Preventive Dentistry Course 2 Notes PDF

Sayfa 1 DENT1006 - Preventive Medicine & Introduction to Public Oral and Dental Health CONCEPTS OF HEALTH “Joseph John (2018), Textbook of Preventive and Community Dentistry, Public Health Dentistry 3rd Ed CBS Publishers CHAPTER 1 “...

Sayfa 1 DENT1006 - Preventive Medicine & Introduction to Public Oral and Dental Health CONCEPTS OF HEALTH “Joseph John (2018), Textbook of Preventive and Community Dentistry, Public Health Dentistry 3rd Ed CBS Publishers CHAPTER 1 “ DISEASE Webster defined disease as “a condition in which body’s health is impaired, a departure from a state of health, an alteration of the human body interrupting the performance of vital functions” The simplest definition is, of course, that disease is just the opposite of health, i.e. any deviation from normal functioning or state of complete physical or mental well-being. Concepts of Causation Up to the time of Louis Pasteur, various concepts of disease causation were in vogue, e.g. the supernatural theory of disease, the theory of humors, etc. Discoveries in microbiology marked a turning point in our aetiological concepts. 1. Germ Theory of Disease This concept gained momentum during the 19th and the early part of the 20th century. The emphasis had shifted from empirical causes (bad air) to microbes as the sole cause of disease. The concept in the germ theory of disease is generally referred to as one-to- one relationship between causal agent and disease. The disease model is as follow: Disease agent → Man → Disease It is now recognized that a disease is rarely caused by a single agent alone, but rather depends upon a number of factors, which contribute to its occurrence. Therefore, Sayfa 2 modern medicine has moved away from the strict adherence to the germ theory of disease. 2. Epidemiological Triad The germ theory of disease has many limitations. For example, not everyone exposed to tuberculosis develops tuberculosis. The same exposure, however, in an undernourished or otherwise susceptible person may result in clinical disease. There are other factors relating to the host and environment, which are equally important to determine whether or not disease will occur in the exposed host. This demanded a broader concept of disease causation that synthesized the basic factors of agent, host and environment. The causative factors of disease may be classified as agent, host and environment. These three factors are referred as epidemiological triad. The mere presence of agent, host and favourable environmental factors in the prepathogenesis phase is not sufficient to start a disease in man. What is required is interaction of these three factors to initiate the disease process in man (Fig. 1.1). The agent, host and environment operating in combination determine not only the onset of disease which may range from a single case to epidemics but also the distribution of disease in community. Fig. 1.1: Interaction of agent, host and environment Sayfa 3 1. Agent factors: Agent is defi ned as a substance, living or non-living, or a force, tangible or intangible, the excessive presence or relative lack of which may initiate or perpetuate a disease process. Disease agents are classified as: 1. Biological agents: These agents include virus, bacteria, fungi, rickettsiae, protozoa and metazoa. These exhibit infectivity, pathogenicity and virulence. 2. Nutritional agents: These can be proteins, fats, carbohydrates, vitamins, minerals and water. Any excess or deficiency results in nutritional disorder. 3. Physical agents: Exposure to excessive heat, cold, humidity, pressure, radiation, electricity and sound may result in illness. 4. Chemical agents: A. Endogenous: Chemicals produced in the body such as urea, ketones, uric acid, etc. B. Exogenous: Agents arising outside the human host, e.g. metals, allergens, fumes, dust and gases. 5. Mechanical agents: Exposure to chronic friction and other mechanical forces may result in crushing, tearing, sprains, dislocations and even death. 6. Social agents: These include smoking, poverty, abuse of drugs and alcohol, unhealthy lifestyles, social isolation, maternal deprivation, etc. 2. Host factors: Host is defi ned as a person or other animal, including birds and arthropods that affords subsistence or lodgment to an infectious agent under natural conditions. The host factors can be classifi ed as: 1. Demographic characteristics such as age, sex. 2. Biological characteristics such as genetic factors. Sayfa 4 3. Social and economic characteristics such as education, occupation, and marital status. 4. Lifestyle factors such as personality traits, living habits, and physical exercises. 3. Environmental factors: Environment is defi ned as man’s external surroundings. It is divided into three components: 1. Physical environment: This is applied to non-living things and physical factors with which man is in constant interaction, e.g. air, water, soil, housing, climate, heat, light, noise, debris, and radiation. 2. Biological environment: The biological environment is the universe of living things, which surrounds man, including man himself. 