MODULE 6: EPIDEMIOLOGY

Epidemiology

* study of the distribution of states of health and of the determinants of deviation from health in population. study of the distribution and determinants of diseases and injuries in human population.
* Concerned with Distribution Determinants Of health and disease Of morbidity Of injuries Of mortality In a population (Friis and Sellers, 2004, p. 5)
* Epidemiology: from Greek words:

•epi, meaning “on or upon,”

•demos, meaning “people,” and

•logos, meaning “the study of.”

Distribution of disease

Describes health status in terms of demographic profile “who” characterized by measures of disease frequency IR, PR, and “How many”

USES OFEPIDEMIOLOGY

* History of the health of the population,
* Useful projections
* Diagnose the health of the community
* Measure the true dimensions and distribution of ill health
* set health problems in perspective
* study the working of health services
* estimate from the group experience whatare
* study syndromes the individual risk
* complete the clinical picture
* search for causes

Importance of epidemiology

It serves as the backbone of theprevention of the diseases

Scope of epidemiology

* health
* Biological processes
* Human population
* Disease

STAGE I: Stage of Susceptibility

* *disease has not developed but the groundwork has been laid by the presence of factors that favor its occurrence. factors whose presence is associated with increased probability that disease will develop later are called risk factors.*
* *some risk factors can be altered like smoking, alcoholism, fatigue some risk factors are not subject to change eg.,age, sex, race and family history*

**STAGE II: Stage of Pre-symptomatic Disease**

* there is no manifest disease yet, but usually through the interaction of factors, pathogenic changes have started to occur.

STAGE III: Stage of Clinical Disease

* sufficient end organ changes have occurred so that there are recognizable signs and symptoms of the disease

STAGE IV  Stage of Disability

* any temporary or long term reduction of a person’s activity as a result of an acute or chronic condition

THE NATURAL HISTORY OF DISEASE

* Disease Process
* Pathogenesis
* Pre-pathogenesis

PREVENTION

Measures the interrupt or slow the progression of the disease

PRIMARY PREVENTION

* prevention of disease by altering susceptibility or reducing exposure for susceptible individuals
* APPROPRIATE FOR STAGE OF SUSCEPTIBILITY
* general health promotion
* specific protective measures

Promotes health and wellness and prevents disease in the population




1. Health Promotion - efforts to improve the quality of life and enhance the level of physical and mental health
2. Specific Protection - specific procedures of disease prevention

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SECONDARY PREVENTION

* early detection and treatment of disease
* APPROPRIATE IN THE PRECLINICAL AND CLINICAL STAGES
* cure the disease, slow its progression, prevent complications, limit disability and reverse communicability of infectious diseases.
* Halts the progress of a disease at its incipient stage and prevents complications



1\. Early Diagnosis


* case finding measures (individual and mass
* screening surveys
* periodic examination
* selective examination of people at high risk

2\. Treatment and Disease Control

TERTIARY PREVENTION

* alleviation of disability resulting from disease and attempts to restore effective functioning 
* APPROPRIATE IN THE STAGE OF ADVANCED DISEASE OR DISABILITY
* Prevents potential consequences of disease


1. Disability Limitation
2. Rehabilitation

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Disease Causation

an understanding of disease causation helps in the formulation of prevention and control measures

THE ECOLOGIC CONCEPT OF DISEASE

* *Disease arises within an ecological system, hence, an ecologic approach is necessary to explain  disease causation and occurrence*
* *Ecology is the study of the relationship of organisms to each other as well as to all other aspects in the environment.*

MULTIPLE CAUSATION OF DISEASE  MULTIFACTORIAL ETIOLOGY OF DISEASE

* disease cannot be attributed to the operation of any one factor, rather more than one factor  is  required to be present for the disease process to be initiated.

PREMISES OF THE MULTIPLE CAUSATION THEORY

* Disease results from an imbalance between the host and the disease agent.
* The nature and extent of the imbalance depends upon the nature and characteristic of the host and the agent.
* The characteristic of the host and the agent are directly related to and largely dependent on the nature of the physical, biologic and socio-economic environment.

HOST FORCES and FACTORS (Intrinsic)

Intrinsic characteristics-the attributes of the host determine to a considerable extent the onset, type and course of certain diseases

* physical constitution
* anatomic structure

physiologic state

nutritional status

concurrent infection

DISEASE AGENT

an element or substance or force, either animate or inanimate the presence or absence of which may following contact with the susceptible host under proper environmental  conditions serve as a stimulus to initiate or perpetuate a disease process.

BIOLOGIC AGENT

1. INFECTIVITY
2. PATHOGENICITY
3. VIRULENCE
4. IMMUNOGENECITY

INFECTIVITY

ability to enter and multiply in the host

PATHOGENICITY

ability to cause disease after entering the body depending on the multiplication process and the extent of tissue damage.

