Principle of epidemiology | CHAPTER 20 | Community Health Nursing

Principle of epidemiology – This book covers the entire syllabus of “Neuron Community Health Nursing” prescribed by the Universities of Bangladesh- for Basic and diploma nursing students. We tried to accommodate the latest information and topics.

This book is an examination-friendly setup according to the teachers’ lectures and examination questions. At the end of the book previous university questions are given. We hope in touch with the book students’ knowledge will be upgraded and flourish. The unique way of presentation may make your reading of the book a pleasurable experience.

Principle of epidemiology

(Infectious disease epidemiology)

Endemic:

En in; demos People It refers to the constant presence of a disease or infectious agent within a given geographic area or the population groups without importation from outside; may also refer to the “usual” of expected frequency of the disease within such area or population group.

Example

• Cholera is endemic in Bangladesh.
• Malaria in hill tracts, and goiter at Rangpur.

[Ref: K Park/26/1011]

Sporadic:

When a disease occurs in an inchoate fashion in different areas of a country without having a common source of causative agent. When a disease occur irregularly, haphazardly from time to time, and generally infrequently without having a common source of infection is known as sporadic. Example: Polio, tetanus, herpes-zoster and meningococcal meningitis.

[Ref: K Park/26/1011]

Pandemic:

An epidemic usually effecting a large proportion of the population, occurring over a wide geographic area such a section of nation, the entire nation, a continent or the world.

Example

• Influenza pandemics of 1918 & 1957
• Cholera Eltor in 1962 (still continuing) and acute haemorrhagic conjunctivitis in 1971 & 1981.

[Ref: K Park/26/101]

Definition of Epidemic:

An epidemic is the rapid spread of infectious disease to a large number of people in a given population within a short period of time, usually two weeks or less. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.

[Ref: K Park/26/101]

Verities of endemic:

a. Hyperendemic: The term “hyperendemic” expresses that the disease is constantly present at a high incidence and/or prevalence rate and affects all the age groups equally.

b. Holoendemic: The term “holoendemic” means a high level of infection beginning early in the life & affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children, as in the case of malaria.

[Ref: K Park/26/101]

Differences between sporadic, endemic, epidemic and pandemic:

[Ref: K Park/26/1011]

Infection:

The entry and development or multiplication of an infectious agent in the body of man or animals. It also implies that body responds in some way to defend itself against invader either in the form of immune response or disease. An infection does not always cause illness.

Or,

Infection is the invasion of an organism’s body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

[Ref: K Park/26/100]

Types/levels of infections:

1. Colonization: Normally present in the body & lodgment e.g. S. aureus in skin & normal nasopharynx.

2. Sub clinical infection or in apparent infection No sign & symptoms e. g. Polio

3. Latent infection:e.g. virus of herpes simplex.

4. Manifest or clinical infection: Clinical sign & symptoms present

[Ref: K Park/26/100]

Infection chain:

Communicable diseases are transmitted from the reservoir or source of infection to susceptible host which may he designed as follows:

This is infection chain. Basically, there are three links in the chain of transmission-

i. Source of infection or reservoir

ii. Mode of transmission

iii. Susceptible host

[Ref: K Park/26/1011]

Define reservoir

Source:

The person, animal, object or substance from which an infectious agent passes or disseminated to the host.

Reservoir:

It is defined as any person, animal, arthropod, plant, soil or substance in which an infectious agent lives & multiplies in which it depends primarily for survival & where it reproduces itself in such a manner that it can be transmitted to a susceptible host.

Example:

i. Hook worm: Source – soil, Reservoir-Man

ii. Tetanus: Source -soil, Reservoir – soil

iii. Typhoid fever: Source faces. Urine or contaminated food, Reservoir case or carrier.