3. Psychosocial environment: It is difficult to define psychosocial environment due to the varied social, economic and cultural contexts. It includes a complex of psychosocial factors which are defined as those factors affecting personal health, health care and community well-being that stem from the psychosocial make-up of an individual and the structure and functions of social groups. A stable and harmonious equilibrium between man and his environment is needed to reduce man’s vulnerability to diseases and to permit him to lead a more productive and satisfying life. 3. Multifactorial Causation : Pettenkofer of Munich was an early proponent of this concept. As a result of advances in public health, chemotherapy, antibiotics and vector control, communicable diseases began to decline—only to be replaced by new types of diseases, the so-called “modern” diseases of civilization, e.g. lung cancer, coronary heart disease, chronic bronchitis, mental illness, etc. These diseases could not be explained on the basis on the germ theory of disease nor could they be prevented by the traditional methods of isolation, immunization or improvements in sanitation. The realization began to dawn that the “single cause idea” was an over simplification and that there are other factors in the aetiology of diseases—social, economic, cultural, genetic and psychological which are equally important. Diseases such as coronary heart disease and cancer are due to multiple factors. For example, excess of fat intake, smoking, lack of physical exercise, and obesity are all involved in the pathogenesis of coronary heart disease. Most of these factors are linked Sayfa 5 to lifestyle and human behaviour. Thus this model de-emphasizes the concept of disease “agent” and stress multiplicity of interactions between host and environment. The multifactorial concept offers multiple approaches for the prevention and control of disease. 4. Web of Causation This model of disease causation was suggested by Mac Mahon and Pugh. This model is ideally suited in the study of chronic disease, where the disease agent is often not known, but is the outcome of interaction of multiple factors. The “web of causation” considers all the predisposing factors of any type and their complex interrelations with each other. The web of causation does not imply that the disease cannot be controlled unless all the multiple causes or chains of causation or at least a number of them appropriately controlled or removed. Sometimes removal or elimination of just one link or chain may be sufficient to control disease, provided that link is sufficiently important in the pathogenetic process. Natural History of Disease Disease results from a complex interaction between man, an agent and the environment. The term natural history of disease is a key concept in epidemiology. It signifies the way in which a disease evolves over time from the earliest stage of its prepathogenesis phase to its termination as recovery, disability or death, in the absence of treatment or prevention. Each disease has its own unique natural history, which is not necessarily the same in all individuals, so much so, any general formulation of the natural history of disease is necessarily arbitrary. It is customary to describe the natural history of disease as consisting of two phases: Prepathogenesis/ Pathogenesis. Sayfa 6 a. Prepathogenesis Phase This refers to the period preliminary to the onset of disease in man. The disease agent has not yet entered man, but the factors which favour its interaction with the human host are already existing in the environment. This situation is frequently referred to as ‘man in midst of disease’ or ‘man exposed to risk of disease’. Potentially, we are all in the prepathogenesis phase of many diseases, both communicable and non-communicable. b. Pathogenesis Phase The pathogenesis phase begins with the entry of disease ‘agent’ in the susceptible human host. The further events in the pathogenesis phase are clear-cut in infectious diseases, i.e. the disease agent multiplies and induces tissue and physiologic changes, the disease progresses through a period of incubation and later through early and late pathogenesis. The final outcome of the disease may be recovery, disability or death. The pathogenesis phase may be modified by intervention measures such as immunization and chemotherapy. Risk Factors When the disease agent is not firmly established, the aetiology is generally discussed in terms of “risk factors”. The term risk factor means an attribute or exposure that is significantly associated with the development of a disease. The presence of a risk factor does not imply that the disease will occur, and in its absence, the disease will not occur. The important thing about risk factors is that they are observable or identifiable prior to the event they predict. The combination of risk factors in the same individual may be purely additive or synergistic. For example, smoking and occupational exposure (dye and leather industry) were found