measurement of health course

Madda Walabu University Faculty of Health Sciences By Diriba Dibaba (BSc, MPH in Epidemiology )

COURSE OUTLINE • Understanding cause of diseases • Models for disease causation in epidemiology • Epidemiological study Design

What is a cause disease ? CONCEPT OF DISEASE • A condition of the body or some part or organ of the body in which its functions are derranged. • It is a mal-adjustment of an organism to the environment. • It is deviation from normal function & purpose. • A cause :Webster’s dictionary defines a cause as ‘something which has an effect’. • In epidemiology a cause can be considered to be something that alters the frequency of disease, health & production status or associated factors in a population. • Or the cause of a specific disease occurrence is an antecedent event, condition, or characteristic that was necessary for the occurrence of the disease at the moment it occurred, given that other conditions are fixed.

Models of disease causation in epidemiology • In epidemiology, there are several models of disease causation that help understand disease process. The most widely applied models are: • The epidemiological triad (triangle), the wheel, and the web, and the sufficient cause and component causes models (Rothman’s component causes model).

The epidemiologic triad Model • The epidemiologic triangle or triad is the traditional model of infectious disease causation. • It has three components: • an external agent, • a susceptible host, and environmental factors that interrelate in a variety of complex ways to produce disease & alter production & utility in animals.

Agent factors :originally referred to an infectious microorganism or pathogen: a virus, bacterium, parasite, or other microbe and others (poisonous creatures). Generally, these agents must be present for disease to occur as essential causal factor. however, presence of that agent alone is not always sufficient to cause disease.

Nutritive: Excesses or deficiencies (Cholesterol, vitamins, proteins) Chemical agents: (carbon monoxideand medications) Physical agents :radiation While the epidemiologic triad serves as a useful model for many diseases, it has proven inadequate for cardiovascular disease, cancer, and other diseases that appear to have multiple contributing causes without a single necessary one (chronic disease).

Figure shows Epidemiologic Triad

Host factors :refers to the human who can get the disease. • A variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual’s exposure, susceptibility, or response to a causative agent. • Opportunities for exposure are often influenced by behaviors such as sexual practices, hygiene, and other personal choices as well as by age and sex Host factors which affect susceptibility & response to an agentare: • Genetic composition, nutritional and immunologic status, • anatomic structure, • presence of other disease or medications, etc.

Environmental factors: Environmental factors are extrinsic factors which affect the agent as well as the host and the opportunity for exposure. Environmental factors include: Physical factors such as geology, climate(Malaria,common cold) etc. Biologic factors such as insects that transmit an agent; and Socioeconomic factors such as crowding, sanitation, and the availability of health services

Web of Causation model • Web of Causation is mostly used to address chronic disease but can also be applied to communicable disease) due to multi-factorial/ multi-etiologic nature of causation in many diseases. • There is no single cause / multi-factorial causes . • Causes of disease are interacting in various pathways • Illustrates the interconnectedness of possible causes • Here the disease is usually well-defined from a clinical point of view (e.g. lung cancer, Mastitis) but the etiologic perspective is more complex

Web of causation Model

The Wheel model of disease causation The Wheel of Causation which pionered (Mausner & Kramer,1985) by de-emphasizes the agent as the sole cause of disease. It emphasizes the interplay of physical,biologicaland social (production) environments. It also brings genetics into the mix.

Figure shows a wheel a disease causal model

Rothman(1976) causal pie model Necessary and sufficient causes A necessary cause is a causal factor whose presence is required for the occurrence of the effect. If disease does not develop without the factor being present, then we term the causative factor “necessary”. Sufficient cause is a “minimum set of conditions, factors or events needed to produce a given outcome. The factors or conditions that form a sufficient cause are called component causes. Example :The tubercle bacillus is required to cause tuberculosis but, alone, does not always cause it. Thus tubercle bacillus is a necessary, not a sufficient cause. This true for most the infectious causes.

Rothman's model has emphasized that the causes of disease comprise a collection of factors. These factors represent pieces of a pie, the whole pie (combinations of factors) is the sufficient cause for a disease. It shows that a disease may have more than one sufficient cause, with each sufficient cause being composed of several factors.

The factors represented by the pieces of the pie in this model are called component causes. Single component cause is rarely a sufficient cause by itself, but may be necessary for causation of the disease. The complete pie, which might be considered a causal pathway, is called a sufficient cause Control of the disease could be achieved by removing one of the components in each "pie" and if there were a factor common to all "pies“ (necessary cause) the disease would be eliminated by removing that alone

Figure shows Rothman's pie model

Types of Causal Relationships A)Necessary and sufficient – without the factor, disease never develop. With the factor, disease always develops (this situation rarely occurs) Necessary but not sufficient – the factor in and of itself is not enough to cause disease.  Multiple factors are required, usually in a specific temporal sequence (such as carcinogenesis).

