Epidemiology of Infectious Diseases

Epidemiology of Infectious Diseases

-Ma. Eufemia M. Collao, MD DPAFP

Objectives: At the end of the session, the student must be able to:

1. Define the following: • sporadic, endemic, epidemic, pandemic, • herd immunity, • attack rates

2. Differentiate:• Communicable from non-communicable

disease• Isolation from quarantine• Mechanical from biological vector• Incubation from generation period

Cont’n…

3. Describe the intrinsic properties of disease agents

4. Discuss the different host agent interaction.

5. Discuss the different pathogenetic mechanism

6. Discuss the different mechanism of transmission.

Infectious Diseases

Also known as communicable diseases.Transmitted directly to man from man (anthropo-zoonoses) or from animals (zoonoses) to man.Characterized by cyclic periodicities and seasonal predilectionsGenetics only an indirect role = predispositions

The EPIDEMIOLOGIC TRIANGLE

MAN

DISEASE AGENT

ENVIRONMENT

Disease Transmission is Affected By :

Factors intrinsic to man

Factors intrinsic to the disease

Role of vectors

Factors intrinsic to the environment

The study of these factors and their dynamics in disease transmission is

EPIDEMIOLOGY

The methodology for this study consists of

Data generation

Data analysis

Evidence-based intervention planning

The main tool used for data generation is

DISEASE SURVEILLANCE

SOURCES OF DATA

FIELD HEALTH SERVICE INFORMATION SYSTEM

PROGRAM-BASED REPORTS

NESS REPORTS

Terms:• Endemic: • Sporadic: occurrence of

disease, on and off, cases seemingly unrelated to each other, low incidence level

• Pandemic: a marked increase in disease incidence of world-wide or continental distribution, usually of sudden occurrence

• Herd immunity: resistance of a group to invasion and spread of an infectious agent

• Secondary attack rate: rate thru which spread of disease is measured

• Secondary attack rate: number of cases of a disease developing during a stated time period among those members of a closed group who are at risk = # of new cases in a group – initial _________case(s)___________# of susceptible persons in the grp –initial cases

EPIDEMICS

Also called outbreaks

Occurrence of a disease over and above its expected levels of occurrence.

Should be qualified by the presence of clustering : chronological and geographic

ESTABLISHNG LEVELS OF OCCURRENCE

Taking the averages (over a period of 3-4 years)

Consider highs and lows

Use of incidence and prevalence

EPIDEMICS

Are calamities in themselves

May result from the occurrence of natural or physical calamities

Occurs as a result of environmental sanitation breakdown, displacement of populations and more conducive environmental conditions for disease prevalence and transmission

NECESSARY…

EARLY DETECTION AND MANAGEMENT OF CASES

SURVEILLANCE : monitor trends

IDENTIFY CAUSE

PLAN PREVENTIVE MEASURES

Origin/ Cause of Epidemics:

New DiseaseIntroduction of disease to the community for the first time

Dse of lower animals affecting man for the first time

Dse discovered for the first time

Old DiseaseIntroduction of new strain of the disease

Immigration of a large number of susceptible individuals

Decrease resistance of population due to catastrophe such as famine, earthquakes, floods, etc

Increased virulence/ pathogenicity

Increased agent population

Termination of Epidemic

Exhaustion of susceptibles

Elimination of the agent

Closure of secondary transmission

Differentiate:

• Communicable vs non-communicable disease

• Isolation vs quarantine• Mechanical vs biological

reservoir• Incubation period vs generation

period

Communicable disease: due to a specific infectious agent or its toxic products, arising through transmission of that agent or its products from reservoir to susceptible host, either directly as from an infected person or animal or indirectly by the agency of an intermediate plant or animal host, a vector, or the inanimate environment; also includes infestation

• Incubation period: from the time the agent enters the host to the time the disease becomes manifest.

• Generation period: period between the receipt of infection by a host and maximal communicability of the host

• Isolation vs quarantine• Mechanical vs biological vector

Describe the intrinsic properties of disease agents

Discuss the different host-agent interaction.

Discuss the different pathogenetic mechanism

Discuss the different mechanism of transmission.

The Ecologic Model of Disease Causation

1. The lever or Balance

Host agent

Environment

The Ecologic Model of Disease Causation

1. The lever or Balance

Host

agent

Environment

Ecologic Model

Host

agent environment

Agent: any element, substance, or force whether living or non-living, the presence of which can initiate or perpetuate a disease process

Types:Living vs non-living

Physical , mechanical

Chemical, nutrients

Characteristics:Inherent: physical features, biologic requirements, chemical make-up, viability, resistance

Those directly related to man: infectivity, pathogenicity, virulence, antigenicity

Those related to the environment: reservoirs and sources of infection and mode of transmission

Those directly related to man

Infectivity: ability to gain access and adapt to the human host to the extent of finding lodgment and multiplication

Pathogenicity: measures the ability of the agent to cause a specific reaction

Virulence: severity of the reaction, usually measured in terms of fatality

Antigenicity: ability to stimulate a response

Different pathogenetic mechanisms

Direct tissue invasion

Production of a toxin

Immunologic enhancement or allergic reaction leading to damage to the host

Persistent or latent infection

Enhancement of host susceptibility to drugs of otherwise minimal toxicity

Immune suppression

Different mechanisms of transmission

Direct transmissionIndirect transmission:

Vehicle-borneVector-borne:

1. Mechanical2. biological

Airborne: 1-5 um1. Droplet nuclei2. Dust

Disease Causation

1. Agent increase in virulence or taken in massive doses2. Agent remains the same but there is a change in the characteristic of the host- a) Decrease in host resistance b) Change in the activity of the host – increase in contact with the diseased person

Isolation & Quarantine

Isolation – separation during the period of communicability of infected persons or animals from others .

To prevent the spread of the disease to those who are susceptible.

Isolation & Quarantine

Categories of Isolation : 7 categories

A) Hands must be washed after contact with the patient or potentially contaminated articles and before taking care of another patient.

B) Articles contaminated with infectious materials should be appropriately discarded or bagged & labeled before being sent for decontamination & reprocessing.

Isolation & Quarantine

Categories of Isolation : 7 categories1. Strict Isolation – to prevent highly contagious and virulent infections that may spread by both air and contact .Specification:Private roomUse of mask , gloves & gown for all persons entering the room.Special ventilation requirements with the room at negative pressure to surrounding areas is desirable

Isolation & Quarantine

Categories of Isolation : 7 categories

2. Contact Isolation –

For less highly transmissible or serious infections

Disease or conditions which are spread primarily by close or direct contact.

Isolation & Quarantine

Categories of Isolation : 7 categories

3. Respiratory Isolation – diseases spread through the air

4. Tuberculosis Isolation-

5. Enteric Precaution

6. Drainage Secretion precaution

7.Blood & Body fluid precaution