1. DYNAMICS OF DISEASE
TRANSMISSION
Dr. Sanat Rathod
Assistant Professor
GMERS Medical College
Gotri
Vadodara

2. 2

3. 1st - The Infectious Agent
-any disease-causing
microorganism
(pathogen)
Infectivity
Pathogenicity
Virulence
3

4. 2nd: Source or Reservoir
• The starting point for the occurrence of a communicable
disease
• Source of infection :
• the person, animal, object or substance from which an
infectious agent passes or is disseminated to the host
(immediate source)
• RESERVOIR:
• “any person, animal, arthropod, plant, soil, or
substance, or a combination of these, in which
an infectious agent normally lives and multiplies,
on which it depends primarily for survival, and
where it reproduces itself in such a manner that
it can be transmitted to a susceptible host.
It is the natural habitat of the infectious agent.” 4

5. Types of Reservoirs
Human
reservoir
Animal
reservoir
Non-living
reservoir
•5

6. cases
•Primary case
•Index case
•Secondary cases
•Clinical cases
(mild/severe-typical/atypical)
•Sub-clinical cases
•Latent infection cases
:Type
•Incubatory
•Convalescent
•healthy
Duration:
•Temporary
•Chronic
carriers
:Portal of exit
•Urinary
•Intestinal
•Respiratory
•others
6

7. “A person in the population or
study group
Identified as having particular
disease, health disorder or
condition
Under investigation”
7

8. The clinical illness maybe mild or moderate,
typical or atypical, severe or fatal.
Epidemiologicall y, mild cases may be
mor e impor tant sour ces of
infection than severe cases because they
are ambulant and spread the infection
wherever they go, whereas severe cases
usually confined to bed.
8

9. Subclinical cases
Inapparent, Covert, Missed or Abortive Cases
• disease agent multiply in the host but does
not manifest by S/S.
• But contaminates the environment in the
same way as clinical cases.
•
Subclinical cases play a dominant role in maintaining the
chain of infection in the community.
9

10. detected only by
laboratory
tests
subclinical cases
occurs in most
infectious disease.
Eg






Rubella,
Mumps,
Polio,
Hepatitis A and B,
Influenza,
Diphtheria
10

11. Latent infection
• infectious agent lies in a non-infectious formdormant within the host without symptoms with
no shedding (and often without demonstrable
presence in blood, tissues or bodily secretions
of the host)

eg.

HSV and VZV: nerve ganglia cells,
CMV: kidney and salivary glands cells,

EBV: lymphocytes
11

12. 
Index Case
◦ Person that comes to the
attention of public health
authorities

Primary Case
◦ First case of a communicable
disease introduced into the
population unit bring studied
◦ Attack rate

Secondary Case
◦ Person who acquires the disease
from an exposure to the primary
case
◦ Secondary attack rate
12

13. Carriers

An infected person or animal that harbors a
An infected person or animal that harbors a
specific infectious agent in the absence of
specific infectious agent in the absence of
discernible (visible) clinical disease and
discernible (visible) clinical disease and
serves as a potential source of infection to
serves as a potential source of infection to
others
others
Reason :
due to inadequate treatment or immune response
the disease agent is not completely eliminated
leading to a carrier state.
13

14. Three elements in a carrier state:
14

15. CARRIERS
As a rule carriers are less infectious than cases,
but epidemiologically, they are more
dangerous than cases
because
◦ escape recognition
◦ continuing to live a normal life among population
or community
◦ readily infect the susceptible individuals
◦ over a wider area and longer period of time under
favorable conditions.
15

16. Classification of Carrier
Incubatory
Carriers: those who shed the
infectious agent during the incubation period.
This usually occurs during last few days of IP
 Measlesthe period of communicability is 4 days before the rash.
 Mumpsusually 4-6 days before onset of symptoms
 Polio7-10 days before onset of symptoms
 Hepatitis Bfor a month before jaundice
 Pertusis
 Influenza
 Diphtheria
16

17. Carrier May Be Classified :
By Type
Convalescent Carriers:
 those who continue to shed the disease agent during the
period of convalescence
 In the disease, clinical recovery does not coincide with
bacteriological recovery.
 Serious threat to HH members
 Highlights importance of bacteriological surveillance of
carriere state after recovery
◦
◦
◦
◦
◦
typhoid fever
cholera,
diphtheria,
bacillary dysentery
pertusis
17

