps in viral infection (pathogenesis)?

Answer 8

1) Entry of virus and primary replication
2) Spread and infection of target organs
3) Virus cell interactions
4) Tissue organ injury
5) Shedding

Obstacle to the virus’ success is the host’s immunity

Question 9

What are the routes of entry into the host via the skin?

Answer 9

• A cut breach
• Bite of arthropods
• Bite of infected animal
• Contaminated objects (like needles)

Question 10

What are the routes of entry into the host via the mucous membrane?

Answer 10

• Conjunctiva
• Oropharynx
• Genitourinary Tract, Rectum
• Defenses include IgA and Virucidal proteins

Question 11

Other routes of entry into the host

Answer 11

• GI tract (via contaminated food/water)

* Respiratory tract

Question 12

How does the virus spread in the host?

Answer 12

•At the epithelium: Local Spread of virus on
epithelial surfaces, causes localized infection which may or may not proceed to subepithelial layer/ underlying tissues (need to overcome host defense first).
• At subepithelial level: Viruses get access to
lymphatics, phagocytic cells and tissue fluids (these may help carry virus to the blood stream).
•In the blood stream: virus will become viremic (presence of virus in the blood) where it will spread through the body.

Question 13

What is primary viremia?

Answer 13

The initial spread of virus into the blood via either:
• from subepithelial tissue/lymphatics
•directly injected in blood, through bite of mosquitoes, or syringes

Question 14

What is secondary viremia?

Answer 14

Virus has replicated/multiplied in major organs & once more entered the circulation.

Question 15

What is disseminated viremia?

Answer 15

Infection spreads beyond the primary site of infection.

Question 16

What is systemic infection?

Answer 16

If a number of organs or tissues are infected.

Question 17

How are viruses spread via nerves?

Answer 17

• Through peripheral nerves (Rabies virus)
• Through receptor neurons in the nasal olfactory epithelium (HSV-1)
• Virus can cross bloodbrain barrier & infect CNS (West Nile virus)

Question 18

What is a neurotropic virus?

Answer 18

• Viruses that can infect neural cells.

* Infection may occur by neural or hematogenous spread.

Question 19

What is a neuroinvasive virus?

Answer 19

Viruses that enter the central nervous system (spinal cord and brain) after infection of a peripheral site.

Question 20

What is a neurovirulent virus?

Answer 20

Viruses that cause disease of nervous tissue, manifested by neurological symptoms and often death.

Question 21

Explain how a virus “trojan horse” works

Answer 21

The viruses enter a monocyte (the trojan horse) in the lumen of the blood vessel. The monocyte then crosses the blood tissue barrier. Once it is in the tissue, the virus leaves the monocyte.

Question 22

What is tropism?

Answer 22

The specificity/affinity of a virus for a particular host organ/tissue

Question 23

What is a pantropic virus?

Answer 23

A virus that can replicate in more than one organ/tissue in the host.

Question 24

What are the virus cell interactions?

Answer 24

• Inhibition of Host-Cell Nucleic Acid Synthesis
• Inhibition of Host-Cell RNA Transcription
• Inhibition of Processing of Host-Cell Messenger RNAs
• Inhibition of Host-Cell Protein Synthesis
• Cytopathic Effects of “Toxic” Viral Proteins
• Interference with Cellular Membrane Function

Question 25

What are the outcomes for the cell during a viral injury?

Answer 25

• Cell lysis/bursting (allows for the spreading of newly formed viruses)
• Apoptosis (programmed cell death used as a last resort by the host to prevent the viral factory from making new viruses)
• Oncoviruses (cells that cause cancer like papillomavirus)

Question 26

What is a persistent infection?

Answer 26

•When viruses remain latent or dormant in host cell
for long periods, escaping detection by the host immune system. Some of these viruses also cause chronic stimulation of the immune system of host, resulting in
immunopathology.
• Will shed at lower titers for months to years.

Question 27

What is viral shedding?

Answer 27

•The expulsion and release of virus progeny following successful reproduction during a host-cell infection.
• Crucial to the maintenance of infection in
populations

Question 28

What happens to the shedding in an acute infection?

Answer 28

There will be intensive shedding over short time period

Question 29

What are the different virus injury that can be done to the skin?

Answer 29

• Ulcers (from foot and mouth disease)
• Nodules (from lumpy skin disease in the cattle)
• Benign skin growths resulting from the virus affected the top layer of the skin called warts (from papillomavirus)
• Reddening of the skin called erythema (from hog cholera)

Question 30

How do viruses attack the gastrointestinal tract?

Answer 30

Once ingested or carried by the blood, it destroys the intestinal enterocytes. This leads to gastrointestinal disease, malabsorption, diarrhea. Pronounced dehydration, acidosis, hemoconcentration

Question 31

How do viruses affect the respiratory system?

Answer 31

• Inflammation
• Exudation
• Influx of inflammatory cells
• Obstruction of air passages
• Hypoxia & Respiratory distress
• Secondary Bacterial Infection

Question 32

How do viruses affect the nervous system?

Answer 32

• Lytic(destruction/bursting) infections of neurons
• Neuronal necrosis (Necrosis is death of body tissue)
• Neuronophagia (killing/devouring of neuronal cells by phagocytic cells)
• Perivascular cuffing (Inflammatory cells around blood vessels in CNS)

Question 33

How do viruses affected the hemopoietic system?

Answer 33

• Damages the endothelium (by either causes small spot hemorrhages called Petechiae hemorrhage or large spot hemorrhages called Ecchymoses hemorrhage).

Question 34

What is disseminated intravascular coagulation?

Answer 34

• Formation of blood clots in the small blood vessels throughout the body.
• Triggered by viral infection or effect of virus infection on blood vessels.
• Some of these clots can clog the vessels and cut off blood supply to organs such as the liver, brain, or kidneys, damaging vital organs.
• Over time, the clotting proteins in your blood are consumed or “used up.” When this happens, severe bleeding start occurring from various sites.

Question 35

What is vasculitis?

Answer 35

• Inflammation and damage to blood vessels
• Can be mediated with Immune-Complex (antigen-antibody complex) formation and seen in Feline Infectious Peritonitis Virus (FIPV)

Question 36

What is a teratogenic virus?

Answer 36

• A virus that causes cause developmental defects of embryo or fetus after in-utero infection.
• Depends on species and gestational age.