The term modes of transmission refer to how an infectious agent, also called a pathogen, can be transferred from one person, object, or animal, to another.
Viruses, bacteria, parasites, or fungi can spread infectious diseases. Understanding the modes of transmission for an infectious disease is an important way to limit its spread.
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Terminology for Modes of Transmission
Chain of Infection
In epidemiology, a triad model called the chain of infection states that infectious diseases occur because of the interaction between an infectious agent, a host, and their environment.
The chain of infection refers to the series of events that result in a new person (also called a susceptible host) becoming infected with an infectious agent.
A fomite is an object or surface that is capable of transmitting disease and infectious agents. Fomites can also be referred to as passive vectors.
Fomites can include pens, phones, work surfaces, countertops, tabletops, handrails, and doorknobs.
Portal of Entry
A portal of entry is how an infectious agent enters a susceptible host. For the pathogen to multiply, the portal of entry has to provide access to tissues. Portals of entry are often the same as the portal of exit in the disease host.
This is seen with the influenza virus. Flu exits through the respiratory tract in the host and enters through the respiratory tract in the susceptible host. Other portals of entry include through the skin, mucous membranes, and blood.
Portal of Exit
A portal of exit refers to the path through which an infectious agent leaves a host. The portal of exit usually matches where an infectious agent is found inside a person’s body.
For instance, flu viruses leave through the respiratory tract, enterovirus 70, a cause of hemorrhagic conjunctivitis, leaves through secretions from the eyes, and the mite that causes scabies uses skin-to-skin contact as its portal of exit.
In epidemiology, people, animals, objects, or environments carrying an infectious agent are called reservoirs. This is where the infectious agent lives, grows and proliferates.
Sexually transmitted diseases, skin conditions, and respiratory illnesses are all found in human reservoirs. However, humans may not always show signs of infection or illness.
These types of people are called asymptomatic or passive carriers. Those who can transmit infectious agents before they experience symptoms of infection themselves are called incubatory carriers.
Convalescent carriers are people who have experienced illness because of an infectious agent and are still able to transmit it to others.
Chronic carriers are people who are capable of transmitting infections to others months or years after they first become infected.
Symptomatic carriers are less likely to spread disease, as they are aware of the risks they pose to other people. Asymptomatic carriers are less likely to be careful about who or what they come into contact with, and as such can spread disease unknowingly.
Humans can also be infected by animal reservoirs. Zoonotic diseases are diseases that can be spread from animals to humans under natural conditions and include rabies, anthrax, and SARS.
The susceptible host is the final stage in the chain of infection. There is a complex range of parameters that determine who may be a susceptible host for a certain infectious disease.
For instance, a person’s genetic makeup may make them more or less susceptible to disease. They may have specific immunity to a disease due to antibodies from previous infections or vaccination.
Socioeconomic factors may also determine how likely it is that someone can become a susceptible host for an infectious disease.
Factors that lower the risk of infection can include:
- The skin
- Mucosal membranes
- Gastric acidity
- Cough reflexes
- Nonspecific immune responses.
Increased risk of infection can be caused by:
- A disease that lowers the immune system
- Therapy that impairs immune responses.
The Different Modes of Transmission
Direct contact takes place through skin-to-skin contact, as well as kissing and sexual intercourse. However, direct contact does not only refer to contact between humans.
Direct contact with contaminated soil is also possible as well as through contact with fomites. Infection through respiratory droplets is a form of direct contact, such as through sneezing, coughing, or talking.
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Droplet transmission can occur when a person comes within 1 meter of an infected person. Infectious diseases can be spread through respiratory droplets released into the air when a person coughs or sneezes at this close proximity.
Infectious diseases can be spread through droplets of different sizes, with droplet particles over 5-10 μm in diameter being respiratory droplets, and droplet particles under 5 μm in diameter being droplet nuclei.
Respiratory droplets can enter the body through the mucosal membranes of the body, and so a susceptible host is at risk of contracting an infectious disease if respiratory droplets come into contact with a susceptible host’s mouth, nose, or eyes.
In SARS-CoV-2, the virus that causes COVID-19, the mode of transmission is through respiratory droplets. Evidence has shown that COVID-19 is typically transmitted through respiratory droplets and direct contact with infected people and indirect contact through fomites.
To combat this, people are always advised to stay at least 2 meters from other people and wash and disinfect their hands and common fomites such as phones and doorknobs regularly.
Indirect contact allows a pathogen to spread to a host through suspended air particles, fomites, or vectors (insects such as mosquitoes and fleas). Airborne transmission is possible when droplet nuclei are suspended in the air.
Airborne dust or particles from soil is also capable of spreading pathogens when it is blown into the air. Droplet nuclei can travel long distances through the air, whereas respiratory droplets quickly fall to the ground.
Indirect contact is often facilitated when unclean hands contaminate surfaces and objects that are then passed around other people. For instance, stethoscopes are common objects that spread infectious agents in hospitals and doctor’s offices.