epidemiology of infectious disease in microbiology

1. EPIDEMIOLOGY OF INFECTIOUS DISEASE
PRESENTED BY- Dr. Sarita Kumari Sahu
PG 1st Year
Department of Microbiology
SCB Medical College ,Cuttack

2. EPIDEMIOLOGY
 Epidemiology is derived from the Greek word EPI(among) + DEMOS(people)+
LOGOS(study)
 Epidemiology has been defined by John M. Last in 1988 as “The study of the
distribution and determinants of health related states or events in specified
populations and application of this study to the control of health problems.
Learning Objectives…
 To recognize the infectious cycle (epidemiological triad)
 To understand the chain of infection.
 To understand the natural history of the disease

3. AIMS OF EPIDEMIOLOGY
According to international epidemiological association(IEA), Epidemiology has 3 main aims :
I. To describe the distribution and magnitude of health and disease problems in human
population
II. To identify etiological factors in pathogenesis of disease
III. To provide the data essential to planning, implementation and evaluation of services for
the prevention, control and treatment of disease and to setting up of priorities among
those services

4. EPIDEMIOLOGICAL TRIAD
o Disease is the result of
forces within a dynamic
system called an
“epidemiological triad”
which consisting of :-

5. COMMUNICABLE DISEASE
 Diseases transmitted from one case
to another.
 There is a chain for transmission of
infection from one host to another.

9. HUMAN RESERVOIR
 CASES: A case is a person in the population or study group identified as having a
particular disease .
The case may be a -
 PRIMARY CASE
 INDEX CASE
 SECONDARY CASE
 SUSPECTED CASE
 CARRIERS: A person who is harboring the organism (allows its multiplication inside his
body) without showing signs & symptoms and is capable of transmitting the disease
to other person.
Carriers are less infectious than cases but epidemiologically more dangerous than
cases.
They escape recognition and continuing to live a normal life and readily infect
susceptible individuals.

10. TYPES OF CARRIER
TIMING OF
TRANSMITTING
AGENT
DURATION PORTAL OF EXIT
INCUBATORY
CONVALESCENT
HEALTHY
TEMPORARY
CHRONIC
RESPIRATORY
FECAL
URINARY

11. ANIMAL RESERVOIR
Diseases that can be transmitted under natural conditions from vertebrate animals
to humans are called “Zoonosis” (e.g. rabies, yellow fever, plague, anthrax,
brucellosis).
Famous animal reservoirs: cat, dog, horse, cattle, bat, poultry and rodents

12. RESERVOIR IN NONLIVING THINGS
Soil can also act as reservoir of infection (e.g. soil may harbor agents that cause tetanus &
anthrax).
• Water.
• Food: meat, milk, vegetables.

13. PORTAL OF EXIT
Path by which an agent leaves its human or animal source host
‘
Respiratory tract e.g. influenza virus
Genitourinary tract e.g. sexually transmittal diseases
Alimentary tract e.g. hepatitis A virus (HAV)
Blood e.g. hepatitis B virus (HBV)
In-utero transmission e.g. rubella, CMV

16. MODE OF TRANSMISSION
DIRECT TRANSMISSION
1. Direct contact
2. Droplet infection
3. Contact with soil
4. Inoculation into skin or mucosa
5. Transplacental
INDIRECT TRANSMISSION
1. Vehicle born
2. Vector borne
3. Air borne
4. Fomite borne
5. Unclean hands and fingers

17. SUSCEPTIBLE HOST
Successful parasitism include 4 stages
i. The infectious agent must find an appropriate portal of entry.
ii. After entry it must reach suitable site of tissue.
iii. Disease agent must find a way out of body to reach a new host.
iv. After leaving human body the organism must survive in the external
environment.

18. HOST SUSCEPTIBILITY
The host’s susceptibility pattern to infectious agent can be described
by-
1.Host immunity
2.Incubation period
3.Serial interval
4.Generation time
5.Communicability period
6.Secondary attack rate

19. 1.HOST IMMUNITY
Intrinsic factor that influence an individual's exposure, susceptibility, or response to a
causative agent is the host immunity.
HOST IMMUNITY
VIRULENCE OF ORGANISMS

21. VACCINES
Immunity is induced by immunization by vaccines ( Artificial acquired active immunity )
which stimulates the body to form immunoglobulin.
Duration of immunity depends on the type of vaccine
Live attenuated vaccine
o Give lifelong immunity
Killed vaccine
o Give short time or low level of
immunity

22. HERD IMMUNITY
 Describes the immunity level that is present in a population group.
 Provides an immunological barrier to the spread of diseases in the community.
 May be acquired after frequent mass vaccinations
The higher the herd immunity
the higher the power to defense to an epidemic occurrence in the community.

