Horizontal or lateral transmission

HORIZONTAL OR LATERAL TRANSMISSION

  • Transmission of zoonotic pathogen from infected population (animals) to the susceptible population (human beings) or cross-sectional spread of diseases among members of same group or different groups of the same generation. It may be
  • Examples are rabies, brucellosis, anthrax 

Horizontal_transmission

Diagrammatic representation of horizontal transmission

Direct transmission

  • Transfer of infectious agents from the infected animals to the susceptible human population by direct contact without involvement of intermediate vectors.
  • Direct physical contact with infected animals and their discharges
    • Physical touch and sexual contact lead to the transmission pathogen.
    • Examples are brucellosis, leptospirosis, vibriosis, anthrax, foot and mouth diseases, cow pox, orf
  • Direct air-borne transmission
    • Droplets and or droplet nuclei which carry the infectious pathogen in the environment leads to the spread of zoonotic diseases.
    • While sneezing or coughing or breathing infectious pathogens are sprayed with droplets and or droplet nuclei in the air which in turn inhaled or contacted with visible mucus membrane of immediately surrounding susceptible population.
    • Epidemiologists do not consider the spread as a true air-borne transmission; they regard it as a form of direct-contact transmission.
    • The close-proximity, over-crowding and poor ventilation enhance the chances of spread of infectious pathogens by this mode.
    • Examples are New castle disease virus conjunctivitis, tuberculosis
  • Contact with contaminated soil (saprophytic transmission)
    • Contact with fomites or saprophytes results in disease transmission.
    • Examples are anthrax, cutaneous larva migrans, visceral larva migrans
  • Bite or scratches by infected animals
    • Animal bites or scratches by infected animals transmit some zoonotic diseases.
    • Examples are rabies, cat-scratch fever

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Indirect transmission

  • Transmission of infectious pathogens from the infected host to the susceptible host by an intermediate vector either animate or inanimate.
  • Fomite-borne transmission
    • Infections are transmitted through the inanimate objects (contaminated soil, cloths, utensils, surgical instruments, feeders, waterers, syringe and needles, milking machines, farm equipment).
    • Examples are anthrax, foot and mouth disease
  • Air-borne transmission
    • Transmission of infectious pathogens between infected and susceptible host through droplets and droplet nuclei in the air.
    • Examples are avian influenza, foot and mouth disease, tuberculosis
  • Vehicle-borne transmission
    • Transmission of infectious pathogens between infected and susceptible host through non-living things or substance (food, water, blood, body fluids, serum, plasma, tissues, organs).
      • Mechanical transmission: Mechanical transmission of pathogens. Example is hepatitis from serum.
      • Developmental transmission: If essential development takes place in the vehicle without multiplication. Example is ancylostomiasis.
      • Propagative transmission: If the agent multiplies in the vehicle before being injected into the host. Examples are milk-borne streptococcosis, staphylococcosis, food poisoning (salmonellosis).
      • Cyclo-propagative transmission: If the agent undergoes development and multiplication in the vehicle before being injected into the host. Example is strongyloidosis.
  • Vector-borne transmission
    • An invertebrate host usually an arthropod is responsible for the transmission of an infectious agent from the infected host to the susceptible host.
      • Mechanical transmission: Mechanical transmission of pathogens. Examples are anthrax, salmonellosis
      • Biological transmission: It involves ingestion of infective materials from the infected population resulting in development or multiplication or both of an etiological agent in the biological vectors (ticks, flies, mosquitoes) before being transmitted to the human population.
      • The time period between ingestion of infectious agents and availability of infective agents after essential development or multiplication or both in the vector is referred to as ‘extrinsic incubation period’.
      • Developmental transmission: If essential development takes place in or on the vector without multiplication. Example is Dirofilaria immitis in mosquitoes.
      • Propagative transmission: If the agent multiplies in or on the vector before being injected into the host. Example is plague.
      • Cyclo-propagative transmission: If the agent undergoes development and multiplication in the vector before being injected into the host. Example is Trypanosoma cruzi in bugs.

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Transmission of zoonotic pathogens associated with ticks and flies

  • Trans-ovarian transmission
    • The infection may be transmitted in vectors vertically from one generation to the next generation before being injected into the host.
    • Examples are Rift valley fever, West Nile fever, Russian spring summer encephalitis
  • Trans-stadial transmission
    • The infection may be transmitted in vectors from stage to stage before being injected into the host.
  • Salivarian transmission
    • If an infectious agent undergoes development in the midgut of the vector and transformed into infective stage in the salivary gland of that vector, from which the infective form of the agent is released into the host by biting by the vector is referred to as salivarian transmission.
    • Example if African trypanosomiasis by the vector Glossina.
  • Stercorarian transmission
    • If an infectious agent undergoes development in the midgut of the vector and transformed into infective stage in the hindgut of that vector, from which the infective form of the agent is excreted through its faeces leads to infection of the host by faecal contamination is referred to as stercorarian transmission.
    • Example is Chaga’s disease (Trypanosoma cruzi) in man by bugs.

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Last modified: Wednesday, 16 May 2012, 6:32 AM