The presentation includes the topic of reservoirs, sources, and various modes of transmission of disease and their epidemiological significance in the prevention and control of diseases. the topic also includes various examples to know the natural history of a given disease and how it will be applicable in a given epidemic situation

1. CM7.2 Enumerate and describe and
discuss the modes of transmission of
communicable disease
Dr. Anjali Mall
GGMC & JJH, MUMBAI-08

2. Specific Learning Objectives
• At the end of lecture 2nd year MBBS students must-know-(60mts)
I. Introduction(10mts)
II. Understand the chain of infection -reservoir, sources, and portal of exit of
infection. (15mts)
III. Enumerate various modes of transmission of communicable diseases.(15mts)
IV. Describe Direct Transmission with Suitable examples.(10mts)
V. Describe Indirect Transmission with Suitable examples.(10mts)

3. I. Introduction
• The communicable disease occurs as a result of the interaction of infectious agents,
the transmission process, the host, and the environment.
• Source a person, animal, object, or substance from which an infectious agent
passes or is disseminated to the host”
• A Reservoir is defined as "any person, animal, arthropod, plant, soil or substance
(or combination of these) 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"

4. II. Chain of infection

5. Chain of Infection
Reservoir of
source
Modes of
transmission
Susceptible
host

6. Examples of
Reservoir and
source
For Example
• Source- Soil contaminated with infective larvae
• Reservoir- man
In Hookworm infection
• Source- Soil
• Reservoir- Soil
In Tetanus
• Source- faeces OR Urine of patients OR
contaminated food, milk or water.
• Reservoir- May be a case OR Carrier
In Typhoid

8. cases
•Clinical cases
(mild/severe-typical/atypical)
•Sub-clinical cases
•Latent infection cases
•Primary case
•Index case
•Secondary cases
carriers
t
8
:Type
•Incubatory
•Convalescen
•healthy
Duration:
•Temporary
•Chronic
:Portal of exit
•Urinary
•Intestinal
•Respiratory
•others

9. Cases
• A person in the population orstudy group identified as
havingaparticulardisease, health disorder, or condition
under investigation” The clinical illness may be mild or moderate,
typical or atypical, severe or fatal.
• Epidemiologically, mild cases may be more important
sources of infection than severe cases because they are
ambulant and spread the infection wherever they go, whereas
severe cases are usually confined tobed.

10. 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.

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

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 acquiresthe disease
from an exposure to the primary
case
◦ Secondary attack rate
12

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

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 thepopulation or
community
◦ Readily infect the susceptible individuals
◦ Over a wider area and a longer period of timeunder
favorable conditions.
15

16. Incubatory Carriers: those who shed the
infectious agent during the incubation period.
This usually occurs during last few days ofIP
Measles-
Mumps-
Polio-
the period of communicability is 4 days before the rash.
usually 4-6 days before onset of symptoms
7-10 days before onset of symptoms
for a month beforejaundice
Hepatitis B-
Pertusis
Influenza
Diphtheria 16
Classification of Carrier

17. Carrier May Be Classified :
ByType
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 maybe classified :
• Healthy Carriers:
• victims of subclinical infection who have developed a 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
BYTYPE
18
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 bacilli- so
not labelled as the carrier.

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

20. Chronic carriers
20
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 severalyears.
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.

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

23. Animal reservoirs
• infection that is transmissible under
natural conditions from animals to
man.
• e.g.
– Bacterial: Leptospira, plaguefrom Rat.
– Viral : Rabies from dog.
– Protozoa: Leishmaniasisfrom dog.
– Helminths : Hydatiddiseasefromdog
– 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
25
• Route of escape of the pathogen from
the reservoir-IA entersintosurrounding
env-transfertohostattheirportalof entry
Examples:
respiratorysecretions,
GI
bloodexposure,
breaksin skin

26. III. Modes of Transmission
A. Direct transmission
1.Direct contact
2.Droplet infection
3.Contact with soil
4.Inoculation into skin or mucosa
5.Transplacental (vertical)
B. Indirect transmission
1.Vehicle-borne
2.Vector-borne
i. Mechanical
ii. Biological
3.Air-borne
i. Droplet nuclei
ii. Dust
4.Fomite-borne
5.Unclean hands and fingers

27. IV. Direct
transmission

28. 1. Direct contact
Skin To Skin
Mucosa To Mucosa
Mucosa To Skin Of The Same
Or Another Person

29. • Skin-to-skin contact –
• Touching, kissing or sexual
intercourse or continued close
contact.
• Diseases transmitted by direct contact
include STD and AIDS, leprosy,
leptospirosis, skin and eye infections.

30. 2. Droplet infection

31. Diseases transmitted
by droplet spread
• Respiratory infections,
• Eruptive fevers,
• Infections of the nervous system,
• Common cold
• Diphtheria, Whooping cough,
• Tuberculosis, Meningococcal
Meningitis, etc.

