The document discusses various modes of transmission of communicable diseases, including direct contact, droplet transmission, vehicle-borne transmission, vector-borne transmission, and airborne transmission. Direct contact involves skin-to-skin or respiratory droplet transmission over short distances. Vehicle-borne transmission involves pathogens being carried from a reservoir to a susceptible host by contaminated food, water, blood, or fomites. Vector-borne diseases are transmitted biologically or mechanically by arthropods or other living carriers. Airborne transmission occurs through dust or suspended droplet nuclei that can remain in the air for long periods.

1. Modes of transmission
6/7/2023 Epidemiology of Communicable Diseases 1

2. 4. Modes of transmission
• Many different ways
• As a rule only one route for each pathogen
oTyphoid fever ----- vehicle transmission
oCommon cold---------direct contact
• But there are others that may be transmitted by several route
oAIDs, Salmonellosis, Hepatitis B, brucellosis, Q fever,…..etc.
6/7/2023 Epidemiology of Communicable Diseases 2

3. Mode of transmission
Direct transmission Indirect transmission
Direct contact
Droplet infection
Contact with soil
Inoculation into skin or mucosa
Trans-placental (vertical)
Vehicle-borne
•Vector-borne:
•Mechanical
•biological
Air-borne
Fomite-born
Unclean hands and fingers
propagative
Cyclo-prop.
Cyclo-develop.
6/7/2023 Epidemiology of Communicable Diseases 3

4. Modes
of
Transmission
6/7/2023 Epidemiology of Communicable Diseases 4

5. Pathogen
Transmission
6/7/2023 Epidemiology of Communicable Diseases 5

6. Direct transmission
Direct Contact
•There is essentially immediate transfer of the agent from a reservoir to a
susceptible host by direct contact or droplet spread.
• Direct contact occurs through: Skin-to-skin contact, kissing, and sexual
intercourse.
• Direct contact refers also to contact with soil or vegetation harbouring
infectious organisms.
•STDs, AIDs, leprosy, Skin and Eye infections
6/7/2023 Epidemiology of Communicable Diseases 6

7. Contact Transmission
Rhinovirus?
6/7/2023 Epidemiology of Communicable Diseases 7

8. Direct transmission
Droplet Infection
•Direct projection of a spray of droplets of saliva and naso-pharyngeal
secretions
•Produced by sneezing, coughing, or even talking.
•Particles <5mmm can penetrate deeply and reach the alveoli.
•Usually limited to a distance of max 1 meter (30-60cm).
o Close proximity
o Overcrowding
o Lack of ventilation
•Common cold, TB, Meningococcal Meningitis, Whooping cough.
6/7/2023 Epidemiology of Communicable Diseases 8

10. Droplet Transmission
Measles?
6/7/2023 Epidemiology of Communicable Diseases 10

11. Direct transmission
Contact with soil
• Direct exposure of the susceptible tissues to the disease agent in soil
• E.g. Tetanus, Hookworm.
Inoculation into skin or mucosa
• The agent is directly inoculated into skin or mucosa.
• E.g. rabies, hepatitis B,
Trans-placental
• TORCH, Varicella, Hepatitis B, AIDs
6/7/2023 Epidemiology of Communicable Diseases 11

12. Indirect Transmission
From a reservoir to a susceptible host by:
• Vehicle borne: inanimate vehicle
• Vector borne: animate vector
• Airborne transmission: suspended air particles
The infectious agent must be capable of surviving outside the human
host in the external environment.
6/7/2023 Epidemiology of Communicable Diseases 12

13. Vehicle borne:
• An infectious agent is carried from a reservoir to a susceptible host by an
inanimate intermediary.
• Vehicles include:
1. Contaminated food and water: typhoid, paratyphoid, food poisoning, dysentery
and cholera.
2. Biologic products (blood): Hepatitis B, AIDS, Syphilis.
3. Fomites (inanimate objects such as: door knobs, toys, handkerchiefs,
bedding, or surgical instruments). Influenza.
6/7/2023 Epidemiology of Communicable Diseases 13

14. Foodborne
Transmission
Hepatitis A
6/7/2023 Epidemiology of Communicable Diseases 14

15. Waterborne
Transmission
Cryptosporidium parvum?
6/7/2023 Epidemiology of Communicable Diseases 15

16. Common-Source
Outbreak
Cholera!
The Broad Street
Pump.
6/7/2023 Epidemiology of Communicable Diseases 16

17. Indirect-Contact Transmission
Beddings are an example of a Fomite,
an inanimate object that can transmit
pathogens between people.
Methicillin-resistant
Staphylococcus
aurius (MRSA)?
6/7/2023 Epidemiology of Communicable Diseases 17

18. Indirect-Contact Transmission
Door knobs are
another good example
of a fomite.
Influenza virus?
6/7/2023 Epidemiology of Communicable Diseases 18

19. Vector-borne
• Vector is an arthropod or any living carrier (e.g. snail) that transport an
infectious agent to a susceptible individual.
• Arthropod include flies and mosquitoes, fleas, cockroaches, ticks and
mites, sucking lice, eyc.
• Mechanical transmission: the agent does not multiply or undergo
physiologic changes in the vector.
For example, flies carry Shigella on appendages.
• Biologic transmission: When the agent undergoes changes and/or
multiplication within the vector before it is transmitted.
6/7/2023 Epidemiology of Communicable Diseases 19

20. Modes
of
Transmission
6/7/2023 Epidemiology of Communicable Diseases 20

21. Airborne
Airborne transmission occurs by particles that are suspended in air.
There are two types of these particles:
- dust and
- droplet nuclei.
1. Dust particles:
-result from re-suspension of particles that have settled on floor or bedding,
- infectious particles blown from the soil by the wind. Example: Fungal spores.
6/7/2023 Epidemiology of Communicable Diseases 21

22. 2. Droplet nuclei
• They represent the dried residue of droplets that have been
coughed or sneezed into the air.
• They are very tiny particles less than 5 µ (microns) in size and may
remain suspended in the air for long periods.
Examples:
• Tuberculosis is transmitted more often indirectly, through droplet
nuclei, than directly, through direct droplet spread.
• Legionnaires’ disease and histoplasmosis also spread through
airborne transmission.
6/7/2023 Epidemiology of Communicable Diseases 22

23. Airborne Transmission
Mycobacterium
tuberculosis?
6/7/2023 Epidemiology of Communicable Diseases 23