to have an additive effect as risk factors for bladder cancer. On the other hand, smoking was found to be synergistic with other risk factors such as hypertension and high blood cholesterol. Risk factors may be truly causative or merely contributory. Some risk factors can be modified (smoking); others cannot be modified (age, sex). Epidemiological methods (case control Sayfa 7 and cohort studies) are needed to identify risk factors and estimate the degree of risk. The detection of risk factors will help in the prevention and intervention of diseases. Spectrum of Disease The term “spectrum of disease” is a graphic representation of variations in the manifestations of disease. At one end of the disease, spectrum are subclinical infections, which are not ordinarily identified and at the other end are fatal illnesses. In the middle of the spectrum, lie illnesses ranging in severity from mild to severe. These different manifestations are simply reflections of individual’s different states of immunity and receptivity. Leprosy is an excellent example of the spectral concept of disease. The sequence of events in the spectrum of disease can be interrupted by early diagnosis and treatment or by preventive measures. Iceberg of Disease A concept closely related to the spectrum of disease is the concept of the iceberg phenomenon of disease. According to this concept, disease in a community may be compared with an iceberg (Fig. 1.2). The tip of the iceberg represents what the physician sees in the community, i.e. clinical cases. The vast submerged portion of the iceberg represents the hidden mass of disease, i.e. latent, inapparent, presymptomatic and undiagnosed cases and carriers in the community. The “water line” represents the demarcation between apparent and inapparent disease. In some diseases [e.g. hypertension, diabetes, anaemia, malnutrition, mental illness], the unknown morbidity [i.e. the submerged portion of the iceberg] far exceeds the known morbidity. The hidden part of the iceberg thus constitutes an important, undiagnosed reservoir of infection or disease in the community, and its detection and control is a challenge to modern techniques in preventive medicine. Sayfa 8 Fig. 1.2: The iceberg of disease Disease Control The term “disease control” describes operations aimed at reducing: The incidence of disease The duration of disease, and consequently the risk of transmission The effects of infections, including both the physical and psychosocial complications; and The financial burden to the community. Control activities may focus on primary prevention or secondary prevention; most control programmes combine the two. Sayfa 9 In disease control, the disease ‘agent’ is permitted to persist in the community at a level where it ceases to be a public health problem according to the tolerance of the local population. A state of equilibrium becomes established between the disease agent, host and environment components of the disease process. Disease Elimination The term “elimination” is used to describe interruption of transmission of disease, as for example, elimination of measles, polio and diphtheria from large geographic regions or areas. Disease Eradication Eradication literally means to “tear out by roots”. Eradication of disease implies termination of all transmission of infection by extermination of the infectious agent. The word eradication is reserved to cessation of infection and disease from the whole world. Totally, smallpox is the only disease that has been eradicated. Three diseases have been seriously advanced as candidates for global eradication within the foreseeable future; polio, measles and dracunculiasis. The feasibility of eradicating polio appears to be greater than that of others. SCREENING FOR DISEASES Screening has been defined as “the search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals”. The original screening programmes were for individual diseases such as tuberculosis, syphilis, etc. Over the years, the screening tests have steadily grown in number. Today screening is considered a preventive care function, and some consider it a logical extension of health care. Sayfa 10 Screening differs from periodic health examinations in the following respects: 1. Capable of wide application. 2. Relatively inexpensive 3. Requires little physician-time. In fact the physician is not required to administer the test, but only to interpret it. A screening test is not intended to be a diagnostic test. It is only an initial examination. Those who are found to have positive test results are referred to a physician for further diagnostic work-up and treatment. Aims and Objectives of Screening The basic purpose of screening is to sort out from a large group of apparently healthy persons those likely to have the disease or at increased risk of the disease under study, to bring those who are “apparently abnormal” under medical supervision and treatment. Criteria for Screening The criteria for screening are based on two considerations: The disease to be screened, and the test to be applied. Disease The