Methods of Infection Transfer

Recall the Koch’s postulates in 1884

1. Same pathogen has to be present in every case of the disease

2. The pathogen is isolated from the host and grown in culture

3. The pathogen from the culture must cause the same disease when injected in a healthy lab animal

4. Pathogen isolated from the infected lab animal must be the same as the one from the host

What are some limitations of Koch’s postulates?

1. The pathogen can sometimes be found in healthy individuals

2. The pathogen can cause different diseases

3. A disease with the same symptoms can be caused by different pathogens (cystitis)

What are reservoirs?

Reservoirs are places where pathogens can live to keep causing infections and spread to new hosts

What are the 3 reservoirs?

1. Human reservoirs

2. Animal reservoirs

3. Non-living reservoirs: pathogens in an environment (soil, contaminated water)

What are the 3 ways virus can spread through infection?

1. Contact transmission

2. Vehicle transmission

3. Vector transmission

What are the 3 methods of contact transmission?

1. Direct contact

2. Indirect contact

3. Droplet transmission

Describe Direct contact (contact transmission)

1. Direct contact: direct transmission of a pathogen by physical contact

Describe Indirect contact (contact transmission)

2. Indirect contact: pathogen is spread from its reservoir to a host through a non-living object (fomite)

Describe Droplet transmission (contact transmission)

3. Droplet transmission: pathogen is spread in droplets that travel short distances (less than 1 meter) through coughing, sneezing, etc.

What are the 3 methods of vehicle transmission?

The three ‘bornes’

1. Waterborne

2. Foodborne

3. Airborne

Describe Waterborne (vehicle transmission)

1. Waterborne: pathogens are spread through contaminated water (ex: cholera)

Describe Foodborne (vehicle transmission)

2. Foodborne: pathogens are spread through food that is not cooked or stored properly (ex: S. aureus, tapeworms)

Describe Airborne (vehicle transmission)

3. Airborne: pathogens are spread through small droplets or dust that can travel long distances (ex: fine spray from mouth containing influenza virus, coronavirus, and fungal spores)

What is a vector?

Vectors are animals or insects that carry pathogens from one host to another

What are the two methods of vector transmission?

1. Mechanical transmission

2. Biological transmission

What is mechanical transmission? (vector transmission)

1. Mechanical transmission is the carrying of pathogens by an insect without the pathogen growing or reproducing inside the insect

What is Biological transmission? (vector transmission)

2. Biological transmission is when a pathogen enters an animal or insect, reproduces inside it, then is passed to another host

What are some examples of biological transmission?

Examples of biological transmission:

• Plasmodium in mosquito

• Rabies virus in bats