18. Carrier may be classified :
BY TYPE
Healthy Carriers:
 victims
of subclinical infection who have
developed carrier state without suffering from
overt disease, but are nevertheless shedding the
disease agent
◦
◦
◦
◦
◦
poliomyelitis,
cholera,
meningococcal meningitis,
salmonellosis,
diphtheria.
Note:- Person whose infection remains subclinical may or may not act as
carrier (eg.- in polio inf may remain subclinical but person act as temp carrier
due to shedding of virus in stool..while TB most of us with +ve Mt, do not
disseminate bacillie- so not labelled as carrier.
18

19. Temporary
carriers are those who shed the
infectious agent for short period of time.
Chronic
carriers are those who excretes the
infectious agent for indefinite periods
19

20. Chronic carriers
Chronic carriers are far more important sources of
infection than cases.
The longer the carrier state, the greater the risk of
community-- reintroduce disease into areas which
are otherwise free of infection
The duration of the carrier state varies with the
disease.
 In typhoid fever and hepatitis B, the chronic
carrier state may last for several years.
 In chronic dysentery it may last for year or longer.
 In diphtheria, the carrier state is associated with
infected tonsils, in typhoid fever with gall bladder
disease.
20

21. 


Mary Mallon (1869 –1938), better
known as Typhoid Mary, was the
first person in the US identified as
an asymptomatic carrier of the
pathogen associated with
typhoid fever.
She was presumed to have
infected some 50 people, three of
whom died, over the course of her
career as a cook.
She was forcibly isolated twice by
public health authorities and died
after a total of nearly three
decades in isolation.
21

22. Carrier classified :
By Portal Of Exit of Infectious Agent

Respiratory carrier: e.g. influenza

Fecal (intestinal) carrier: e.g. typhoid, cholera

Blood carrier: e.g. hepatitis B and HIV

Urinary : e.g. Typhoid

sexual Carrier: gonococcus and HIV
22

23. Animal reservoirs
• infection that is transmissible under
natural conditions from animals to
man.
• e.g.
–
–
–
–
–
Bacterial: Leptospira, plague from Rat.
Viral : Rabies from dog.
Protozoa: Leishmaniasis from dog.
Helminths : Hydatid disease from dog
Tape worms : Cattle , Pig.
•23

24. Reservoir in non-living
things
Some organisms are able to
survive and multiply in
nonliving environments
such as soil and water
Clostridium that causes tetanus
and botulism can survive many
years in the soil
Hookworms deposit their eggs
into the soil
Water contaminated by human
or animal feces cause GI tract
disease (list includes bacteria,
viruses, protozoa)
•24

25. 3rd - The Portal of Exit
• Route of escape of the pathogen from
the reservoir-IA enters into surrounding
reservoir
env-transfer to host at their portal of entry
Examples:
respiratory secretions,
GI
blood exposure,
breaks in skin
25

26. 4th –Mode of Transmission
Direct
transmission
Direct contact
Droplet infection
Contact with soil
Inoculation into skin or mucosa
vertical
Indirect
transmission
Vehicle-borne
:Vector-borne •
Air-borne
Fomite-born
Unclean hands
and fingers
26

27. Direct Transmission
•27

28. Direct Contact
•Inf spread by direct contact of
skin-skin, skin mucosa, mucosamucosa of same or other person
•by touching, kissing,,
bites, or sexual intercourse
•Direct & immediate transfer of IA
from reserviour –host (no intermediate
agency)
•So it introduces larger dose of IA
•No time interval of survival in
environment..
•Overcrowded place or where
place with lack of ventilation
Scabies
Pediculosis
STD’s
Skin/eye inf
leprosy
•28

29. Droplet spread:
•
Direct projection of
droplets of
saliva/nasopharynge
al secretion by
Sneezing, Speaking,
Coughing
•
Droplets directly
impinge on
conjunctiva, nasal
mucosa or skin
•29

30. •
•
•
•
H1N1
Tubercle bacilli
Measles •30
Chickenpox

31. Inoculation:
Pathogen injected into tissues.
•
–
–
Tetanus spores
Arboviruses (Insects).
•31

32. Vertical transmission
Transplacental
 To R C H
 HIV
 HBV
15
•32

33. Indirect Transmission
5 ‘F’
food, flies, fomite, finger, fluid
33

34. Vehicle transmission
•
•
Water: Cholera, H A V , H E V, Typhoid etc.
FOOD: Staphylococci, Cl. Botulinum.
•
•
Blood/serum-HIV, HBV,HCV
Organ-cmv



Clustering of cases
Distance bw secondary cases more
Common source can be traced
34

35. Vectorinsects
Mechanical
Hf
Biological
Diarrhea
Dysentery
Typhoid
Trachoma
Propagative
Only multiplication
No developmental
Plague bacilli in rat
flea
Cyclo propagative
Multiplication
developmental
Malaria parasites in
mosquito
Cyclodevelopmental
No multiplication
developmental
Filaria parasite
In mosquito
35