disease to be screened should fulfill the following criteria before it is considered suitable for screening: The condition sought should be an important health problem (in general, prevalence should be high). There should be a recognizable latent or early asymptomatic stage. The natural history of the condition, including development from latent to declared disease, should be adequately understood (so that we can know at what stage the process ceases to be reversible). Sayfa 11 There is a test that can detect the disease prior to the onset of signs and symptoms. Facilities should be available for confirmation of diagnosis. There is an effective treatment. There should be an agreed-on policy concerning whom treat as patients (e.g. lower ranges of blood press borderline diabetes). There is good evidence that early detection and treatment reduces morbidity and mortality. The expected benefits (e.g. the number of lives saved early detection) exceed the risks and costs. When the above criteria are satisfied, then it would be appropriate to consider a suitable screening test. Screening Test The test must satisfy the criteria of acceptability, repeatable and validity, besides others such as yield, simplicity, safety rapidity, ease of administration and cost. 1. Acceptability Since a high rate of cooperation is necessary, it is important that the test should be acceptable to the people at whom it is aimed. In general, tests that are painful, discomforting and embarrassing (e.g. rectal or vaginal examinations) are not in likely to be acceptable to the population in mass campaign. 2. Repeatability An attribute of an ideal screening test or any measurement (e.g. height, weight) is its repeatability (sometimes called reliability, precision or reproducibility). That is, the test must give consistent results when repeated more than once on the same individual or Sayfa 12 material, under the same conditions. The repeatability of the test depends upon three major factors namely observer variation, biological (or subject) variation and errors relating to technical methods. For example, the measurement of blood pressure is poorly, producible because it is subjected to all these three major factors. 3. Validity (Accuracy) The term validity refers to what extent the test accurately measures which it purports to measure. In other words, validity expresses the ability of a test to separate or distinguish those who have the disease from those who do not. Validity has two components—sensitivity and specificity. When assessing the accuracy of a diagnostic test, one must consider both these components. Both measurements are expressed as percentages. Sensitivity and specificity are usually determined by applying the test to one group of persons having the disease, and to a reference group not having the disease (Table 1.1). Sensitivity and specificity, together with “predictive accuracy” are inherent properties of a screening test. Table 1.1: Screening test result by diagnosis The letter “a” (Table 1.1) denotes those individuals found positive on the test who have the condition or disorder being studied (i.e. true positives). The group labelled “b” includes those who have a positive test result but who do not have the disease (i.e. false positives). Group “c” includes those with negative test results but who have the disease Sayfa 13 (i.e. false negatives). Finally, those with negative results who do not have the disease are included in group “d” (i.e. true negatives). Evaluation of a Screening Test The following measures are used to evaluate a screening test: Sensitivity = a/(a + c) × 100 Specificity = d/(b + d) × 100 Predictive value of a positive test a/(a + b) × 100 Predictive value of a negative test = d/(c + d) × 100 Let us rewrite Table 1.1 substituting hypothetical figures (Table 1.2) and calculate the above measures: Table 1.2: Screening test result by diagnosis Sensitivity (true positive) = (40/140) × 100 = 28.57% Specificity (true negative)= (9840/9860) × 100 = 99.79% Predictive value = (9840/9940) × 100 = 98.9% of a negative test. The term sensitivity was introduced by Yerushalmy in 1940s as a statistical index of diagnostic accuracy. It has been defined as the ability of a test to identify correctly all those who have the disease, that is “true positive”. A 90% sensitivity means that 90% of the diseased people screened by the test will give a “true positive” result and the remaining 10% a “false negative” result. Sayfa 14 Specificity It is defined as the ability of a test to identify correctly those who do not have the disease, i.e. “true negatives”. A 90% specificity means that 90% of the non-diseased persons will give “true negative” result, 10 per cent of nondiseased people screened by the test will be wrongly classified as “diseased” when they are not. Sensitivity Sensitivity may be increased only at the expense of specificity and vice versa. An ideal screening test should be 100% sensitive and 100% specific. In practice, this seldom occurs. Predictive Value of Test Result For interpreting the test result (done on an