Sufficient but not necessary – the factor alone can cause disease, but so can other factors in its absence  Benzene or radiation can cause leukemia without the presence of the other Neither sufficient nor necessary – the factor cannot cause disease on its own, nor is it the only factor that can cause that disease  This is the probable model for chronic disease relationships

Real-World Examples Necessary but Not Sufficient: A person must be infected with HIV before they can develop AIDS . HIV infection is, therefore, a necessary cause of AIDS. However, since every person with HIV does not develop AIDS, it is not sufficient to cause AIDS. You may need more than just HIV infection for AIDS to occur. Sufficient but Not Necessary: Decapitation is sufficient to cause death; however, people can die in many other ways. Therefore, decapitation is not necessary to cause death. .

Neither Necessary nor Sufficient: Gonorrhea is neither necessary nor sufficient to cause pelvic inflammatory disease. A person can have gonorrhea without ever developing PID. They can also have PID without ever having been infected with gonorrhea. Both Necessary and Sufficient: A gene mutation associated with Tay-Sachs( It is genetic disorder which shows absence of an enzyme that helps break down fatty substances)is both necessary and sufficient for the development of the disease. Everyone with the mutation will eventually develop Tay-Sachs. No one without the mutation will ever have it.

Natural History of disease 3 Natural History.pptx

Level of Disease prevention Objective of this lecture will be : • Understanding different level of disease prevention and its objective • Application of level of disease prevention to real health problem.

Level of diseases prevention • The major purpose in investigating the epidemiology of diseases is to learn how to prevent and control them. • Disease prevention means to interrupt or slow the progression of disease. • Epidemiology plays a central role in disease prevention by identifying those modifiable causes and their risk factors. - preventing sick people from becoming disabled or becoming chronically sick or dying.

There are several stages during the course of a disease at which we can intervene in order to control the disease. 1. Primary prevention- • is preventing healthy people from becoming sick. In order to carry out effective primary prevention • We must know who most at risk of getting the disease. • The objectives here are to promote health, prevent exposure, and prevent occurrence of disease.

A. Health promotion: This consists of general non-specific interventions that enhance health and the body’s ability to resist disease. • The improvement of socioeconomic status through the provision of adequately; – nutrition, – paid jobs, – education, -housing, etc.

B. Prevention of exposure: • Prevention of exposure is the avoidance of factors which may cause disease if an individual is exposed to them. • There are many examples of interventions aimed at this stage, relatively they are specific to individual diseases compared to health promotion Example Provision of vector control; - Provision of a safe environment at home - Vaccination - Use of bed nets - Consistent use of condom - Provision of safe and adequate water, proper excreta disposal

C. Prevention of disease This is when the intervention aims to prevent initiation of disease, in persons who already are exposed to agent Immunization can be taken as a good example. Passive immunization- providing the host with the antibodies necessary to fight the disease. It is commonly given after exposure. Example: Rabies, Tetanus.

2. Secondary prevention:- involves detecting people who already have the disease as early as possible in order to cure the patient and prevent chronic disease and disability. • In pH context, secondary prevention also results in reduction of prevalence of diseases and risk of transmission. • Secondary prevention thus involves the detection and treatment of those people in the sub clinical stage. • The objective here is to stop or slow the progression of disease

and to prevent or limit permanent damage. • Strategy at this stage is through early detection and treatment of disease. Limit severity and spread of disease, that is decrease secondary cases. 3. Tertiary prevention- is the prevention of more disability and death. The aims are:- Disability limitation and Rehabilitation

=> In a patient who can not be cured, tertiary prevention have many benefit. objectives: Objectives of rehabilitation: 1. Restore function- e.g. surgical correction of leprosy =>medical rehabilitation 2. Vocational training- so that the individual can earn his/her livelihood =>vocational rehabilitation 3. Integrate family with society – social rehabilitation 4. Instill self confidence- psychological rehabilitation

Application of the three level of prevention Example: Trachoma:- primary prevention:-improving basic sanitation, including availability and use of soap and water, avoid common use of towel Secondary prevention:-early detection and early treatment to prevent blindness Tertiary prevention:-Rehabilitation for blind person (limit the physical, psychological, social, and financial impact of disability)

Summary: Levels of prevention: definition, timing, objectives

EXERCISE 1. The Pap smear test detects cervical cancer in women at an early stage of the disease when there are no symptoms and the disease is not evident on visual examination. Which of the following statements applies to a woman with no history of cervical cancer who undergoes a Pap smear? A. She is practicing primordial prevention.B. She is practicing primary prevention.C. She is practicing secondary prevention.D. She is practicing tertiary prevention.E. This is a screening test, not a preventive procedure.

Reference Books • 1. Fletcher. Principles of Epidemiology • 2. Charles H Hennekens and Julie E Buring. Epidemiology in Medicine • 3. Rothman, Kenneth J.; Greenland, Sander; Lash, Timothy L. Modern epidemiology. 3rd • edition. 2008 • 4. David G. Kleinbaum, Kevin M. Sullivan. A pocket guide to epidemiology. 2007 • 5. YemaneBerhane, DamenHailemariam and Helmut Kloos. Epidemiology and ecology of • health and disease in Ethiopia. 2006 • 6. Daniel. Biostatistics: a foundation for analysis in health sciences. • 7. Pagano. Principles of Biostatistics • 8. Colton. Statistics in Medicine

measurement of health course