Community and Public Health (Week 5)

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EPIDEMIOLOGY

Overview of the session• The students are introduced to the concepts

of Epidemiology and its application in Public Health.

Objectives• At the end of the session, the students should be able to define epidemiology and understand thedifferent study designs as applied to publichealth.

EPIDEMIOLOGY• Study the occurrence and distribution of

diseases as well as distribution of determinants of health state or events in specified population and the application of this study to control health problems

• Field of science dealing with the relationship of the various factors which determine the frequencies and distribution of an infectious process. A disease or a physiological state in human community.

• Study of the behavior of disease in the community rather than in individual patients and includes the study of reservoirs and sources of human disease.• Studies the patterns of disease

occurrence in human populations and the factors that influence this pattern. The term obviously is related to epidemic (derived form the Greek word” upon the people” meaning leading the people)

EPIDEMIOLOGYA science concerned with the various

factors and conditions which influence the occurrence and distribution of health, disease, defect, disability and death among groups of individuals.

The present usage includes studies of all conditions and circumstances important to man’s health.Examples:Accidents, Suicides, Medical care and Diseases

Three Major Aims of Epidemiology:1. Construct or complete the natural history of 

disease so that adequate measures for diagnosis and treatment, and prevention can be formulated

2. To study immediate and special problems in the field of health. The study could provide the necessary data upon which program may be based; to guide the program, assessing its progress and pinpointing failures, etc.

3. To evaluate the effectiveness of therapy, preventive measures and programmes.

Epidemic- an increase in the frequency (incidence) of a disease above the usual and expected rate, which is called the endemic rate., thus epidemiology count cases of a disease, and when they detect the sign of epidemic, they ask who, when and where questions.

? Who is getting the disease? Where and when the disease is occurring? From this information, they can often make

informed guesses as to why it is occurring.

Notifiable disease- surveillance made by the government before many people start dying.

The timely reporting of cases of notifiable disease allows public health authorities to detect an emerging epidemic at an early stage.

A typical Epidemiologic Investigation- Outbreak of hepatitis

• It is caused by virus that contaminates food or water, it is important to identify the source of outbreak so that wider exposure to the virus can be prevented. 

• What are the possible questions to asked?• Who, where and when• Who: interviewed and asked question on 

what date did the first symptoms appear?(knowing the hepatitis A virus has an incubation period of 30 days, it is possible to work back on the estimated date of exposure)

• The where question is the hardest: where did the victim obtain their food and water during the period and what sources did they have in common?

• It may be that they had eaten at the same restaurant. The epidemiologist would visit the restaurant and might and investigate. They may found out that the chef had developed Hepatitis. And was hospitalized and so solution of the problem is easy.

What is the Goal of epidemiology?

The ultimate goal is to use this knowledge to control and prevent the spread of disease.

John Snow-father of modern epidemiology

• Study about cholera – Snow would not have been formulated his hypothesis without the data he gathered

Two Main Areas of Investigation1. Describes the distribution of health status in 

terms of age, gender, race, geography, and time.2. Patterns of disease distribution in terms of 

causal factor• In epidemiology of any disease or event, one 

studies the factor which contribute to its causation and behavior- AGENT, HOST, ENVIRONMENT

• Epidemiology concept maintains that there can be no single cause of disease

THE AGENT FACTORS OF DISEASE

Agent is any element, substance or force whether living or non-living thing; the presence or absence can initiate or perpetuate a disease process.

The Epidemiologic Lever  

AGENTS            HOST                                                                                                   

ENVIRONMENT

Types of Agent1. This could be living or non-living 

things, physical or mechanical in nature such as extremes of temperature, light electricity.

2. They could be chemicals- endogenous (within the body) or exogenous (poison)

Characteristics of Agent of disease1. Inherent characteristics- physical

feature, biological requirement, chemical composition, resistance

2. Characteristic in relation to the environment- refers to the reservoir and source of infection and modes of transmission.

