this presentation will help the students to learn on dynamics of disease transmission and chain of infection and its control

1. P R ES E N T E D BY
M RS . P O U L E E N A R E D DY
S I M S CO L L EG E O F N U RS I N G
G U N T U R
Dynamics of infectious disease
transmission

2.  Communicable diseases are transmitted from the
reservoir/source of infection to susceptible host.
 Basically there are three links In the chain of transmission
 A. reservoir
 Mode of trans mission
 C. susceptible host

3. 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 .

4. I. Source or reservoir
• These are natural habitat of infectious agents in which an
infectious agent normally lives and multiplies.
• Definition of source ; “the person , animal, object or
substance from which an infectious agent passes or is
disseminated to the host “
• reservoir : defined as any “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 produces itself in such
manner that it can be transmitted to a susceptible host “

5. • Examples are;
1. Human reservoirs
2. Animal reservoirs (zoonotic diseases)
3. Reservoir in non-living things.

6. 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
• Suspect case
Type:
• Incubatory
• Convalescent
• healthy
• Duration:
• Temporary
• Chronic
• Portal of exit:
• Urinary
• Intestinal
• Respiratory
• others

7. Cases may be:
1. Primary case: The first case of an infectious disease introduced
into the population.
2. Index case: The first case that come to attention of investigators.
3. Secondary case: Cases that develop from primary cases.
4. Suspect case: case in an individual (or a group of individuals) who
has all of the signs and symptoms of a disease or condition, yet
has not diagnosed as having he disease condition

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

9.  Whatever may be the “ gradient of infection” all infected
persons, whether clinical or subclinical are potential
sources of infection.

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. Elements in carrier state are
I. A.the presence in the body of the disease agent
II. The absence of recognizable symptoms and signs of
disease
III. The shedding of the disease agent in the discharges or
excretions

12. Classification of carriers
 1.. type
1.incubatory
2.convalsecent
3.healthy
2. Duration
a. temporary
b.chronic
3. Portal of exit
1. urinary
2. Intestinal
3. Respiratory
4. others

13. Cont.
Carriers may be classified by:
1. Types:
i. Incubatory Carrier: Person capable of transmitting an infectious agent
during incubation period.
ii. 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.
iii. Healthy Carrier: Emerge from subclinical cases. They are victims of
subclinical infectionn who have developed carrier state without
suffering from overt disease .

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

15. Cont.
3. Portal of exit:
• Carriers may be classified according to portal of exit of
infectious disease.
1. Urinary
i. Intestinal
ii. Respiratory
iii. Open wounds and
iv. Blood.

16. 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.

17. 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.

18. CO M M U N I C A B L E D I S EA S ES T R A N S M I T T E D F RO M T H E R ES E RVO I R O R
S O U RC E O F I N F EC T I O N TO A S U S C E P T I B L E H O ST I N M A N Y WAYS .
II: MODES OF
TRANSMISSION OF DISEASE

19. 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:
a.Mechanical
b. biological
Air-borne;
a. Droplet
b. dust
Fomite-born
Unclean hands
and fingers
propagative
Cyclo-prop.
Cyclo-develop.

21. 1. Direct transmission:
I. Direct physical contact: Infect5ion May Be Transmitted
By Direct Contact From Skin To Skin.
From source/ reservoir to a susceptible host,
Direct contact not only reduces the period for which the
orgenism will have to survive outside the human host
but also ensures a larger dose of infectoin.
e.g. skin to skin contact, STDs AIDS, eye infections etc.

22. II. Droplet infection: THIS is direct projection of a spray
of droplets of saliva and naso-pharyngeal secretions
during coughing , sneezing. The expelled droplets
may impinge directly upon the conjunctiva , oro
respiratory mucosa or skin of a close contact.
1. Particles of 10mmm or greater in diameter are
filtered off by nose. By saliva & nasopharyngeal
secretions e.g. flu, diphtheria, whooping cough,
tuberculosis, meningococcal meningitis etc.

23. iii. Contact with soil: the diseas agent may be acquired by direct exposure of
susceptable tissue to the disease agent in soil. compost or decaying
vegetable matter in which it normally leads saprophytic existence
The disease agent is in soil e.g. hook worm infection, tetanus, mycosis etc.
iv. Inoculation into skin or mucosa: The disease agent inoculated directly into
skin or mucosa e. g. rabies, hepatitis B infection through contaminatd needles
and syringes, etc.
v. Trans placental (vertical): disease agent can be transmitted trans
placentally. This a is another from of direct transmission, e.g. Toxoplasmosis,
others, rubella, cytomegalovirus, herpes simplex (TORCH),virecella virus,
syphilis, hepatitis B.

