DYNAMICS OF DISEASE TRANSMISSION

1. THEORIES AND MODELS OF
DISEASE CAUSATION

2. • GERM TEORY
• THEORY OF EPIDEMIOLOGICAL
TRIAD
• MULTIFACTORIAL CAUSATION
THEORY
• WEB OF CAUSATION
• DEVERS EPIDEMIOLOGICAL MODEL

3. 1. GERM THEORY
• It becomes popular during the 19th and
early part of 20th century.
• This theory attributes micro-organisms
as the only cause of disease.
Causitive
agent
Man Disease
Cause Effect

4. 2. THEORY OF EPIDEMIOLOGICAL
TRIAD
• This model is also called as ecological model
and evolved through the study of infectious
disease.
• Acc. To this model there are three elements
or major factors which are responsible for a
particular disease causation. i.e. agent, host
& environment.

5. • The agent refers to primary factor, without
which a particular disease can not occur.
• The host refers to human beings who come
in contact with the agent.
• The environment includes all, that is
external to the host and agent but that may
influence interaction between them.
agent
host environment

6. 3. MULTIFACTORIAL CAUSATION
THEORY
• Disease caused by multiple factors .
example:- coronary heart disease.

7. 4. WEB OF CAUSATION
Given by macmohan and pugh. according to
him disease never depends upon single
isolated cause rather it develops from a chain of
causation in which each link itself is a result of
complex interaction of preceding events these
chain of causation which may be the fraction of
the whole complex is known as web of
causation.

9. 5.DEVERS EPIDEMIOLOGICAL
MODEL
• This model is composed of four major
categories of factors
– human biology
– lifestyle,
– environment and
– health system.
all these factors influence health status
positively or negatively.

10. Human biological --- epidemiological triad and
include genetic inheritance, complex
physiological systems, factors related to
maturation and ageing
Life style factors include daily living
activities, customs, traditions, health habits etc
. Environmental factors include physical,
biological, social and spiritual components

11. Health care system factors include
availability, accessibility, adequacy and use
of health care services at all levels.

12. DYNAMICS OF DISEASE
TRANSMISSION

13. Communicable diseases are transmitted
from the reservoir/ source of infection to
susceptible host.
There are three links in the chain of
transmission
1. Reservoir
2. Mode of transmission
3. Susceptible host

14. SOURCE
OR
RESERVOIR
MODE OF
TRANSMISSION
SUSCEPTIBLE
HOST

15. 1. SOURCE & RESERVOIR
• SOURCE
• The source defined as the “person,
animal, object or substance from which
an infectious agent passes or
disseminated to the host”.

16. • RESERVOIR
• A reservoir is defined as “any person,
animal, arthropod, plant, soil or
substance in which an infectious agent
lives and multiplies , on which it
depends primarily for survival, and
where it reproduces itself in such
manner that it can be transmitted to a
susceptible host”.

17. • Eg:-
In hookworm infection, the reservoir is
man and the source of infection is soil
contaminated with infective larvae.
In typhoid fever the reservoir is a case
or carrier and the source of infection is
faeces or urine of patient or
contaminated food and water.

18. Types of reservoir
1. Human reservoir
2. Animal reservoir
3. Reservoir in non living things

19. 1. HUMAN RESERVOIR
The most important source or reservoir
of infection for human is man himself.
Human may be
Case
Carrier

20. a. CASES
• A case is defined as “ a person in the
population having the particular
disease, health disorder or condition
under investigation”.
• The presence of infection in host may
be :-
Clinical
Sub clinical
Latent

21. • 1. clinical illness:-
• Clinical illness may be mild or
moderate, typical or atypical, severe or
fatal.
• Mild cases may be more important
source of infection than severe cases.

22. • ii. Sub clinical cases
• Sub clinical cases are also known as in
apparent, missed or abortive cases.
• The disease agent may multiply in the
host but does not manifest itself by
signs and symptoms.
• Sub clinical infection may be detected
only by laboratory tests.

23. • iii. Latent infection
• Infectious agent lies dormant within the
host without symptoms.

24. Primary case::- first case of a
communicable disease introduced into
the population.
Secondary case:- develops from
contact with primary cases.
Index case:-first case which comes to
the attention of investigator

25. b. CARRIERS
• In some diseases, either due to
inadequate treatment or immune
response, the disease agent is not
completely eliminated, leading to a
carrier state.
• A carrier is defined as an infected person
or animal that harbours a specific
infectious agent and serves as a
potential source of infection for others.

26. • The elements in a carrier state are:-
The presence of disease agent in the body.
The absence of recognizable signs and
symptoms
Spread of disease agent in the discharges
or excretions.

27. Classification of carriers
A. TYPE
 Incubatory
 Convalescent
 Healthy
B. DURATION
 Temporary
 Chronic
C. PORTAL OF EXIT
 Urinary
 Intestinal
 Respiratory
 Others

28. A. TYPE
• (a) Incubatory carriers:
• Carriers which spread the infectious
agent during the incubation period of
disease.
• This usually occurs during the last few
days of incubation period.
• Eg:- measles, mumps, polio, influenza,
hepatitis B

29. • (b) Convalescent carriers:-
• Carriers which continue to spread
disease during the period of
convalescence.
• Eg ;-Typhoid, Dysentery, Cholera,
Diptheria

30. (c) Healthy carriers:-
• They are victims of sub clinical infection
who have developed carrier state
without suffering from overt disease.
• A person whose infection remains
subclinical may or may not be a carrier.

