TABLE OF CONTENTS

TITLE PAGES

INTRODUCTION 2 - 4

DESCRIPTION OF DISEASES 5 - 10

RELATION OF THE MADEL TO THE

DISEASES 11 - 16

CONCLUSION 17

REFERNECES 18 - 19

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INTRODUCTION

THE EPIDEMILOGICAL TRAID MODEL

This is a traditional model of infectious disease causation. The triad consists of an

external agent; a host and an environment in which host and agent are brought together,

causing the disease to occur in the host. A vector, an organism which transmits infection by

conveying the pathogen from one host to another without causing disease itself, may be part of

the infectious process.

The triad model consists of three corners called vertices:

Agent or microbe that cause the disease ( the what of the triangle)

Host or organism harbouring the disease ( the who of the triangle)

Environment those external factors that cause or allow disease transmission ( the where

of the triangle)

Illustrated below is a diagram showing their interrelations:

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The Agent—“What”

The agent is the cause of the disease. When studying the epidemiology of most infectious

diseases, the agent is a microbe—an organism too small to be seen with the naked eye. Disease-

causing microbes are bacteria, virus, fungi, and protozoa (a type of parasite). They are what most

people call “germs.”

Bacteria: Bacteria are single-celled organisms. Bacteria have the tools to reproduce themselves,

by themselves. They are larger than viruses (but still much too small to be seen with the naked

eye). They are filled with fluid and may have threadlike structures to move themselves, like a

tail.

Virus: A virus may have a spiny outside layer, called the envelope. Viruses have a core of

genetic material, but no way to reproduce it on their own. Viruses infect cells and take over their

reproductive machinery to reproduce.

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Fungi: Fungi are like plants made up of many cells. They are not called plants because they

cannot produce their own food from soil and water. Instead, they live off animals, including

people, and plants. Mushrooms and yeast are fungi.

Protozoa: Protozoa are very small. Most live in water. They are parasites, which mean they live

off other organisms, in some cases humans. Malaria is a parasitic protozoan, as is Guardia.

The Host—“Who”

Hosts are organisms, usually humans or animals, which are exposed to and harbour a disease.

The host can be the organism that gets sick, as well as any animal carrier (including insects and

worms) that may or may not get sick. Although the host may or may not know it has the disease

or have any outward signs of illness, the disease does take lodging from the host. The “host”

heading also includes symptoms of the disease. Different people may have different reactions to

the same agent.

The Environment—“Where ”

The environment is the favourable surroundings and conditions external to the host that cause or

allow the disease to be transmitted. Some diseases live best in dirty water. Others survive in

human blood. Still others, like E. coli, thrive in warm temperatures but are killed by high heat.

Other environment factors include the season of the year

TimeIn the center of the Triangle is time. Most infectious diseases have an incubation period—the

time between when the host is infected and when disease symptoms occur. Or, time may

describe the duration of the illness or the amount of time a person can be Sick before death or

recovery occurs. Time also describes the period from an infection to the threshold of an epidemic

for a population.

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An outbreak or an epidemic exists when there are more cases of a particular disease than

expected in a given area, or among a specific group of people, over a particular period of time.

Another other term you might come across is endemic, when a population has a high level of the

disease all the time. For example, giardiasis and even malaria are endemic in parts of the world.

DECRIPTION OF DISEASES CHOSEN

PERTUSSIS ( INFECTIOUS)

INFECTIOUS DISEASE

An infectious disease is caused by a pathogen, such as a virus, bacteria or a fungus. This type of

disease can be spread from one person to another or from one species to another. The infection

can be spread through via air, bodily fluids, food, drink or by touch.

PERTUSSIS

Pertussis more commonly known as whooping cough is an infection of the upper respiratory tract

which is caused by the bacterium Bordetella pertussis, which lives in the mouth, nose and throat.

It is a highly contagious thus, making it an infectious bacterial disease that causes uncontrollable,

violent coughing. The coughing can make it hard to breathe. A deep “whooping” sound is often

heard when the patient tries to take a breather.

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How is pertussis spread?