24. Indirect transmission
 This embraces a variety of mechanisms
including the traditional 5 Fs- “ flies, fingers,
fomites, food and fluids.
 An essential requirement for indirect
transmission is that the infectious agent must
be capable of surviving outside the human
host in the external environment and retain its
basic properties of pathogenesis and virulence
till it finds new host.

25. 2. Indirect transmission
1. 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.
 The infectious agent may have multiplied or developed in the
vehicles(e.g.,s.aureus in food) before being transmitted; or only passively
transmitted in the vehicle(e.g, hepatitis A virus in water)
 Disease transmitted by water and food include chiefly infections of the
alimentary tract
 E.g. diarrhea, typhoid, cholera, polio, hepatitis A, brucellosis, etc.

26.  The epidemiological features of vehicle transmission
are :
 A. if the dose of contamination is heavy the outbreak may be
explosive as in the case of cholera and hepatitis A. epidemic
 B. cases are initially confined to those who are exposed to the
contaminated vehicle.
 c. when secondary cases occur, the primary case may be obscured
 D. the distance travelled by the infectious agent may be great e.g.
outbreak of food poisoning
 E. it is not always possible to isolate the infectious agent in the
incriminated vehicle

27. 2. Vector-borne transmission: Living carriers that
transports infectious agents to a susceptible individual. It
may be; mechanical transmission or biological
transmission

28. Classification of vector – borne disease
classification
In vertebrate
type
Diptera – flies and mosquitoes
Siphonaptera- fleas
Ortoptera – cockroaches
Anoplura -sucking lice
Hemiptera – bugs,including kissing bugs
Vertebrate
type
Mice
rodents,bats
Copepoda - cyclops

29. II. By transmission chain
 Vector – borne disease are classified under heterogeneous infection chain
and involve three principal patterns :
 A. man and a on – vertebrate host
1. man – arthropod – man (malaria)
2. man – snail – man (schitosomiasis )
b. man , another vertebrate host, and a non – vertebrate host
1. mammal – arthropod – man
2. birds – antropods – man (encephalitis)
C. Man and 2 intermediate hosts
1. MAN – cyclops – fish- man (fish tape warm)
2. Man- snail- fish- man (clonorchis sinensis)
3. Man – snail- crab- man (paragonimiasis)

30. III. By methods in which vectors transmit agent
 A. biting
 B. regurgitation
 C. scratching – in of infective faeces
 D. contamination of host with body fluids of vectors

31. IV. By methods in which vectors are involved in
the transmission and propagation of parasites
i. Mechanical transmission: When a vector simply
carries pathogenic microorganisms on their body and
transfers them to food. E.g. flies and cockroaches.

32. ii. Biological transmission: the agent multiplies OR
develops in a vector. Requires an incubation period
before vector can transmit.
Biological trans mission three types
1. Propagative transmission: the agent merely multiplies in vector but no
change in the form .Multiplication without cyclic change such as DHF,
yellow fever, etc.
2. Cyclo-development transmission: No multiplication but cyclic change
such as filariasis in mosquito etc.
3. Cyclo propagative transmission: Multiplication and change in form such
as malaria, etc.

33.  Transovarial transmission : when the infectious agent is
transmitted vertically from the infected female to her
progeny in the vector , it is known as transovarial
transmission.
 Transstadial transmission ; transmission of the disease
agent from one stage of the life cycle to another e.g.
nymph to adult

34. Factors influencing the ability of vectors to transmit
disease
 A. host feeding preferences
 B. infectivity - that is ability to transmirt the disease agent
 C. susceptibility, that is ability to become infected
 D. survival rate of vectors in the environment
 E. domesticity , that is degree of association witrh man
 F. suitable environmental factors

35. 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.

36. Diseases may be transmitted as:
1. 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.

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

39. Successful parasitism
Four stages
1. Portal of entry
2. Site of election
3. Portal of exit
4. After leaving the human body the organism must
survive in the external environment for sufficient period
till a new host is found.

40. Stages of infectious disease

41. 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.

43. 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

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

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

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

47. 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.

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

49. 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

50. 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

51. 4. Modes of transmission
• 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.
i. Person to person: Touching, kissing, sexual intercourse,
coughing, sneezing, talking etc
ii. Trans – placental: Transfer of infectious agents from
pregnant woman to fetus via placenta.

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

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

54. 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

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

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

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