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Modes of Transmission

Biology (Year 12) - Spread of Pathogens

Ben Whitten

What are the different modes of transmission? In the life cycle of a pathogen, a mode of transmission is required, otherwise the pathogen will die when the host does. Transmission is defined as the method of how the pathogen is transferred from a host/reservoir to a susceptible host, which can involve developmental stages in the environment or in vectors. The two overarching categories of transmission are direct and indirect, and have subcategories. Direct transmission is the transfer from an infected host directly to another susceptible host, where indirect transmission may require one or more steps and does not go straight from an infected host to a susceptible host.

  • Direct transmission

  • Direct contact

  • Close contact

  • From a reservoir

  • Indirect transmission

  • Living vectors

  • Airborne droplets

  • Soil, water, food and fomites

How does direct transmission work? Direct contact This involves the transmitting of a pathogen through physical touch between the infected host and the susceptible host through either skin or body fluids. There are a number of body fluids, including sweat, tears, vomit, nasal secretions, blood, saliva, sexual fluids and urine. For example, the ABL (Australian bat lyssavirus) via an infected bat, influenza or COVID-19. Close contact Pathogens can be transmitted via airborne droplets when an infeced host and susceptible host are in close proximity, within 1.5m, through sneezing, singing or coughing. Millions of microbes can be ejected into the air through these means, in droplets of mucus or saliva. If these droplets land on a susceptible person's mucous membranes, they may catch the disease. Airborne droplet transmission through close proximity is considered direct. For example, influenza and COVID-19. From a reservoir Transmission may also occur from a reservoir directly to a susceptible host. A reservoir is defined as a living or non-living site which a pathogen normally resides and, and has the possibility to replicate. It may be dormant at the site. A living reservoir may be a human, where a non-living reservoir may be something such as soil. For example, tetanus, influenza or COVID-19 (via fomite transmission). How does indirect transmission work? Living vectors A living vector is often a vertebrate or arthropod which has the purpose of transmitting a pathogen from an infected host to a susceptible host. Some vectors may be infected with the pathogen, or may be carriers and aren't infected. A vector may also allow a pathogen to penetrate the outer defences of the potential host which would otherwise be impossible. For example, malaria from mosquitos, Ross River virus from mosquitoes and tetanus via dog-bite saliva. Airborne droplets Pathogens may also be transmitted inside airborne droplets (aerosols) which are sneezed/coughed into the air, and suspended for a particular period of time prior to inhalation or landing on a surface (fomite). The transmission is considered indirect as it does not occur until later. For example, tuberculosis, influenza or COVID-19. Soil, water, food and fomites Non-living (inanimate) objects can also carry a pathogen in the pathogen life cycle. Fomites are any surface or object which carry an infectious agent; it may be a car tyre, your clothes or even your mobile phone! Some diseases also have the potential to live in soil and water prior to finding a suitable host, such as crown gall and phytopthora dieback.

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