Virulence and Pathogenicity- virology Flashcards Preview

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Flashcards in Virulence and Pathogenicity- virology Deck (14):


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



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.


Factors affecting virulence of a virus: Virus vs. Host


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


Viral Tropism

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


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


Lethal dose 50

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


Lower LD50= more virulent virus



Five steps of Pathogenesis 

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.


Routes of virus entry to host (4)

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


Viral spread in host

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)


3. spread via nervous system



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


Virus spread via nerves

  • Neurotropic virus
  • Neuroinvasive virus
  • Neurovirulent virus

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


Virus-cell interactions: mechanisms 

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


Virus-cell interactions: Outcomes (5)

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


Virus injury to organs/tissues (6)

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 


Virus shedding

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.