VIRULENCE

the ability to induce severe disease due to    INVASIVENESS and TOXICITY

IMMUNOGENECITY

the ability to induce specific immunity in the host.

RESULT OF HOST-AGENT RELATIONSHIP

* INFECTION (which may be inapparent or apparent)


* NO INFECTION

NUTRITIVE

Lack or excess of virus on patients body system

CHEMICAL agent

* dusts, gas, vapors, fumes, mists, pesticides, food additives

PHYSICAL agent

* atmospheric pressures, humidity,, intense sound (noise), vibration, heat, light, sound, radiation

The physical agents of disease

* HEAT


* COLD


* Atmospheric Pressure Changes


* **NOISE**


* **VIBRATION**

HEAT

Sources:   prolonged exposure to the heat of the sun, indoors, hot processes,

and/or inadequate ventilation

Effects: burns, heat stress, heat illnesses

COLD

Sources: cold processes or operations,  low  outdoor temperature.

Effects: heat loss, freezing, frost bites etc.

Atmospheric Pressure Changes

Sources: low/high altitude,  disturbance in the pressure

Effects: Dysbarism, decompression

NOISE

intense sound above tolerable levels

•institutions-45 decibels

•residential-55 decibels

•commercial-80 decibels

•industrial-90 decibels

 

*Sources: people’s activities, machineries*

*effects: impairment of hearing, increase in BP*

VIBRATION

oscillatory conditions within a mechanical system

*Sources:   use of tools like drills*

*effects:   muscular pains, visual acuity effects*

 THE ENVIRONMENT (extrinsic factor)

* is the sum total of all external conditions and influences that affect the life and development of an organism.  
* It affects both the agent and the host

3 component subdivisions

* Physical Environment
* Biologic Environment
* Socio-economic Environment

Biologic Environment

* infectious agent, reservoir of infection,vectors, plants and animals
* universe of living things that surround man, except man himself and the infectious agent. 

Eg.   animal and plants as reservoir of infection, insects or arthropods as vectors of infection

Physical Environment

* non living or inanimate things that surround man.  
* They can either be predisposing factors or transmitters or vehicles of the infectious agent.

Socio-economic Environment

* overall economic and political organization of a society and of institutions by which individual are integrated into the society at various stages in their lives.

\-the association of man with his fellow men

•occupation

•urban-rural

•economic development

•social disruption

•revolutions

•war

•earthquakes/typhoons

ECOLOGIC MODELS or INTERRELATIONS OF FACTORS

* developed to depict ways  in which these interactions influence the occurrence of disease.

The EPIDEMIOLOGIC TRIANGLE

widely used model for many years. 

* is considered to consist of three components, HOST, ENVIRONMENT AND AGENT.
* the model implies that each must be analyzed and understood for comprehension and predictions of patterns of a disease.
* it highlights as a separate component, the AGENT of disease. 
* when the focus of epidemiologic  studies was limited to infectious diseases, the infecting organism was accorded a status separate from other environmental factors and identified as AGENTS of the disease.

The wheel

depicts human-environment interactions or relations.

* it consists of a hub (host or human), which has genetic make up as its core.
* surrounding the host is the environment, schematically divided into biological, social and physical.
* the relative sizes of the different components of the wheel depend upon the specific disease problem under consideration
* it implies the need to identify the  multiple etiologic factors of disease without emphasizing the agent of disease

The WEB of CAUSATION

* developed by MacMahon in 1970
* it implies that the effects never depend on  single isolated causes, but rather develop as the result of chains of causation in which each link itself is the result of a complex “genealogy of antecedents”
* a web, its complexity and origins lies beyond our understanding

TYPES OF COMMUNITY REACTIONS

* sporadic
* Endemic
* Epidemic
* Pandemic

SPORADIC

* occurrence is the intermittent occurrence of a few isolated an unrelated cases in a given locality.
* the cases are few and scattered, so that there is no apparent relationship between them.
* they occur on and off, through a period of time.
* eg.Rabies

ENDEMIC

* continuous occurrence throughout a period of time, of the usual number of cases in a given locality.
* disease is therefore always occurring in a given locality and the level of occurrence is always constant through a period of time.  It may be high or low, provided any given level is continuously maintained.
* eg.  Schistosomiasis is endemic in Leyte and Samar Filariasis in Sorsogon

EPIDEMIC

* occurrence of an unusually large number of cases in a relatively short period of time.
* there is a disproportionate relationship between the number of cases and the period of occurrence, the more acute is the disproportion, the more serious and urgent is the problem.

PANDEMIC

* is the simultaneous occurrence of epidemics of the same disease in several countries.
* pattern of occurrence in an international perspective.
* influenza pandemic in 1918 and 1965 cholera-1961-1965, Southeast Asia