24. 2.INCUBATION PERIOD (IP)
The time interval between contact with an agent and the 1 st clinical
evidence of the disease.
It depends on;
Portal of entry (defense mechanism).
Ability of multiplication (infectivity).
 Number of agents.
 Level of antibody in the host

25. The IP varies individually according to :-
 Defense mechanism “Ability to react against agent invasion in the body”.

26. Diseases can have
1.Short IP
2.Medium IP
3.Longer IP
Importance of IP:
 Tracing the source of infection & contact
 Period of surveillance
 Immunization
 Identification of point source or propagated epidemics
 Prognosis -
e.g. in rabies & tetanus, the shorter the IP is, the worse the prognosis of
disease

27. 3.Serial interval
Gap in the time between the onset of primary case and secondary case.
4.Generation time
The interval of time between receipt of infection by a host and maximal infectivity of that
host.
5.Communicability period
Time during which an infectious agent may be transferred directly or indirectly from an
infected person to another person.
6.Secondary attack rate
SAR = NO.OF PERSONS DEVELOPING DISEASE WITH IN THE RANGE OF IP X 100
TOTAL NO.OF SUSCEPTIBLE OE EXPOSED CONTACTS

28. CLINICAL PICTURE AND COMPLICATION OF INFECTION
 Differs from one disease to another & from one case to another.

29. SPREAD OF COMMUNICABLE DISEASES

30. EPIDEMIC INVESTIGATION
 Set of procedures used to identify; the cause responsible for the disease, the people affected,
the circumstances & mode of spread of the disease, and other relevant factors involved in
propagating the epidemic disease .
It is a challenging task for health workers.

31. STEPS OF EPIDEMIC INVESTIGATION
1.Verify the diagnosis or causes:
- Careful analysis of the initial reports.
- Confirm diagnosis by performing clinical & lab studies.
- Putting the criteria for case detection.
2.Establish the existence of an epidemic:
Comparing current level (incidence) with past level of the disease in that locality & population
4. Develop hypothesis to explain occurrence of the epidemic.
5. Test the hypothesis
6. Identification of susceptible population.
7. Management of the epidemic.
8. Formulation of the report & communicate findings & recommendation to higher levels in
the health system.

33. PREVENTION OF COMMUNICABLE DISEASE
The goal of preventive medicine is to promote, preserve and restore health
when it is impaired and to minimize suffering and distress.
Levels of prevention can be –
I. Primordial prevention
II. Primary prevention
III.Secondary prevention
IV.Tertiary prevention

34. LEVELS PREVENTION OF COMMUNICABLE DISEASES

35. PRIMORDIAL PREVENTION
It is prevention of emergence or development of risk factors in countries or
population groups in which they have not yet appeared.
Here efforts are directed towards discouraging children from adopting
harmful lifestyles.
The main intervention in primordial prevention is through individual and
mass education.

36. PRIMARY PREVENTION
 Here Action taken prior to the onset of disease which removes the possibility that a
disease will ever occur.
 It signifies intervention in prepathogenesis phase.
 It includes concept of ‘positive health’, a concept that encourages achievements and
maintenance of an acceptable level of health that will enable every individual to lead a
socially and economically productive life.
 It includes interventions like –
1. Health promotion and
2. Specific protection

37. 1.HEALTH PROMOTION
 Health promotion is the process of enabling people to increase control over and to
improve health
 It is not directed against a particular disease
 It includes-
I. Health education
II. Environmental modification
III. Nutritional intervention
IV. Life style and behavioral changes

40. 2. SPECIFIC PROTECTION
• To avoid disease is the ideal intervention but it is possible in limited no of
cases
• Some currently available interventions are-
 Immunization
 Use specific nutrients
 Chemoprophylaxis
 Protection against occupational hazards
 Protection from carcinogens
 Avoidance of allergens etc

41. SECONDARY PREVENTION
Action which halts the progress of a disease at its incipient stage and
prevent complication.
For that early diagnosis and treatment is very necessary.
The earlier a disease is diagnosed and treated ;the better is the outcome
from prognosis point of view and thereby preventing the occurrence of
further cases and any long term disability.

42. TERTIARY PREVENTION
It signifies intervention in late pathogenesis phase.
It is all about the measures available to-
 Minimize suffering.
 Optimize function.
 Prevent further deterioration.
 Help complicated cases to cope with their handicap.

43.  These can be achieved by disability limitation and rehabilitation.
DISABILITY LIMITATION :-
• The concept of disability is;
DISEASE IMPAIRMENT DISABILITY HANDICAP
REHABILITATION :-
• The combined and coordinated use of medical, social, educational and vocational measures for
training and retraining the individual to highest possible level of functional ability.
• It can be –
a) MEDICAL REHABILITATION
b) VOCATIONAL REHABILITATION
c) SOCIAL REHABILITATION
d) PSYCHOLOGICAL REHABILITATION