1. Dynamics of infectious disease transmission
Dr Noreen Shah
Senior Lecturer
Dept of Community Medicine
KGMC

2. Objectives: At the end of this session the students will be able to;
Define concepts of infectious disease epidemiology
Describe different modes of transmission of infectious diseases
Explain the chain of infection
Describe the ways to break Chain of Infection

3. Epidemiologic Triad
The standard model of infectious disease causation.
It has 3 corners (vertices).
Agent: A microbe that causes disease.
Host: An organism that harbors disease.
Environment: External factors that allows disease transmission.

5. Dynamics of transmission
I. Source/
Reservoir
II. Modes of
Transmission
III. Susceptible
Host
Communicable diseases are transmitted from the reservoir/source of infection to susceptible host through .

6. I. Source or reservoir
These are natural habitat of infectious agents in which an infectious agent normally lives and multiplies.
Examples are;
Human reservoirs
Animal reservoirs (zoonotic diseases)
Reservoir in non-living things.

7. 1. Human reservoirs Cases Carriers
According to spectrum of disease:
Clinical cases
(Mild/severe-typical/atypical)
Sub-clinical cases
Latent infection cases
Primary case
Index case
Secondary cases
Type:
Incubatory
Convalescent
healthy
Duration:
Temporary
Chronic
Portal of exit:
Urinary
Intestinal
Respiratory
others

8. Cases may be:
Primary case: The first case of an infectious disease introduced into the population.
Index case: The first case that come to attention of investigators.
Secondary case: Cases that develop from primary cases.

9. Cont. Cases may be according to spectrum of disease;
Clinical cases: may be mild/severe/typical or atypical depending upon the gradient of involvement.
Subclinical cases: Asymptomatic or mildly infected that does not alert the patients.
Latent cases: Asymptomatic infection capable of showing symptoms under some circumstances if 
activated.

10. Carriers:
Carriers are infected persons that harbor specific infectious agent in the absence of visible clinical disease and serves as a potential source of infection to others.
Carriers are less infectious than cases.
They live a normal life but can infect other individuals.
Epidemiologically they are more dangerous as these escape recognition.

11. Cont. Carriers may be classified by:
Types:
Incubatory Carrier: Person capable of transmitting an infectious agent during incubation period.
Convalescent Carrier: Person who continue to shed the disease agent during the period of convalescence, e.g. typhoid fever, cholera, whooping cough, in these clinical recovery does not coincide with bacteriological recovery.
Healthy Carrier: Emerge from subclinical cases.

12. Cont. 2. Temporary or chronic:
Temporary carriers: Are those who shed infectious agent for short periods of time. All the three incubatory, convalescent and healthy are included.
Chronic carriers: A carrier who excretes the infectious agent for indefinite period. E.g. typhoid fever, hepatitis B, dysentery, malaria, etc.

13. Cont.
3. Portal of exit:
Carriers may be classified according to portal of exit of infectious disease.
Urinary
Intestinal
Respiratory
Open wounds and
Blood.

14. 2. Animal reservoirs (zoonotic diseases)
Infection disease that are transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis etc.

15. 3. Reservoir in non-living things
Soil and inanimate matter can also act as reservoir of infection. E.g. soil may harbor agents that causes tetanus, anthrax etc.

16. II: MODES OF TRANSMISSION OF DISEASE
Communicable diseases transmitted from the reservoir or source of infection to a susceptible host in many ways.

17. Mode of transmission Direct transmission Indirect
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.

19. 1. Direct transmission:
Direct physical contact: From source/ reservoir to a susceptible host, e.g. skin to skin contact, STDs AIDS, eye infections etc.
Droplet infection: By saliva & nasopharyngeal
secretions e.g. flu, diphtheria, whooping cough, tuberculosis, meningococcal meningitis etc.

20. 3. Contact with soil: The disease agent is in soil e. g
3. Contact with soil: The disease agent is in soil e.g. hook worm infection, tetanus, mycosis etc. 4. Inoculation into skin or mucosa: The disease agent inoculated directly into skin or mucosa e. g. rabies, hepatitis B, etc. 5. Trans placental (vertical): e.g. Toxoplasmosis, others, rubella, cytomegalovirus, herpes simplex (TORCH)

21. 2. Indirect transmission
Vehicle borne transmission: 
The agents multiplies or develops in vehicle & transmits through agencies like water, vegetables, fruits, milk, milk products, ice, blood, serum, tissue or organs transplantation.
E.g. diarrhea, typhoid, cholera, polio, hepatitis A, brucellosis, etc.

22. 2. Vector-borne transmission: Living carriers that transports infectious agents to a susceptible individual. It may be;
Mechanical transmission: When a vector simply carries pathogenic microorganisms on their body and transfers them to food. E.g. flies and cockroaches.

23. ii. Biological transmission: the agent multiplies and develops in a vector. E.g. 
Propagative transmission: Multiplication without cyclic change such as DHF, yellow fever, etc. 
Cyclo-development transmission: No multiplication but cyclic change such as filariasis, etc. 
Cyclo propagative transmission: Multiplication and change in form such as malaria, etc.

24. iii. Air borne: Transmission is carried in air in form of droplets and dust e.g. tuberculosis, influenza, chicken pox, measles, viruses, spores of fungi etc.
iv. Fomite borne: These are inanimate substances other than water or food contaminated by the infectious discharge, e.g: diphtheria, typhoid bacillary dysentery, hepatitis A, eye and skin infections.