[Ref: K Park/26/101-21]

Different types of reservoir

Types of reservoir: There are three types of reservoir.

i. Human reservoir: cases or carriers.

ii. Animal reservoir: cases or carriers of zoonotic diseases.

iii. Reservoir in non-living things: soil may harbour agents that causes tetanus, anthrax.

Another types:

i. Homologous reservoir: The agent infects the same type of host i.e.Reservoir& victims are same species e.g. Man to man-cholera.

ii. Hetemlogous reservoir: The agent infects different types of hosts. e.g. Dog to man Rabies.

Ref: K Park/26/101-21

Definition of Case:

A case is defined as “a person in the population or study group identified as having the particular disease, health disorder or condition under investigation”

(Ref: K Park/26/73, 103)

Classify/types of cases

Types of cases:

i. Clinical

• Mild or moderate – More spread as the pt. does not confined to bed.
• Typical or atypical:
• Serve case- Less spread, bad for pt. but good for community

ii. Sub-clinical:

• Agent enters the body, multiply & spread but no sign & symptoms e.g. Rubella, Mumps, Hepatitis, A.B. Influenza
• But measles: No sub-clinical infection.

iii. Latent infection:

• The host does hot shed the infectious agent and lies dormant within the host without sign & symptoms e.g. Herpes simplex.

Other types (Epidemiological terminology):

i. Primary case: The case that develops into the population unit being studied

ii. Secondary case: All cases that develops from primary case within the incubation period.

iii. Index case: The case which is identified or come to attention at first by investigator is called index case.

(Ref: K Park/26/73, 103)

Type of reservoir is most important epidemiologically

Human reservoir is the most important because man can act as a case or carrier.

[Ref: K Park/26/102/]

Sub clinical infections are more dangerous than clinical case because:

• In sub clinical cases the disease agent multiply in the host but does not manifest any sign symptoms
• Sub clinical cases play a dominant role in maintaining the chain of infection (Endemicity) in the community
• Sub clinical infection only detected by laboratory test e.g. antibody response, biochemical and skin sensitivity test

[Ref: K Park/26/102]

Differentiate between source and reservoir.

Source:

The person, animal, object or substance from which an infectious agent passes or disseminated to the host.

Reservoir:

It is defined as Any person” animal, arthropod, plant, soil or substance in which an infectious agent lives & multiplies in which it depends primarily for survival & where it reproduces itself in a such a manner that it can be transmitted to a susceptible host.

[Ref: K Park/26/102]

Define carrier

It is defined as infected person or animal that harbours a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for other.

Example: Typhoid Mary” is a classic example of a carrier.

(Ref: K Park/26/103)

Criteria/Elements of carrier state:

a. The presence in the body of the disease agent

b. The absence of recognizable symptoms and signs of disease

c. The shedding of the disease agent in the discharges or excretions.

[Ref: K Park/26/103/]

Types/classification of carrier:

A. According to time (type):

i. Incubatory carrier: Agent are shedding during incubation period of disease but pt. has not been suffered e.g. Measles, diphtheria. Mumps, Hepatitis-B, Polio, pertussis. Influenza.

ii. Convalescent carrier: Agents are shedding during the convalescent period of disease. Patient has been suffered. e.g. Typhoid fever. Dysentery, cholera, Diphtheria, whooping cough.

iii. Healthy carrier: Disease spread by apparently healthy person due to sub-clinical infection: e.g. Polio, Cholera, Meningococcal, meningitis, typhoid fever, and diphtheria.

B. According to duration:

i. Temporary: Agents are shed for short period. e.g. Incubatory carrier: Convalescents carrier, Healthy carrier.

ii. Chronic: Agents are shed for indefinite periods. e.g. Typhoid fever, Hepatitis-B, Dysentery, Cerebro-spinal meningitis, Gonorrhoea.

C. According to portal of exit:

i. Urinary carrier

ii. Intestinal carrier

iii. Respiratory carrier

iv. Nasal carrier

v. Skin eruption

vi. Blood

vii. Open wound.