individual), the predictive value of positive test and predictive value of negative test are useful. Predictive Value of Positive Test Predictive value of positive test is the probability that a person actually has the disease given that he or she tests positive. It is the probability that the disease is present when the test result is positive. It is calculated as the number of true positive results divided by true positive results and false positive results. For the same test (for a given sensitivity, and specificity) the predictive value of positive test will be higher when the test is done in a population where the disease prevalence is higher compared to when the test is done in a population where the disease prevalence is lower. Predictive Value of Negative Test Predictive value of negative test is the probability that an individual is truly disease-free given that he or she tests negative. It is the probability that the disease is not present when the test result is negative. It is calculated as the number of true negative results divided by the true negative results and false negative results. Sayfa 15 The above test characteristics answer the following questions: 1. If the disease is present, what is the probability that the test result will be positive? (Sensitivity) 2. If the disease is absent, what is the probability that the test result will be negative? (Specificity) 3. If the test result is positive, what is the probability that the disease is present? (Predictive value of positive test) 4. If the test result is negative, what is the probability that the disease is not present? (Predictive value of negative test). Uses of Screening Case detection: It is the identification of unrecognised disease, which does not arise from a patient’s request, e.g. diabetes mellitus, iron deficiency anaemia. Control of disease: People are examined for the benefit of others, e.g. screening of immigrants from infectious diseases such as tuberculosis and syphilis to protect the home population. Research purposes: Screening may sometimes be performed for research purposes. For example, there are many chronic diseases whose natural history is not fully known (e.g. cancer, hypertension). Screening may aid in obtaining more basic knowledge about the natural history of such diseases, provides a prevalence estimate and subsequent screening, an incidence figure. Educational opportunities: Screening provides opportunities for creating public awareness and for educating health professionals. Sayfa 16 INFECTION Definition: The entry and development or multiplication of an infectious agent in the body of man or animals. Dynamics of Disease Transmission Communicable diseases are transmitted from the reservoir or source of infection to susceptible host. Basically, there are three links in the chain of transmission, viz. the reservoir, modes of transmission and the susceptible host. Sources and Reservoir The starting point for the occurrence of a communicable disease is the existence of a reservoir source of infection. Source of infection is defined as “the person, animal, object or substance from which an infectious agent passes or is disseminated to the host.” A reservoir is defined as “any person, animal, arthropod, plant or substance or (combination of these) in which an infectious agent lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host.” The reservoir may be of three types: 1. Human reservoir 2. Animal reservoir 3. Reservoir in non-living things 1. Human Reservoir By far the most important source or reservoir of infection for humans is man himself. He may be a case or carrier. Man is often described as his own Sayfa 17 enemy because most of the communicable diseases, which man contracts, are from human sources. Cases: 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.”1 Carriers: A carrier is defined as “an 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 others.” The elements in a carrier state are: The presence in the body of the disease agent. The absence of recognizable symptoms and signs of disease. The shedding of the disease agent in the discharges or excretions, thus acting as a source of infection for other persons. 2. Animal Reservoir The source of infection may sometimes be animals and birds. The diseases and infections which are transmissible to man from vertebrates are called zoonoses. These are over 100 zoonotic diseases which may be conveyed to man from animals and birds. The best known examples are rabies, yellow fever and influenza. 