3. Characteristic directly related to mana. Infectivity- ability to gain access and adapt

to the human host to the extent of finding of finding lodgement and multiplication

b. Pathogenicity- measures the ability of agent when lodged in the body set up a specific reaction

c. Virulence- refers to the severity of the reaction produce and is usually measured in terms of fatality.

d. Antigenicity- ability to stimulate the host to produce antibody

Modes of Transmission1. Direct transmission- immediate transfer of infectious

agent a receptive portal of entry2. Indirect transmission

a. Vehicle borne- contaminated inanimate objects or materials

b. Vector-borne- from other living organism (ex. Insects)c. Mechanical vectord. Biological vector

3. Airborne- dissemination of microbial aerosols to a suitable portal of entry usually the respiratory tracta. Droplet nuclei- usually small residues which result 

from evaporation of fluid from droplets emitted by an infected host

b. Dust

THE HOST FACTOR OF DISEASE1. Age2. Sex3. Race4. Habits, Customs and religions5. Exposure to agent

Defense mechanism of the hostHumoral defense- these are cells in our 

body like plasma cells and lymphocytes that produce antibodies to neutralize harmful effects of the infectious agents and body fluids in our body that possess substance that have antimicrobial properties

Cellular defense- there are cells in our body like macrophages and neutrophils involve in the process of phagocytocis

Immunity   This is the total property of an individual to 

protect himself from an infectious agent Two types of immunity1. Non specific resistance- present at the 

time of birth or has developed during maturation

2. Specific resistance- acquired as a result of prior exposure with a foreign substance

Two folds of specific resistance Active- what has been introduced to the individual is

the antigen and the body makes the antibody.Naturally acquired active immunity- when we get sick 

the infective agent will gain entry to the body, act as stimulant for antibody formation because the organism acts as antigen.  The immunity is lifelong (ex. Measles, chicken pox, hepatitis A)

Artificially acquired active immunity- when the antigen has been deliberately introduce like injecting vaccines, they act as antigen to stimulate antibody formation. It makes use of vaccine which is suspension of killed or living organism (ex. MMR,OPV,BCG)

Passive- when what has been introduced to the body is already antibodies that provide immediate protection against microorganisms.

Naturally acquired passive immunity- exhibited by the transfer of antibodies from mother’s placenta to the fetus and transfer of antibodies from breast milk to the baby.

Artificially acquired passive immunity- injection of artificially prepared substance like immune serum of gamma globulin. These two are antibodies preparation (ex. Anti-tetanus antibodies, diphtheria antitoxin)

THE ENVIRONMENTAL FACTORS OF DISEASE Environment- sum total of an organism’s

external surrounding conditions and influences that affect its life and development

• Physical Environment• Climate- certain disease have seasonal 

distribution• Geography and location• Biologic Environment- living environment of man 

consist of plants, animals and fellow human beings.

• Socio-economic environment

DISEASE CAUSATION• The occurrence of disease follows biologic

laws which apply to both communicable and non-communicable diseases.

• Disease results from imbalance between the forces of the agent and host

• The nature and extent of imbalance depends on the nature and characteristics of host and agent

• The characteristics of two are influenced considerably the condition of the environment.

Incubation Period• Time between exposure to infectious agent up to

the time of appearance of the earliest signs and symptoms

1. Clinical incubation period- the time between

exposure to a pathogenic organism and the onset of symptoms of a disease.

2. Biological Incubation Period- The time taken by the parasite to complete its development in the definite host (from the time of entry of the infective larvae to the presence of microfilariae) is called the Intrinsic incubation period (Biological incubation).

ISOLATION AND QUARANTINEISOLATION-As applied to patient, separation for the 

period of communicability, of infected persons or animals from others in such places and under such conditions as to prevent or limit the effect of the direct or indirect transmission of the infectious agent from those infected to those who are susceptible or who may spread the disease agent.

Categories of Isolation1.  Strict isolation- this category is designed 

to prevent transmission of highly contagious or virulent infectious that may spread by direct contact or droplet.

Includes:• Private room, masks, gowns and gloves 

for all persons entering the room• Special ventilation requirements with a 

room at negative pressure

2. Contact isolation- for less highly transmissible or serious infections, for disease or conditions which are spread primarily by close or direct contact.

Includes:• Private room may share a room with the 

same pathogen• Mask for those who come close to the 

patient• Gloves are indicate for touching infectious 

materials

3. Respiratory isolation- to prevent transmission of infectious diseases over short distance through the air

Includes:• Private room may share a room with the 

same organisms of disease• Mask for those who come close to the 

patient• Gloves and gowns are not indicated

4.  Tuberculosis isolation (AFB isolation)- for patient with pulmonary tuberculosis who have a positive sputum smear or chest x-rays which strongly suggest active tuberculosis

Includes:• Private room with special ventilation and the 

door closed• Mask only used only if patient is coughing• Gowns are used to prevent gross 

contamination of clothing• Gloves are not indicated

5. Enteric Precautions- for infectious transmitted by direct or indirect contact with feces purulent material or drainage from an infected body site

Includes:• Private room if the patient hygiene is 

poor• Gowns should be used if soiling is likely• Gloves are to be used for touching 

contaminated materials

6. Drainage/secretion Precautions- to prevent infections transmitted by direct or indirect contact with purulent material or drainage from an infected body site.