When an infected person sneezes or coughs tiny droplets containing the bacteria move through

the air and the disease is easily spread from person to person. A person may even catch pertussis

by standing close (less than 3 feet away) to an infected person who is coughing or sneezing.

Infection usually lasts six weeks. An infected person is contagious from just before onset of

symptoms until up to three weeks after symptoms start. Treatment with appropriate antibiotics

shortens the contagious period to about five days.

Who is susceptible to contracting pertussis?

Despite the effectiveness of vaccination, pertussis continues to occur among all age groups.

Anyone who has not had pertussis previously or who has not received the pertussis vaccine can

get the disease. Immunity following disease or vaccination is not lifelong. Older children,

adolescents and adults can become susceptible to pertussis five-to 10-years after their last dose of

pertussis-containing vaccine. Older children and adults can carry the germ and spread it even

though their cold-like symptoms may be so mild they might not seek medical care.

Symptoms of Pertussis

Symptoms usually appear five-to 10-days after exposure, but can take as long as 21 days. The

first symptoms are similar to those of a common cold - a runny nose, sneezing, low-grade fever

and a mild, occasional cough. The cough gradually becomes severe and, after one to two weeks,

the patient has spasmodic bursts of numerous, rapid coughs. The characteristic high-pitched

"whoop," which is more common in children, comes from breathing in after a coughing episode.

During such an attack, the patient may turn blue, vomit and become exhausted. Between

coughing attacks, the patient usually appears normal.

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Coughing attacks occur more frequently at night. The attacks increase in frequency for a couple

of weeks, remain at the same level for two-to three-weeks, and then gradually decrease.

Coughing may last as long as 100 days. Cough medicines usually do not help eliminate this

cough. Recovery is gradual, but coughing episodes can recur for months after the onset of

pertussis.

How is pertussis treated?

Pertussis is usually treated with a multi-day course of appropriate antibiotics, such as

azythromycin, erythromycin or clarithromycin, or an acceptable alternative. Some children may

need to be hospitalized. People in close contact with children or adults with pertussis usually

need to be treated with antibiotics and efforts should be taken to minimize an infant’s exposure

to children and adults with cough illnesses.

Atherosclerosis ( NON – INFECTIOUS)

What is Atherosclerosis

Also known as hardening of the arteries, atherosclerosis is a condition that results from the slow

build up of plaque on the inside walls of the arteries. As the plaque builds up, it can cause serious

health problems, such as heart attack, angina, and sudden death. A person's particular symptoms

will depend on which part of the body is not receiving enough blood and oxygen due to the

narrowing of arteries. These symptoms take some time to develop, as the disease must progress

to the point where an artery is severely narrowed or completely blocked.

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Risk Factors That Can Lead to Atherosclerosis

There are certain risk factors that contribute to the development of atherosclerosis. These risk

factors are commonly categorized as the following:

Being male

If female, being past menopause

High blood pressure

High LDL ("bad") cholesterol or triglycerides (fats in the blood)

Diabetes

Being overweight

Smoking

A family history of heart disease

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Elevated homocysteine levels

Sedentary lifestyle

Diets high in saturated fat and trans fatty acids (trans fats)

Depression

Obstructive sleep apnoea

Signs and Symptoms

chest pain or chest discomfort (angina)

Pain in one or both arms, the left shoulder, neck, jaw, or back

Shortness of breath

Dizziness

Faster heartbeats

Nausea (feeling sick to your stomach)

Abnormal heartbeats

Feeling very tired.

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Effects of Atherosclerosis

The effects of atherosclerosis differ depending upon which arteries in the body narrow

and become clogged with plaque. If the arteries that bring oxygen-rich blood to your

heart are affected, you may have coronary artery disease, chest pain, or a heart attack. If

the arteries to your brain are affected, you may have a transient ischemic attack (TIA) or

a stroke. If the arteries in your arms or legs are affected, you may develop peripheral

artery disease. You may also develop a bulge in the artery wall (aneurysm).

Causes

The formation of plaque referred to as Atherosclerosis may be caused by cholesterol fats,

nicotine, high blood pressure or abnormal sugar content in the blood. You become more

susceptible to the disease as you grow old or if the disease runs in your family. Obesity and

emotional stress also invite the disease.

