theories of disease causation level200

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THEORIES OF DISEASE CAUSATION

Disease is a dynamic process and it is just opposite to the health. Health

denotes perfect harmony and normal functioning of all the body system or

state of complete wellness whereas disease denotes disharmony and

deviation from normal functioning of various body functioning systems. Thefollowing are some of the theories that were believe to be the cause of disease.

Germ Theory of Disease Causation,

Epidemiological Triad of Disease Causation,

Multifactorial Theory of Disease Causation,

GERM THEORY

Germ theory states that many diseases are caused by the presence and

actions of specificmicro-organismswithin the body. The theory was

developed and gained gradual acceptance in Europe and the UnitedStates from the middle 1800s. It eventually superseded

existingmiasmaandcontagiontheories of disease and in so doing

radically changed the practice of medicine. It remains a guiding theory

that underlies contemporary biomedicine.

Awareness of the physical existence of germs preceded the theory by

more than two centuries. Discoveries made by several individuals also

pointed the way to germ theory. On constructing his first simple

microscope in 1677,Antoni van Leeuwenhoekwas surprised to see tiny

organisms - which he called animalcules - in the droplets of water hewas examining. He made no connection with disease, and although later

scientists observed germs in the blood of people suffering from disease,

they suggested that the germs were an effect of the disease, rather than

the cause. This fitted with the then popular theory of spontaneous

generation.

The observations and actions ofIgnaz Semmelweis,Joseph

ListerandJohn Snowwould retrospectively be acknowledged as

contributing to the acceptance of germ theory. But it was the laboratory

researches ofLouis Pasteurin the 1860s and thenRobert Kochin the

following decades that provided the scientific proof for germ theory.

Their work opened the door to research into the identification of disease-

causing germs and potential life-saving treatments.

EPIDERMIOLOGIC TRAID OF DISEASE CAUSATION

A traditional model of infectious disease causation, known as the Epidemiologic

Triad is depicted in Figure 2. The triad consists of an external agent, a host andan environment in which host and agent are brought together, causing the
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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.

A classic example of a vector is the Anopheles mosquito. As the mosquitoingests blood from an infected host, it picks up the parasite plasmodium. The

plasmodium are harmless to the mosquito. However, after being stored in the

salivary glands and then injected into the next human upon which the mosquito

feeds, the plasmodium can cause malaria in the infected human. Thus,the Anopheles mosquito serves as a vector for malaria. Another familiar

example of a vector are ticks of the genus Ixodes which can be vectors forLyme disease.

In the traditional epidemiologic triad model, transmission occurs when the agent

leaves its reservoir orhost through a portal of exit, is conveyed by a mode oftransmission to enter through an appropriate portal of entry to infect

a susceptible host. Transmission may be direct (direct contact host-to-host,

droplet spread from one host to another) orindirect (the transfer of aninfectious agent from a reservoir to a susceptible host by suspended air

particles, inanimate objects (vehicles or fomites), or animate intermediaries(vectors).

Figure 2: Epidemiologic Triad of Disease Causation (Historical)

Can the epidemiologic triad can be applied to a disease that notinfectious? Consider a smoking-related disease (Figure 3). If smoking (or more

specifically, a carcinogen in the smoke of the cigarette) causes the disease,

those who manufacture, sell and distribute cigarettes are vectors, bringing the

disease-causing agent to the susceptible host. Diagramming the epidemiologic

triad also indicates potential interventions to reduce disease in the population. In

this example, clean indoor air legislation, advertising potential harm fromsmoking or establishing workplace smoking cessation programs could change

the environment and reduce the exposure of host to agent. Conversely,

increased advertising from cigarette manufacturers or increased numbers ofvendors would increase exposure of host to agent.


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Fig. 3: Epidemiologic Triad Applied to Smoking-related Disease

Thus, the traditional model of disease transmission can be useful to identify

areas of potential intervention to reduce disease prevalence, whether infectiousor non-infectious.

MULTIFACTORIAL CAUSATION THEORY

Epidemiological theory is not applicable for non infectious and chronicdiseases like coronary artery diseases etc. because it has many causes

or multiple factors. This theory helps to understand the variousassociated causative factors, prioritise and plan preventive and planmeasures to control the disease.

BODY ADAPTIVE MECHANISM

The immune system functions as the bodys defense mechanism

against invasion. The term immunity refers to thebodys specific

protective response to an invading foreign agent or organism. Immunefunction is affected by age and by a variety of other factors,

such as central nervous system function, emotional status, medications,

the stress of illness, trauma, and surgery. Dysfunctions involving

the immune system occur across the life span. Many are

genetically based; others are acquired. The term immunopathologyrefers to the study of diseases resulting from dysfunctions

within the immune system. Disorders of the immune system may

stem from excesses or deficiencies of immunocompetent cells,

alterations in the function of these cells, immunologic attack on

self-antigens, or inappropriate or exaggerated responses to specific


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antigens.

To gain insight into immunopathology and the growing number

of immunologic-based disorders and to assess and care for

people with immunologic disorders, the nurse needs a sound

knowledge base of the immune system and how it functions

Acquired (adaptive) immunityimmunologic responses acquired

during life but not present at birthusually develops as a result of

prior exposure to an antigen through immunization (vaccination)

or by contracting a disease, both of which generate a protective

immune response. Weeks or months after exposure to the disease

or vaccine, the body produces an immune response that is sufficient

to defend against the disease upon re-exposure to it.

The two types of acquired immunity are known as active andpassive. In active acquired immunity, the immunologic defences

are developed by the persons own body. This immunity generallylasts many years or even a lifetime.

Passive acquired immunity is temporary immunity transmitted

from another source that has developed immunity through

previous disease or immunization. For example, immune globulin

and antiserum, obtained from the blood plasma of people withacquired immunity, are used in emergencies to provide immunity

to diseases when the risk for contracting a specific disease isgreat and there is not enough time for a person to develop adequate

active immunity. For example, immune globulin may be

administered to those exposed to hepatitis. Immunity resulting

from the transfer of antibodies from the mother to an infant inutero or through breastfeeding is another example of passive immunity.

Active and passive acquired immunity involve humoral

and cellular (cell-mediated) immunologic responses

REFERENCES

Books

Abbas, A. K., & Lichtman, A. H. (2001). Basic immunology: Functions

and disorders of the immune system. Philadelphia: W. B. Saunders

Guyton & Hall Textbook Of Medical Physiology 11th_Edition2

Donna, D. Ignatavicius . Medical Surgical Nursing


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RESOURCES AND WEBSITES

Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta,

GA 30333; (404) 639-3311, (800) 311-3435; http://www.cdc.gov.

National Institute of Allergy and Infectious Disease, Office of Communication

and Public Liaison, Building 31, Room 7A-50, 31 Center Drive

MSC 2520, Bethesda, MD, 20892-2520; http://www.niaid.nih.gov/.

National Institutes of Health, Bethesda, Maryland; http://nih.gov. For

toll-free information line for NIH departments: http://www.nih.gov/

health/infoline.htm.

Group members Index number

1.KOOMSON SOPHIA ED/SHS/13/0176

2. ABIGIAL COBBINA ED/SHS/13/0117

3.AMOATENG ANTWI AGYEI ED/SHS/13/0196

4.

5.

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