25. Diseases may be transmitted as:
Horizontal transmission: From one individual to another in the same generation.
Direct contact (licking, touching, biting)
Indirect contact (cough, sneeze, vectors, fomites, fomites, allow the transmission of disease without physical contact.

27. iii. Susceptible host:
The person who is at risk for developing an infectious disease.
Factors making person susceptible are,
age (young and elderly are more at risk)
underlying chronic diseases such as diabetes
Immuno suppression conditions like HIV chemotherapy, and malnutrition.

28. Stages of infectious disease

29. Chain of Infection
Definition: Chain of infection is a model (a circle of links) used to understand the infection process.
Each circle represents a component in the cycle.
Each link must be present and in the sequential order for an infection to occur.
Breaking any link in the chain can disrupt the infection.

31. Different Ways to break Chain
Diseases have certain weak points
The basic approach in controlling the disease is to
Identify these weak points and
Break the weakest links in chain of transmission

32. Chain of Infection: An infectious agent leaves its
It is the process that begins when,
An infectious agent leaves its
Reservoir or host through a
Portal of exit and then
Is conveyed by some mode of transmission to
Enter through an appropriate portal of entry to
Infect a susceptible host

33. 1. Infectious agent:
Infectious agent is the pathogen (germ) that causes diseases

34. Breaking the Chain Infectious agent Early diagnoses and treatment
Cleaning, disinfection and sterilization
Infection prevention policies
Pest control

35. 2. Reservoir or host
Principal habitat where infectious agents live and multiply
From there they may spread to cause infection.
Reservoirs may be humans, animals or environmental.

36. Breaking the Chain
2. Reservoir of infection Cleaning, disinfection and sterilization
Infection prevention policies
Pest control

37. 3. Portal of exit:
It is the site from where micro-organisms leave the host to enter another host to cause infection. E.g.
Upper respiratory tract: saliva, sneeze, cough
Gastrointestinal tract: feces, vomit
Blood: infected blood
Uro - genital tract: semen, vaginal secretions, urine
Skin and mucous membranes: discharges

38. Breaking the Chain 3. Portal of exit Hand hygiene
Personal protective equipment
Control of aerosols and splatter (aerosols can transmit respiratory infections like tuberculosis, splashing of mucosa is also a potential risk)
Respiratory etiquette
Waste disposal

39. 4. Modes of transmission Modes of transmission are,
The route by which an infectious agent is transmitted from a reservoir to another host.
6 Fs, food, fluid, fingers, fomites, feces, flies.
Modes of transmission are,
Direct transmission.
Person to person: Touching, kissing, sexual intercourse, coughing, sneezing, talking etc
Trans – placental: Transfer of infectious agents from pregnant woman to fetus via placenta.

40. Indirect transmission.
Air borne transmission: The infectious agent is present in air as dried secretions from respiratory tract, e.g. tuberculosis.
Vehicle borne transmission: any non-living substance that can be contaminated by an infectious agent, which then transmits it to a new host. 
Vector borne transmission: An organism, usually an arthropod, which transmits an infectious agent to a new host. E.g. houseflies, mosquitoes, lice and ticks.

41. Breaking the Chain 4. Mode of transmission Hand hygiene
Personal protective equipment
Food safety
Cleaning, disinfection, sterilization
Isolation

42. 5. Portal of entry:
The site through which micro-organisms enter the susceptible host.
Inhalation: influenza
Ingestion: gastro enteritis
Needle prick: hepatitis B
Sexual contact: HIV/AIDS,
Open wound or punctures: tetanus

43. Breaking the Chain 5. Portal of entry: Hand hygiene
Personal protective equipment
Personal hygiene
First aid

44. 6. Susceptible Host: A organism that can be infected by pathogens.
Vulnerable Populations:
Very young and the very old,
Immune suppressed (due to genetics, transplant drugs, malnutrition, or viral infection like HIV).
Occupational exposure.
Non-immune.

45. Breaking the Chain 6. Susceptible Host: Immunization
Treatment of under laying disease
Patient education

46. Quiz;
1. Identify the part(s) of chain of infection that has been eliminated by the following action. Catheter care.
Mode of transmission
Portal of exit
Portal of entry
2. Identify the part(s) of chain of infection that has been eliminated by the following action. Food handling
Infectious agent
Source /reservoir

47. 3. Identify the part(s) of chain of infection that has been eliminated by the following action. Hand washing.
Susceptible host
Mode of transmission
Portal of entry
4. Identify the part (s) of chain of infection that has been eliminated by the following action. Identification of organism.
Portal of exit
Infectious agent

48. 5. Identify the part (s) of chain of infection that has been eliminated by the following action. Personal protective equipment
Source/ Reservoir
Portal of exit
Portal of entry
6. Identify the part (s) of chain of infection that has been eliminated by the following action. Sterilization
Source or reservoir/mode of transmission
Portal of entry/Portal of exit
Mode of transmission / Infectious agent

49. 7. Identify the part (s) of chain of infection that has been eliminated by the following action. Employees health
Source/ Reservoir
Susceptible host
Infectious agent
8. Identify the part (s) of chain of infection that has been eliminated by the following action. Recognition of high risk patients
Portal of entry
Source or reservoir

50. 9. Identify the part (s) of chain of infection that has been eliminated by the following action. Recognition of high risk patients
Source reservoir
Susceptible host
Infectious agent
10. Identify the part (s) of chain of infection that has been eliminated by the following action. Early recognition of signs of infection
Portal of entry
Source or reservoir

51. Answers: c b a