3. Reservoir in Non-living Things Soil and inanimate matter can also act as reservoirs of infection. For example, soil may harbour agents that cause tetanus, anthrax, coccidioidomycosis and mycetoma. Sayfa 18 Modes of Transmission Communicable diseases may be transmitted from the reservoir or source of infection to a susceptible individual in many different ways, depending upon the infectious agent, portal of entry and the local ecological conditions. The mode of transmission of infectious diseases may be classified as below: Direct transmission 1. Direct contact 2. Droplet infection 3. Contact with soil 4. Inoculation into skin or mucosa 5. Transplacental (vertical) Indirect transmission 1. Vehicle-borne 2. Vector-borne Mechanical/ Biological 3. Air-borne Droplet nuclei / Dust 4. Fomite-borne 5. Unclean hands and fingers Sayfa 19 A. Direct Transmission 1. Direct contact: Infection may be transmitted by direct contact from skin-to-skin, mucosa-to-mucosa, or mucosa to skin of the same, or another person. This implies direct and essentially immediate transfer of infectious agents from the reservoir or source to a susceptible individual. Diseases transmitted by direct contact includes STD and AIDS, leprosy, leptospirosis, skin and eye infections. 2. Droplet infection: This is direct projection of a spray of droplets of saliva and nasopharyngeal secretions during coughing, sneezing, or speaking and spitting, talking into the surrounding atmosphere. In infectious diseases, these droplets, which may contain millions of bacteria and viruses can be a source of infection to others. When a healthy susceptible person comes within the range of these infected droplets he is likely to inhale some of them and acquire infection.Diseases transmitted by droplet spread include many respiratory infections, eruptive fevers, many infections of the nervous system, common cold, diphtheria, whooping cough, tuberculosis, meningococcal meningitis, etc 3. Contact with soil: The disease agent may be acquired by direct exposure of susceptible tissue to the disease agent in soil, compost or decaying vegetable matter in which it normally leads a saprophytic existence, e.g. hookworm larvae, tetanus, mycosis, etc. 4. Inoculation into skin or mucosa: The disease agent may be inoculated directly into the skin or mucosa, e.g. rabies virus by dog bite, hepatitis B virus through contaminated needles and syringes, etc. 5. Transplacental or vertical transmission: Disease agents can be transmitted transplacentally. This is another form of direct transmission. Examples include the so- called TORCH agents (Toxoplasma gondii rubella virus, ctyomegalovirus and herpes virus). B. Indirect Transmission This embraces a variety of mechanisms including the traditional 5 Fs—“flies, fingers, fomites, food and fluid”. An essential requirement for indirect transmission is that the infectious agent must be capable of surviving outside the human host in the external environment and retainits basic properties of pathogenesis and virulence till it finds a new host. Sayfa 20 1. Vehicle-borne: Vehicle-borne transmission implies transmission of the infectious agent through the agency of water, food (including raw vegetables, fruits, milk and milk products), ice, blood, serum, plasma or other biological products such as tissues and organs. Of these, water and food are the most frequent vehicles of transmission, because every one uses them. The infectious agent may have multiplied or developed in the vehicle. Diseases transmitted by water and food include chiefly infections of the alimentary tract, e.g. acute diarrhoea, typhoid fever, cholera. 2. Vector-borne: In infectious disease epidemiology, vector is defined as an arthropod or any living carrier (e.g. snail) that transports an infectious agent to a susceptible individual. Transmission by a vector may be mechanical or biological. In the latter case, the disease agent passes through a developmental cycle or multiplication in the vector. 3. Air-borne: Droplet nuclei: “Droplet nuclei” are a type of particles implicated in the spread of air-borne infection. They are tiny particles that represent the dried residue of droplets. The droplet nuclei may remain air-borne for long periods of time, some retaining and others losing infectivity or virulence. Diseases spread by droplet nuclei include tuberculosis, influenza, chickenpox, measles. Dust: Some of the larger droplets which are expelled during talking, coughing or sneezing, settle down by their sheer weight on the floor, carpets, furniture, clothes, bedding, linen and other objects in the immediate environment and become part of the dust. A variety of infectious agents (e.g. streptococci, other pathogenic bacteria, viruses and fungal spores) and skin squamae have been found in the dust of hospital wards and living rooms. Dust particles may also be blown from the soil by wind; this may include fungal spores. 4. Fomite-borne: Fomites are inanimate articles or substances other than water or food contaminated by the infectious discharges from a patient and capable of harbouring and transferring the infectious agent to a healthy person. Fomites include soiled clothes, towels, linen, handkerchiefs, cups. The fomites play an Sayfa 21 important role in indirect infection. Diseases transmitted by fomites include diphtheria, typhoid fever, bacillary dysentery, hepatitis A, eye and skin infections. 