[4] Virus Transmission

• Movement of virus from one host to another

  Transmission
• Virions are metastable so they can survive outside the host cell
• Means only a small number of the host are infected

  Enzootic
• Refers to high infection that can spill over to other species

  Epizootic
• These can promote the spread of emerging diseases

  Genetic changes
• Anthropogenic processes can facilitate the introduction and establishment of novel pathogens, and increase their transmission
• Modes of Transmission:

  1. Direct or non-vector
  2. Indirect or vector
• Common routes of transmission

  1. Enteric
  2. Respiratory
  3. Zoonotic
  4. Sexual
• Horizontal transmission refers to the transmission of the virus from one individual to another
• Routes of horizontal transmission:

  1. Respiratory tract
  2. Intestinal tract
  3. Abrasions and wounds
  4. Genital tract
  5. Shed epithelial cells
• Examples of respiratory tract viruses:

  1. Influenza
  2. Cold
  3. Measles
• Examples of intestinal tract viruses:

  1. Influenza
  2. Rotavirus
• Examples of abrasion and wounds viruses:

  1. Papillomavirus
  2. Rabies
• Examples of genital tract viruses

  1. HIV
  2. Papillomaviruses
• Examples of shed epithelial cells viruses:

  1. Cold viruses
  2. Measles
  3. Chicken pox virus
• Vertical transmission involves the transfer of virus from parent to offspring
• Vertical transmission can occur via the placenta (rubella) or milk (HIV) of the mother
• These are inanimate vectors where virions can survive on

  Fomites
• Factors for viral survival on fomite:

  1. Virus properties
  2. Fomite properties
  3. Surrounding environment
• Study 1:

  1. Specific viruses were studied in specific buildings and surfaces
  2. In hospitals: needle sticks were significant vectors
• Study 2:

  1. Looked at SARS-Cov2 Survival on fomites while still being infectious
  2. The presence of proteins were more important than the type of fomite (porous or nonporous)
• These are large protein structures with embedded virions


  Occlusion bodies
• Occlusion bodies are produced by cypoviruses and baculoviruses
• Plant viruses are often vectored by organisms that bite plants and penetrate their cell walls
• Invertebrate viruses can be spread by other insect or arthropod vectors
• These vectors are significant in spreading animal viruses

  Arthropod vectors
• Arthropod borne arbovirus vectors:

  1. Mosquitoes
  2. Midges
  3. Ticks
• Significant arboviruses in the Philippines:

  1. Dengue
  2. Malaria
• Examples of arboviruses:

  1. ssRNA+
  2. ssRNA-
  3. dsRNA
• No matter where dengue replicates, it must end up in the salivary gland

  
• This term means the virus can enter cells due to presence of a receptor

  Susceptible
• This means host cells can facilitate the replication of the virus

  Permissible
• Detectable amounts of virus in the blood

  Viremia
• These are hosts that cannot transmit the virus

  Dead-end hosts
• These are vectors that can infect dead-end hosts

  Secondary vectors

  
• The West Nile Virus is vectored by mosquitoes
• Viremia is crucial to a species' role in transmission:

  • Birds have high enough viremia for transmission, but not to be enough affected themselves

  
• WNV manifests as encephalitis and meningitis
• Unlike crows and blue jays, robins are efficient hosts as they do not die to the WNV
• Refers to when a virus is maintained in a particular region or locality in non-human sources

  Enzootic amplification