Virulence and Pathogenicity- virology

Question 1

Pathogenicity

Answer 1

Pathogenicity: the ability of a virus to cause disease in host.

*a pathogen is the virus that causes disease

*pathogenesis is the mechanism of development of a disease

Question 2

Virulence

Answer 2

Virulence: a quantitative measure of the degree of pathogenicity of the infecting virus. virulence depends on both the virus and the host. it is not an absolute property of a virus.

Question 3

Factors affecting virulence of a virus: Virus vs. Host

Answer 3

Virus factors

• genetic variation
• route of virus entry
• affinity of virus to host organs
• dose of infection
• immuno evasion

Host factors

• host species
• host immunity
• host physiological factors
• fever

Other factors

• environment
• dual infections

Question 4

Viral Tropism

Answer 4

Viral Tropism= the affinity of a virus for a particular host tissue

Pantropic virus: can replicate in more than one host organ/tissue

Question 5

Lethal dose 50

Answer 5

Lethal dose 50 (LD50): dose of the virus required to kill 50% of lab animal population.

Lower LD50= more virulent virus

Question 6

Five steps of Pathogenesis

Answer 6

Pathogenesis= steps of viral infection

1. Entry of virus and primary replication
2. spread and infection of target organs
3. virus-cell interactions
4. tissue and organ injury
5. shedding

Virus spread in host: types

Disseminated infection: infection spreads beyond primary site of infection

Systemic infection: a number of organs or tissues are infected.

Question 7

Routes of virus entry to host (4)

Answer 7

Step 1 of pathogenesis= Routes of virus entry

• skin
  
  • through cut
  • injection (bite of bug or animal)
  • injection by contaminated objects (needle)
• mucous membrane
  
  • conjunctiva (eye)
  • oropharynx
  • genitourinary tract
• GI tract
  
  • contaminated food/water
• respitory tract
  
  • air transfer

Question 8

Viral spread in host

Answer 8

Step 2 of pathogenesis= Viral spread in host

“like a layer cake”

1. spread on epithelium
2. invasion of subepithelial layer-tissue and lymphatics
3. invasion of blood stream and spread throughout body (viremia)

OR

3. spread via nervous system

Question 9

Viremia

Answer 9

Viremia= presence of virus in blood.

***mechanamism of spread of virus***

Primary viremia- initial entry of virus into blood either through spread from subepithelium or direct injection (mosquito)

Secondary viremia- virus has replicated in major organs and reenters circulation

Question 10

Virus spread via nerves

• Neurotropic virus
• Neuroinvasive virus
• Neurovirulent virus

Answer 10

virus spread via nerves

***mechanism of spread of virus***

Neurotropic virus: infect neural cells

Neuroinvasive virus: enter CNS after infection of a peripheral site

Neurovirulent virus: cause disease of nervous tissue

Question 11

Virus-cell interactions: mechanisms

Answer 11

Step 3 of pathenogenesis= Virus-cell interctions

Mechanisms of viral injury and disease

• Inhibition of host-cell nucleic acid synthesis
• inhibition of host-cell RNA transcription
• inhibition of processing of host-cell mRNA
• inhibition of host-cell protein synthesis
• cytopathic effects of “toxic” viral proteins
• interference with cellular membrane function

Question 12

Virus-cell interactions: Outcomes (5)

Answer 12

Step 3 of pathenogenesis= Virus-cell interctions

Outcomes of viral injury

• cell lysis- following viral replication to release virus
• apoptosis- cell suicide to prevent success of viral rep
• oncoviruses- cause cancer
• persistent infection- viruses dont kill cells immediately. remain dormant for long periods and evade immune system.
• immunosupression- virus impairs immune function

Question 13

Virus injury to organs/tissues (6)

Answer 13

Step 4 of pathenogenesis= injury to organs/tissues

• Skin
  
  • can be localized or disseminated
    
    • ulcers, nodules, warts, erythema
• GI tract
  
  • dehydration, diarreha, ect.
• Respitory tract
  
  • inflammation, resp. distress, ect.
• CNS
  
  • neuronal necrosis, progressive demyelination, neuronal vacuolation, ect.
• Hemopoietic system
  
  • hemorrhages, vasculitis
  • Disseminated intravascular coagulation (DIC)
• Fetus
  
  • teratogenic viruses- cause defects

Question 14

Virus shedding

Answer 14

Step 5 of pathenogenesis= shedding

Virus shedding= shed of infectious virons to infect new host

virus shed is crutial to the maintenence of infection in populations and important in relation to disease transmission.