[Ref: K Park/26/103]

Importance of carrier:

Carrier are less infectious than cases but epidemiologically they are more dangerous than cases as they escape recognition & continuing as they do to live a normal life among the population or community. They may infect the susceptible individuals over a wid rar. and anger period of time. E.g., Typhoid Mary is classical example of carrier.

[Ref: K Par /26/103]

Infection develop into the body:

1. First, the infectious agent must find a portal of entry by which it may enter into the host. There are many portal of entry, e.g. respiratory tract, alimentary tract, genitourinary tract,
skin etc.

2. On gaining entry into the host, the organism must reach the appropriate tissue or site of election in the body where it may find optimum conditions for its multiplication and survival.

3. Thirdly, the disease agent must find a way out of the body (Portal of exit) in order that it may reach a new host and propagate its species. d. After leaving the human body, the organism must survive in the external environment for sufficient period till a new host is found.

[Ref: K Park/26/103]

Definition of INCUBATION PERIOD:

The period between the infection of an individual by a pathogen and the manifestation of the illness or disease it causes.

Or,

Incubation period is the time interval between the entry of disease agent and the onset of clinical manifestation of the disease.

• Short: Few hours to few days
• Long: Months to years
• Intermediate: One week to 3 weeks

Factors upon which incubation period depends:

1. Generation time of particular pt.
2. Infective dose
3. Route of entry
4. Host susceptibility

Importance of IP:

i. Tracing the source of infection & contacts.

ii. Immunization: A knowledge of IP helps us to prevent clinical illness by human 1g & antisera

iii. Measurement of period of quarantine (surveillance)

iv. Identification of point source or propagated epidemics –

• Point source epidemics – same IP
• propagated epidemics – different IP To see the prognosis of disease e.g.
• Rabies-long IP.
• Tetanus short IP.

[Ref: K Park/26/103]

Types of incubation period:

i. Minimum IP

ii. Median IP – Period required to develop 50% cases after exposure

iii. Estimation of average incubation period

(Ref: K Park/26/103)

Difference between extrinsic and intrinsic incubation period:

[Ref: K Park/26/103]

Isolation & Quarantine

Definition of isolation:

It is defined as “separation for the period incommunicability of infected persons or animals from others in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible or who may spread the agent to others.

[Ref: K Park/26/132]

Purpose of isolation:

a) To protect the community by preventing transfer of infection from the reservoir to the possible susceptible hosts.

b) Isolation has a distinctive value in the control of some infectious diseases. e.g., diphtheria, cholera.

[Ref: K Park/26/132]

Types of isolation

Types of isolation:

1. Standard isolation

2. Strict isolation

3. Protective isolation

4. High security isolation.

[Ref: K Park/26/132]

Define Ring Isolation and Chemical isolation.

Ring Isolation:

The infected person is surrounded by immunized persons. So infection cannot spread to susceptible hosts. E.g. Small pox, Measles.

Chemical isolation:

Rapid treatment of cases in their own homes and thus rendering then non-infectious. Physical + Chemical isolation – TB. Leprosy and STD Isolation has a distinctive value in control some infectious diseases – e.g. Diphtheria, Cholera, etc.

Isolation is useless in some disease- e.g. Polio. Hepatitis A, Typhoid fever. Mumps, Leprosy. Tuberculosis. STD. etc.

[Ref: K Park/26/132]

Define Quarantine.

It has been defined as, the limitation of freedom of movement of such well persons or domestic animals exposed to communicable disease for a period of time not longer than the longest usual incubation – period of the disease, in such manner as to prevent effective contact with those not so exposed.

[Ref: K Park/26/132]

The objective, period, and types of quarantine.

Objective:

To prevent transmission of disease across national boundaries. Period: The longest period for that specific disease.

Types:

1. Absolute quarantine

2. Modified quarantine

3. Segregation. -e.g. Removal of susceptible children to homes of immune persons.

[Ref: K Park/26/132]

Application quarantine

Application:

➤ Ship

➤ aircraft

➤ train.