36. Trans-ovarian transmission
 Inf agent vertical transmitted from female
mosquito to her progeny
◦
◦
◦
◦
◦
Scrub typhus
Rickettsial pox
Indian tick typhus
Q fever
RMSF
Trans-stadial transmission Lyme disease, infects tick vector as a larva, and the
infection is maintained when it molts to a nymph and
later develops as an adult
36

37.  host
feeding preference
 infectivity-ability to transmit disease agent
 susceptibility – ability to become infected
 survival rate of vectors in environment
 Domesticity
 Seasonal factors…
37

38. Fomites :
Contaminated Nonliving Objects like Cup, towel,
napkin, linen, Clothing, glass, Toys, Pencils, door handle,
surgical instruments, syringes, dressing materials…
Ex:
Diphtheria,
Trachoma
influenza
scabies
38

39. 5th - The Portal of Entry
-route through which the
pathogen enters its new
host
•39

40. Respiratory System
io
t
la
a
h n
in
 Upper respiratory tract
Diphtheria
 Lower respiratory tract
Tuberculosis
•40

41. Gastrointestinal System
Infectious agent excreted in faeces
& transmitted to the oral
portal of entry through
􀂃 contaminated food, water, milk,
drinks
􀂃 hands
•
•
•
•
•
•
􀁺 Typhoid fever
􀁺 Shigella
􀁺 Cholera
􀁺 Polio
􀁺 Rotavirus
􀁺 Hepatitis A, Hepatitis E
ingestion
Feco-Oral Route
•41

42. Se
co x
nt ua
ac l
t
Urinary & Reproductive
Tracts
Gonorrhea
Syphilis
HIV
•42

43. Breaks in Protective Skin
Barrier
 Percutaneous
Leptospirosis
 Percutaneous
(bite of arthropod)
Yellow fever
•43

44. 6th - The Susceptible Host
A person or an animal that afford
lodgment to an infectious agent
under natural conditions.
•Accepts the pathogen
44
•The support of pathogen life & its
reproduction depend on the degree of the
host’s resistance.

45. •Cancer Patients
•HIV-AIDS Patients
•Transplant
Patients
•On steroids..
•Infant & Elderly
Patients
45

46. HOST
Obligate host :
hos
the only host
Eg: Man in measles & typhoid
Primary /definitive host: in which
parasite attains maturity or passes
its sexual stage
Secondary or intermediate hosts:
the parasite is in a larval or asexual
state
•46

47. Life cycle
Sporozoits
Liver
Mature
Salivary
Gland
Marozoits
RBC
Schizont
Oocyte
Mature
Ring Trophozoits
Ookinete
Zygote
Mosquito
Exflagellation
Gametocyte
Male / Female

48. THE TIME INTERVAL BETWEEN INVASION BY
AN INFECTIOUS AGENT AND APPEARANCE OF
THE FIRST SIGN OR SYMPTOM OF THE
DISEASE IN QUESTION

49.  DOSE
OF INOCULUM
 SITE OF MULTIFICATION
 RATE OF MULTIFICATION
 HOST DEFENCE MECHANISM

51. No of cases
Median incubation time
15
2
3
10
5
0
1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 21 22
Probable exposure
50%
1
50%
Time

52. Period From Disease Initiation To
Disease Detection
For NCDs

53. GENERATION TIME
INTERVAL OF TIME BETWEEN RECEIPT OF
INFECTION BY A HOST AND MAXIMAL
INFECTIVITY OF THAT HOST
No of cases
10
Generation
time
5
0
1
2 3
4
5
6 7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 Zeit

54. THE GAP IN TIME BETWEEN
THE ONSET OF THE
PRIMARY CASE AND THE
SECONDARY CASE

55. It is defined as the time during which
an infectious agent may be transferred
directly or indirectly from an infected
person to another person, from an
infected animal to man , or from an
infected person to an animal, including
arthropods

56. Ate the food (exposed)
Did not eat the food (not exposed)
Ill
Well
Total
Well
Total
Attack
Ill
Attack
Rate
Rate
10
3
13
76%
7
4
Attack Rate = Ill / (Ill + Well) x 100 during a time period
Attack rate = (10/13) x 100 = 76%
( 7/11) x 100 = 64%
11
64%

57. It is defined as the number of exposed
persons developing the disease within the
range of the incubation period, following
exposure to the primary case

58. Total number of cases – initial case(s)
SAR (%) =
Number of susceptible persons in the group –
initial case(s)
x 100

Used to estimate to the spread of disease in a
family, household or other group environment.

Measures the infectivity of the agent and the
effects of prophylactic agents (e.g. vaccine)
58

59. Thank You