 

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RELATION OF THE EPIDEMIOLOGICAL MODEL TO

THE DISEASES CHOSEN

Pertussis

Pertussis is a highly contagious bacterial disease that causes uncontrollable, violent

coughing. The coughing can make it hard to breathe.  A deep "whooping" sound is often

heard when the patient tries to take a breath. Pertussis, or whooping cough, is an upper

respiratory infection caused by the Bordetella pertussis or Bordetella parapertussis bacteria. It is

a serious disease that can cause permanent disability in infants, and even death.

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Agent

Bordetella pertussis is the causative agent of the disease pertussis (Whooping

cough).   Bordetella pertussis is small gram-negative coccobacillus that is non-motile and

pathogenic only to humans (Hoppe 1999).  The bacteria are observed in culture as single or

paired entities which grow slowly and fastidiously (Todar 2004).

Pertussis has a number of different stages.  The first stage is the catarrhal or colonization phase. 

Here, symptoms resemble a cold or an upper respiratory infection, but remain relatively

nonspecific.  Over the first ten days of infection, coughing, fever, and malaise may be observed

(Todar 2004).  The toxemic, or paroxysmal, stage is characterized by severe, prolonged

coughing.  This coughing is accompanied by the aspiratory “whoop” at this stage (Todar 2004,

Wood and Friedman 1998).  The final, convalescent phase is marked by reduction in attacks of

severe coughing over three to four weeks (Wood and Friedman 1998).

Bordetella pertussis can only live independently inside a human host.  It infects the upper airway

(the throat and nose), as well as the lungs (Jenkinson 2005).  Thus, the life cycle of the bacteria

must occur within these structures.   the bacteria depend upon a high rate of transmission to

continue survival.  Transmission of the bacteria is via airborne droplets produced by coughing

(Bjornstad and Harvill 2005). 

Like other bacteria, B. pertussis can divide by binary fission.  The growth of a population of

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these bacteria is slow in culture. Many of the disease pathologies linked to B. pertussis are a

result of its many secreted toxins and adherence factors.

 Tozzi et al. 2005

The original organisms die, are killed by an immune response or an antibiotic, or are transmitted

to another human, where they will colonize the airways, produce toxin and adhesions, and

transmit again. 

Host

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The bacteria that are shed from the discharges from the nose and throat are spread to others

through coughing and sneezing. The bacteria enter the air and can be inhaled by people nearby.

An infected person is contagious from just before onset of symptoms until up to three weeks

after symptoms start. Whooping cough spreads easily through families, childcare centers and at

school. 

Adults are the main transmitter of these bacteria because the symptoms of the illness are usually

milder than in children. The characteristic whoop that occurs after paroxysmal bouts of coughing

is recognized in only 20%-40% of adults with whooping cough thus making it easy for an

infected adult to transmit the bacteria to a child Infected adults are a reservoir (source) of

infection for children.

Environment

Any community is a potential place for Pertussis to spread quickly. Because it survives in the

human body it can survive anywhere there is humans. Family members are the number 1

transmitter of these bacteria. Child care centers and health facilities are the other transmitters of

this disease.

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Arteriosclerosis

Non-infectious diseases to which the disease atherosclerosis belongs are usually caused by

multiple agents who are nonliving. Non infectious disease usually develops over time and the

end result is often a chronic disease for which you can develop no immunity.

Atherosclerosis, as described earlier, is a non-infectious disease resulting from nutritional and

lifestyle choices. When being classified according to epidemiology, atherosclerosis is termed the

agent. The agent is an element that under a particular condition can initiate a health problem and

can be physical, chemical, psychological, and biological or nutrient based. In the case of

atherosclerosis, the agent, the disease is perpetuated by an excess of fats in the diet and lack of

physical activity. Atherosclerosis is fuelled when a person consumes foods that are high in

saturated fats, which in turn causes a build-up of plaque in the arteries. “…The host is a living

organism capable of being infected or affected by an agent. The host reaction is influenced by

family history, age, and health habits…” (Taylor, Lillis, LeMone, Lynn, 69). Atherosclerosis is

mainly based on health habits even though genetics may have some part to play in the acquisition

of the disease. Genetically an individual may be susceptible to the disease but if that individual

monitors their eating habits, the incidence of the disease can be curbed. For example, Shanice’s

