Today we're going to talk about communicable diseases. More specifically, we're going to explore ways in which diseases are spread. We call this modes of transmission.
The first type of disease transmission that we'll discuss is direct transmission. Direct transmission occurs by immediate transfer of infectious agents from a reservoir, in this instance an infected person, to a new susceptible host. And as you can see here, we have a healthy, uninfected person as the host. Direct transmission can happen in two different ways. It can be through either direct contact or droplet spread.
Direct contact includes direct contact with oral secretions, blood, or other potentially infectious fluids such as drainage from a skin lesion, semen, or vaginal secretions. This can happen as a result from touching, biting, kissing, or sexual intercourse. Then there's droplet spread, which is another form of direct transmission. For droplet spread to occur, there needs to be direct contact with infected droplets that contain the infectious agent. This could occur as a result of an infected person coughing or sneezing secretions directly into the face of an uninfected person.
This can transmit respiratory infections such as pertussis, or what we call the whooping cough, by way of droplet spread. And In order for this to occur, though, people need to be in close proximity to each other. This means that in order for droplet spread to happen, people need to be less than 2 meters from one another. This could happen in instances where people are sharing a space, such as a car, or together in a small room.
Now, remember, in order for infection to be spread through a droplet, an individual would need to be in close contact with that infected individual. In the next couple of slides, I'm going to explain how this is different than airborne transmission. But before we move on, I want you to remember that both direct contact and droplet spread are forms of direct transmission. Next, we have indirect transmission. And once again, there are several types of indirect transmission.
One of these is airborne transmission. Airborne transmission occurs when infectious agents are carried by dust suspended in the air. Because of the small size and the weight of the droplet nuclei, they can remain suspended in air for long periods of time before they're inhaled into the respiratory system of a host. With airborne transmission, direct contact or even close proximity is not needed to spread disease.
With airborne transmission, droplet nuclei remain airborne for long periods of time. They may disseminate widely in an environment, and they can be acquired by a host. even if they don't have direct contact with the infected person. So let's take a look at a picture that'll really help differentiate between the two.
Here we see two different diseases, both carried and spread in the respiratory tract. But we can see the difference between the Ebola virus disease, which is only spread through direct contact, through droplet spread. See here, the droplets are large, they're heavy, they don't stay suspended in the air for long periods of time. In order to be infected, you would need to be in close contact with an infected person. On the other hand, we have the measles.
These infectious agents are lightweight, they're small, they stay suspended in the air for long periods of time, and can travel over long distances. In this case, you could come in contact with the agent by simply being in the same, say, building or an apartment or restaurant as an infected person. Hopefully this picture helps differentiate between droplet spread which again is direct transmission, and airborne spread, which is a form of indirect transmission.
And then we have another type of indirect transmission, vehicle-borne transmission. This type of indirect transmission occurs when an infectious agent is transported within a contaminated, inanimate material. These are called fomites. Examples include toys, sharp objects, tissues, even food and water. As I mentioned, food and water are also fomites and can spread illnesses through indirect transmission.
This type of illness occurs when food or water is contaminated. This can occur at the source. For example, animal waste could be introduced directly into a food or water source, contaminating it.
This can also occur due to improper handling of food or improper food storage. Contamination of food that leads to human illness occurs in two different ways, either through infection or through intoxication. Infection occurs when a food has been contaminated directly by the pathogen, and that pathogen is ingested. An example of this would be ingesting food that's been contaminated by E.
coli or salmonella. The human is infected by direct consumption of that pathogen. On the other hand, intoxication occurs when the food that's ingested has been contaminated by a toxin, and that toxin is a byproduct of the pathogen.
For example, I'm sure you've all heard of botulism. Botulism intoxication occurs as a result of a byproduct of the growth of the bacteria, not as a result of the bacteria itself. This is an image depicting the infamous cholera outbreak of the 1850s in London. There was a cholera outbreak which killed more than 600 people.
At the time, germ theory was not widely accepted. We didn't know why people were suddenly getting sick and dying. John Snow was credited with determining the source, contaminated water.
He mapped out the cases of disease to pinpoint the source. He removed the handle of the Broad Street Promp, and the epidemic ended. This is one of the first documented cases of indirect transmission of waterborne illness.
Another form of indirect transmission is vector transmission. A vector is a living organism that transmits an infectious agent from one infected animal to a human or another animal. Vectors are frequently anthropods such as mosquitoes, ticks, fleas, lice.
Vectors can transmit infectious diseases either actively or passively. Biological vectors, such as mosquitoes and ticks, may carry pathogens that can multiply within their bodies and be delivered to new hosts, usually by biting. Mechanical vectors, on the other hand, such as flies, pick up infectious agents on the outside of their bodies and transmit them through physical contact. Vector-borne illnesses are spread when an infected individual transmits the disease to a vector.
The vector then transmits that disease to another individual. However, without the vector, the individuals are not able to transmit disease to each other. Of all of the infectious diseases, vector-borne illnesses are the most complex to prevent and control. Control strategies directed towards vector-borne diseases typically involve community education and environmental measures to hinder the vector from reaching its host. For example, Strategies to protect individuals against malaria, which is transmitted through vector spread, through mosquitoes, include vaccinations, environmental measures such as the use of mosquito nets, or eliminating standing water.
In summary, modes of transmission are ways that communicable diseases are spread. We have direct transmission. That includes direct contact and droplet spread.
Next, we have indirect transmission. That includes airborne spread, vehicle-borne transmission, and vector transmission.