31. B. DURATION
• (a) Temporary carriers:-
• Carriers which spread infectious agent
for short period of time
• (b) Chronic carriers:-
• Carriers which spread infectious agent
for indefinite period

32. C. PORTAL OF EXIT
 Urinary
 Intestinal
 Respiratory
 Others –(skin eruptions, open wounds,
blood)

33. 2. ANIMAL RESERVOIR
• The source of infection may sometimes
be animals and birds.
• The diseases and infections which are
transmissible to man from vertebrate
are called zoonoses.
• Eg :- Rabies, Yellow Fever, Influenza

34. 3. RESERVOIR IN NON LIVING
THINGS
• Soil and inanimate matter can also act
as reservoir of infection

35. 2. MODE OF TRANSMISSION
• Communicable diseases are transmitted
from reservoir to host in many different
ways
SOURCE
OR
RESERVOIR
MODE OF
TRANSMISSION
SUSCEPTIBLE
HOST

36. • 1. Droplet contact
• 2. Droplet infection
• 3. Contact with soil
• 4. Inoculation into skin or
mucosa
• 5. Trans placental
A. DIRECT
TRANSMISSION
• 1. Vehicle borne
• 2. Vector borne
• 3. Air borne
• 4. Fomite borne
• 5. Unclean hands and fingers
B. INDIRECT
TRANSMISSION

37. A. DIRECT TRANSMISSION
• 1. Direct contact
• Infection may be transmitted by direct
contact from skin to skin, mucosa to
mucosa, or mucosa to skin.
• Eg :- STD, AIDS, leprosy, leptospirosis,
skin and eye infections

38. • 2. Droplet infection
• This is direct projection of spray of
droplets of saliva and nasopharyngeal
secretions during coughing, sneezing,
talking or spitting.
• The droplet spread is usually limited to
a distance of 30-60 cm between source
and host
• Eg :-Respiratory Infections, Common
Cold, Tuberculosis, Diphtheria

39. • 3. Contact with soil:-
• The disease agent may be acquired by
direct exposure to the disease agent in
the soil
• Eg :- hook worm, tetanus, mycosis
• 4. Inoculation into skin:-
• Disease agent may be inoculated
directly into the skin or mucosa
• Eg:-rabies virus by dog bite, Hepatitis B
by contaminated needles

40. • 5. Transplacental transmissin:-
• Disease agents can be transmitted
transplacentally.
– S-Syphilis
– T-Toxoplasma
– O-Other infections(AIDS, varicella, Hepatitis B)
– R-Rubella virus
– C-Cytomegalo virus
– H-Herpes virus

41. B. INDIRECT TRANSMISSION

42. 1. VEHICLE BORNE
• Vehicle borne transmission implies
transmission of the infectious agent
through the agency of water, food, raw
vegetables, fruits, milk, blood etc.

43. 2. VECTOR BORNE
• Classification of vector borne disease:-
• 1. By vector
– Invertebrate Eg-arthropods( flies, mosquitoes,
cockroach, ticks, mites, bugs)
– Vertebrate Eg-mice, rodents

44. • 2. By transmission chain
• Man and a non vertebrate host
(man-mosquitoe- man in malaria)
• Man , another vertebrate host and a non
vertebrate host
(bird-arthropod-man)
• Man and 2 intermediate host
(man-cyclops-fish-man)

45. • 3. By methods in which vectors transmit
agent
Biting
Scratching
• 4. By methods in which vectors are
involved in the transmission of parasite
Mechanical transmission
Biological transmission

46. 3. AIR BORNE
• 1. Droplet nuclei
• 2. Dust

47. 4. FOMITE BORNE
• Fomites are inanimate articles or
substances other than water or food
contaminated by infectious agents.
• Eg- soiled clothes, syringes,
instruments etc.

48. 5. UNCLEAN HANDS
• Lack of personal hygiene favour person-
person transmission of infection.

49. 3. SUSCEPTIBLE HOST
SOURCE
OR
RESERVOIR
MODE OF
TRANSMISSION
SUSCEPTIBLE
HOST

50. SUCCESSFUL PARASITISM
• 4 stages are there in successful
parasitism
PORTAL OF
ENTRY
SITE OF
ELECTION
PORTAL OF
EXIT
SURVIVAL IN
THE
ENVORNMENT

51. 1. PORTAL OF ENTRY
• Infectious agent enter the host
• Eg :-
Respiratory tract
Alimentary tract
Genito urinary tract
Skin

52. 2. SITE OF ELECTION
• In the body the disease agent finds
appropriate tissue for multiplication and
survival

53. 3. PORTAL OF EXIT
The disease agent finds a way out of
the body
4. SURVIVAL OUTSIDE
After leaving the human body the
organism must survive in the external
environment for sufficient period till a
new host is found

54. Diseases prevention and control
• Prevention of source or reservoir
• Early diagnosis
• Notification
• Epidemiological Investigation
• Isolation
• Treatment
• Quarantine
• Interruption of the disease transmission
• Prevention of susceptible host
• Immunization
• Health education