32. 3. Contact with soil
• Hookworm larvae
• Tetanus,
• Mycosis, etc.

33. 4. Inoculation into
skin or mucosa
Hepatitis B Virus, etc.
Rabies Virus-

34. 5. Transplacental (vertical)
• Examples-
• TORCH agents (Toxoplasma gondii, rubella
virus, cytomegalovirus and herpes virus)
• Varicella virus, syphilis, hepatitis B, Coxsackie
B and AIDS.
• Some of the non-living agents
• (e.g., thalidomide, diethylstilbestrol) can
also be transmitted vertically.

35. V. Indirect
Transmission

36. A Variety Of Mechanisms Including The Traditional 5 F's –

37. 1. Vehicle borne
• Intermediate agent: water , food , ice , blood , serum, plasma
or other biological products such as tissues & organs.
• Organisms may multiply (Staph. aureus in food ) or develop or
just get passively transmitted (Hepatitis A virus in water)

38. • Through water and food: infections of alimentary tract
• Through blood :Hepatitis B , malaria , syphilis , brucellosis ,
trypanosomiasis , infectious mononucleosis , cytomegalovirus infection
• Organ transplantation: CMV infection through kidney transplants

39. Epidemiological features:
1. Heavy dose of contamination outbreak of disease.
2. Initial confinement to the exposed to contaminated vehicle.
3. Primary case may be obscured by the time secondary cases appear.
4. Not always possible to isolate the infectious agent from incriminated vehicle.
5. Epidemic subsides on control or withdrawal of vehicle.
6. Great distances may be travelled by the infectious agents.
7. Common source is often traceable.

40. 2. Vector-borne
• Vector….an arthropod or
any living carrier that
transports an infectious
agent to a susceptible
individual.
Cyclops
Bat Mice

41. Epidemiological classification:
BY VECTOR :-
a) Invertebrate type :
1)Diptera – flies &
mosquitoes
2)Siphonaptera – fleas
3)Orthoptera – cockroaches
4)Anoplura –sucking lice
5)Hemiptera –bugs
6)Acarina – ticks &mites
7)Copepoda – cyclops
b) Vertebrate type :
mice
rodents,
bats, etc.

42. By Transmission Chain :-
A) Man & A Non-vertebrate Host
1) Man – Arthropod – Man (Malaria)
2) Man – Snail –Man (Schistomiasis)
B) Man , Another Vertebrate Host &
A Non-vertebrate Host
1) Mammal – Arthropod – Man (Plague)
2) Bird – Arthropod – Man (Encephalitis)
C) Man & 2 Intermediate Hosts
1) Man – Cyclops-fish-man (Fish Tapeworm)
2) Man – Snail – Crab – Man (Paragonimiasis)

43. By methods in which vectors transmit agent :-
a) Biting
b) Regurgitation
c) Scratching-in Of Faeces
d) Contamination Of Host With Body Fluids Of Vectors.
By methods in which vectors are involved in transmission & propagation of parasites :-
a) Mechanical
b) Biological

44. • a). Mechanical transmission
Soiling of feet or proboscis or by passage of organism through its GIT and
passively excreted
e.g., houseflies typhoid

45. • b). Biological transmission
replication or development or both may take place
a) Propagative :
e.g., Plague bacilli in rat fleas
b) Cyclo-propagative :
e.g., Malaria parasites in mosquito
c) Cyclo-developmental :
e.g., Microfilaria in mosquito

46. 3. Air-borne
• a). Droplet nuclei- "Droplet nuclei " are a type of particles
implicated in the spread of airborne infection
e.g., tuberculosis , influenza , chickenpox , measles , Q fever
• b). Dust- settling down of larger droplets may become air-
borne Nosocomial infection
e.g., Pneumonia, Tuberculosis, Psittacosis

47. 4. Fomite-borne
• Fomites (singular; fomes) are inanimate articles or substances other than
water or food contaminated by the infectious discharges from a patient and
capable of harbouring and transferring the infectious agent to a healthy
person.
• Diseases transmitted by fomites- diphtheria, typhoid fever, bacillary
dysentery, hepatitis A, eye and skin infections.

48. 5. Unclean hands and fingers
• The transmission takes place both directly (hand-
to-mouth) and indirectly.
• Examples-
• staphylococcal and streptococcal infections
• typhoid fever, dysentery, hepatitis A and intestinal
parasites.
• Lack of personal hygiene coupled with poor
sanitation favour person-to-person transmission of
infection;

49. Self assessment MCQs
1. Cyclo-developmental stage is seen in :
(a) Malaria
(b) Filaria
(c) Plague
(d) Cholera
(e) None

50. 2. Mother-to-child transmission in vectors is called :
(a) Transovarian
(b) Mechanical transmission
(c) Propagative transmission
(d) Cyclo-developmental transmission

51. Thank you