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mother and grandmother both developed atherosclerosis and died from complications associated

with the disease. Shanice, fearful that she may suffer the same fate, monitored her diet eating less

fatty and fried foods and more of her raw fruits and vegetables. Along with this healthy diet she

also exercised reducing the likelihood that she becomes affected by atherosclerosis. The

environment includes all the factors that are external to the host and make the illness more or less

likely to occur (Taylor, Lillis, LeMone, Lynn, 69). Social factors heavily influence the risk of

developing atherosclerosis. In today’s world where people tend to socialize with their peers,

individuals involve themselves in dining out a lot. Dining out often ends up with a person

making an unhealthy food choice. Another influencing social factor is having a job to which you

must arrive at a particular time. Struggling to get to work on time, individuals tend to reach for

foods that are high in fat. For example, a typical on-the-go breakfast for an American would be a

bacon, egg and cheese bagel with coffee. Here in Trinidad, you see the early morning vendors

out with their doubles, potato pies and whatever else they can stuff into the deep fried dough and

a long line of hungry patrons waiting to sink their teeth into greasy heaven. Agent, host and

Environment all interact together to affect a person with atherosclerosis.

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Conclusion

With the growth of vast populations of people across many continents, identifying causal factors

leading to illness and disease within target groups has become a difficult undertaking. The

accuracy and quality of research findings are only as good as the methods used. The cause-effect

model of study used uncovers risk factors and probable points of origin; however, final

conclusions can only be based on what is inferred from the data.

Diseases shown to have multiple risk factors are more difficult to track and assess within a target

population, especially if no cause for the disease is known. Seemingly insignificant risk factors

that show a low incidence of occurrence may play a larger part in the cause-effect process than

what can be observed from the data. As modern day illnesses and diseases become more

contagious and resistant to cures, the methods of epidemiology and its use within the public

sectors will need to accommodate a more global approach in its work.

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References

Braman SS. Postinfectious cough: ACCP evidence-based clinical practice

guidelines. Chest. 2006; 129(1):138S-146S.

US Food and Drug Administration. First Combination Vaccine Approved to Help

Protect Adolescents Against Whooping Cough. Rockville, MD: National Press Office;

May 3, 2005. Talk Paper T05-17.

Cohn AC, et al. Immunizations in the United States: a rite of passage.Pediatr Clin

North Am.2005; 52(3):669-693.

Module05-Epi-Part2. (n.d.). https://njlmn.rutgers.edu/cdr/docs/. Retrieved April 21,

2012, from https://njlmn.rutgers.edu/cdr/docs/Module05-Epi-Part2.pdf

What is epidemiological triad? | ChaCha. (n.d.). Questions & Answers | ChaCha.

Retrieved April 21, 2012, from http://www.chacha.com/question/what-is-

epidemiological-triadWhat is epidemiology. (n.d.). The Q&A wiki. Retrieved April 21, 2012, from

http://wiki.answers.com/Q/What_is_an_epidemiology_.

Suzanne C. Smeltzer and others, Brunner & Suddarth’s Textbook of medical surgical

nursing 11th edition. Lippincott Williams and Wilkins, Philadelphia, 2008.

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Arteriosclerosis and Atherosclerosis – General Overview! retrieved on 20/4/12. http://no-

more-heart-disease.com/arteriosclerosis-and-atherosclerosis-%E2%80%93-general-

overview

Signs and symptoms of Arteriosclerosis. Retrieved on 20/04/12.

http://arteriosclerosis.org/

http://no-more-heart-disease.com/arteriosclerosis-and-atherosclerosis-%E2%80%93- general-overview/

Taylor, C., Lillis, C., LeMone, P., Lynn, P. (2005). Fundamentals of Nursing: The Art and Science of Nursing Care.6e, 69.

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