Includes:• Gowns should be used if soiling is likely• Gloves are to be used for touching 

contaminated materials

7. Blood/body fluid Precautions- to prevent infections that are transmitted  by direct or indirect contact with infected blood or body fluids.

Includes: • Private room if the patient hygiene is poor• Gloves are to be used for touching blood 

or body fluids• Gowns should be used if soiling of 

clothing with blood and body fluids is likely

QUARANTINE- restriction of the activities of a well 

persons or animals who have been exposed to a case of communicable diseases during its period of communicability to prevent disease transmission during incubation of infection should occur.

Categories of Quarantine1. Absolute or Complete Quarantine Limitation of movement of those exposed 

to a communicable disease for a period of time not longer than the longest usual incubation period of that disease.

2. Modified QuarantineSelective, partial limitation of freedom of 

movements of contacts

THE DIFFERENT EPIDEMIOLOGIC STUDIES

1.  Descriptive StudyIt is the description of various epidemiological features of a particular event. 

2. Analytical

• Ecological- this is also called Correlational study. The unit of observation is population or groups of people rather than individual• Cross-sectional- this measures the

prevalence of disease at a certain point in time, and relates it with the basis population characteristic being surveyed such as age, sex, ethnicity, socio-economic, etc.

• Case-Control- this attempt to show the influence of a risk factor in the causation of disease. The procedure starts with the selection of people with a particular disease, and the selection of people without the disease.

• Cohort- this approach is suitable in determining the influence of a particular risk factor in the causation of an event, such as a particular disease. It begins with the identification of a group exposed to a risk factor and a comparable group not exposed.

II. EXPERIMENTAL STUDY 1. Randomized Control Trial It is an epidemiological experimental study of a new 

preventative or therapeutic regimen. For this reason, it is commonly referred to as prophylactic trial, when testing the effectiveness of a drug to prevent a disease, it is referred to as prophylactic trail. Subject in a population are randomly allocated to groups, usually called treatment and control groups, and the results are assessed by comparing the outcome in the two or more groups.

2. Field trialsIt involves people who are disease –free but presumed to 

be a risk. Since the subjects are disease- free and the purpose is to prevent the occurrence of diseases that may occur with relatively low frequency, field trials are often huge undertakings involving major logistics and financial considerations.

 3.  Community trialsIn this form of experiment the treatment groups are the 

communities rather than individuals. This is particularly appropriate for diseases that have their origins in their social conditions, which in turns can most easily be influenced by intervention directed at group behavior as well as at individuals.

THE NATURAL HISTORY OF COMMUNICABLE DISEASE AND THE LEVELS OF DISEASE PREVENTION

• The natural history of diseases comprises the body of knowledge about the agent, host and environmental factors relating to the disease process. It includes the initial forces/factors which initiated the process in the environment or elsewhere through the resulting changes which took place in man, until continuing equilibrium is reached, or defect, disability, or death ensues.

Phases 1. Pre-pathogenesis- This is the phase before man is involved. Through 

the interaction of the agent, the host and environmental factors, the agent finally reaches man.

- It maybe said that everyone is in the period of pre-pathogenesis of many diseases because agents are present in the environment where man lives

 2. Pathogenesis- This phases includes the successful invasion and 

establishment of the agent in the hos

• After a period of incubation, whereby the agent multiplies and develops, or get absorbed and fix in the tissue, sufficient tissue or physiologic changes may have taken place to produce detectable evidence of the disease process in man. 

The Process of Infection There are six requirements for the successful invasion 

of the host by an infectious agent. 1. Condition in the environment must be favorable to 

the agent or the agent must be able to adapt in the environment

2. Suitable reservoirs must be present3. A susceptible host must be present4. Satisfactory portal of entry into the host5. Accessible portal of exit from the host6. Appropriate means of dissemination and 

transmission to a new host