➤ school etc

[Ref: K Park/26/132]

International quarantinable disease:

➤ Cholera (upto 5 days)

➤ Plague (upto 7 days)

➤ Yellow fever (IP- upto 6 days)

NB: Isolation is the oldest communicable disease control measure. Definition of quarantine and absolute quarantine is same.

[Ref: K Park/26/132]

Distinguish between isolation and quarantine:

[Ref: K Park/26/132]

Define disease notification.

Disease notification:

A notifiable disease is any disease required by law to be notified to the government or other health authority. Diseases to be notified to WHO are outlined in the International Health Regulations but most countries have their own list of nationally notifiable diseases.

156 Making a disease legally ‘notifiable’ by doctors and health professionals allows for intervention to control the spread of highly infectious diseases such as influenza, poliomyelitis or yellow fever. In less infectious conditions it improves information about the burden and distribution of disease.

Disease that can be notified in Primary schools and child care centres: Primary School Principals and Directors of Child Care Centres should notify their local public health unit (PHU) by phone (call 1300 066 055) as soon as possible after they are made aware that a child enrolled at the school or facility is suffering from one of the following vaccine preventable diseases:

• Diphtheria
• Mumps
• Poliomyelitis
• Haemophilus influenzae Type b (Hib)
• Meningococcal disease
• Rubella (“German measles”)
• Measles
• Pertussis (“whooping cough”)
• Tetanus

[Ref: K Park/26/132]

Case fatality rate:

In epidemiology, a case fatality rate (CFR) or case fatality risk, case fatality ratio or just fatality rate is the proportion of deaths within a designated population of “cases” (people with a medical condition) over the course of the disease.

Or

Case fatality rate, also called case fatality ratio, in epidemiology, the proportion of people who die from a specified disease among all individuals diagnosed with the disease over a certain period of time. Case fatality rate typically is used as a measure of disease severity and is often used for prognosis (predicting disease course or outcome), where comparatively high rates are indicative of relatively poor outcomes.

Case fatality rate is calculated by dividing the number of deaths from a specified disease over a defined period of time by the number of individuals diagnosed with the disease during that time; the resulting ratio is then multiplied by 100 to yield a percentage.

This calculation differs from that used for mortality rate, another measure of death for a given population. Although number of deaths serves as the numerator for both measures, mortality rate is calculated by dividing the number of deaths by the population at risk during a certain time frame. As a true rate, it estimates the risk of dying of a certain disease. Hence, the two measures provide different information.

As an example, consider two populations. One population consists of 1,000 people; 300 of these people have the specified disease, 100 of whom die from the disease. In this case, the mortality rate for the disease is 100 1,000 0.1, or 10 percent. The case fatality rate is 100+300=0.33, or 33 percent.

The second population also has 1,000 people; 50 people have the disease and 40 die from it. Here the mortality rate is 40+ 1,000 0.04, or 4 percent; the case fatality rate, however, is 40 + 50 0.8, or 80 percent. The incidence of death from the disease is higher in the first population, but the severity of disease is greater in the second.

[Ref: K Park/26/25,66]

Definition of Zoonosis:

A disease that can be transmitted from animals to people or, more specifically, a disease that normally exists in animals but that can infect humans. There are multitudes of zoonotic diseases.
Some examples include:

• anthrax,
• ascariasis,
• brucellosis,
• plague,
• echinococcosis,
• Lassa fever,
• listeriosis,
• Lyme disease
• monkeypox,
• psittacosis,
• rabies,
• salmonellosis,
• trichinosis,
• toxoplasmosis,
• Typhus and, West Nile fever.

Zoonoses may appear suddenly and be relatively virulent, as illustrated by HIV which ignited the AIDS epidemic and the coronavirus responsible.

(Ref: K Park/26/101,316,882)

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