5. Unclean hands and fingers: Hands are the most common medium by which pathogenic agents are transferred to food from the skin, nose, bowel, etc. as well as from other foods. The transmission takes place both directly (hand-to-mouth) and indirectly. SUSCEPTIBLE HOST Four stages have been described in successful parasitism: First, the infectious agent must find a portal of entry by which it may enter the host. There are many portals of entry, e.g. respiratory tract, alimentary tract, genitourinary tract, skin, etc. Some organisms may have more than one portal of entry, e.g. hepatitis B, Q fever, brucellosis. On gaining entry into the host, the organisms must reach the appropriate tissue or “Site of election” in the body of the host where it may find optimum conditions for its multiplication and survival. 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. If there is no portal of exit, the infection becomes a dead-end infection as in rabies, bubonic plague, tetanus and trichinosis. After leaving the human body, the organism must survive in the external environment for sufficient period till a new host is found. In addition, a successful disease agent should not cause the death of the host but produce only a low- grade immunity so that the host is vulnerable again and again to the same infection. The best example is common cold virus. Stages of an Infectious Disease All infectious diseases pass through five stages. 1. Incubation Period An infection becomes apparent only after a certain incubation period, which is defined as “the time interval between invasion by an infectious agent and appearance of the first Sayfa 22 sign or symptom of the disease in question.” During the incubation period, the infectious agent undergoes multiplication in the host. When a sufficient density of the disease agent is built up in the host, the health equilibrium is disturbed and the disease become overt. Non-infectious diseases, such as cancer, heart disease and mental illness, also have incubation periods, which may be months or years. The term latent period is used in noninfectious diseases as the equivalent of incubation period in infectious diseases. Latent period has been defined as “the period from disease initiation to disease detection”. 2. The Onset or Prodromal Stage This commences when the first symptoms appear and continue until the condition is well developed. 3. The Period of Advance or Fastigium All the symptoms are now increasing in severity until a climax is reached. 4. Period of Defervescence All the symptoms are now decreasing in severity. 5. Period of Convalescence The patient has overcome completely the invaders and toxins. Specific Defenses Specific defenses come into play, once microorganisms have breached local defense mechanisms. By virtue of these defenses, the host is able to recognize, destroy and eliminate antigenic material (e.g. bacteria, viruses, proteins, etc.) foreign to his own. A person is said to be immune when he possesses “specific protective antibodies or cellular immunity as a result of previous infection or immunization, or is so conditioned by such previous experience as to respond adequately to prevent infection and/or Sayfa 23 clinical illness following exposure to a specific infectious agent.” The specific defenses may be discussed for convenience under the following heads: Active immunity Humoral immunity Cellular immunity Combination of the above Passive immunity Normal human Ig Specific human Ig Animal antitoxins or antisera 1. Active Immunity It is the immunity which an individual develops as a result of infection or by specific immunization and is usually associated with presence of antibodies or cells having a specific action on the microorganism concerned with a particular infectious disease or on its toxic. Active immunity may be acquired in 3 ways: Following clinical infection (e.g. chicken-pox, rubella and measles) Following subclinical or in apparent infection (e.g. polio and diphtheria) Following immunization with an antigen which may be a killed vaccine, a live-attenuated vaccine or toxoid. Sayfa 24 Humoral immunity: Humoral immunity comes from the B cells (bone marrow derived lymphocytes) which proliferate and manufacture specific antibodies after antigen presentation by macrophages. The antibodies are localized in the immunoglobulin fraction of the serum. These antibodies circulate in the body and act directly by neutralizing the microbe, or its toxin or rendering the microbe susceptible to attack by the polymorphonuclear leucocyte and the monocytes. The complement system, together with antibodies is necessary for efficient phagocytosis of bacteria. Cellular immunity: It is now well-recognized that cellular immunity plays a fundamental role in resistance to infection. It is mediated by the T cells which differentiate into subpopulations able to help B lymphocytes. The T cells do not secrete antibody, but are responsible for recognition of antigen. On contact with antigen, the T cells initiate a chain of responses. For example, activation of macrophages, release of cytotoxic factors, mononuclear infl ammatory reactions, delayed hypersensitivity reactions, secretion of immunological mediators (e.g. immuno interferon), etc. There is growing evidence that cellular immunity is responsible for immunity against many diseases including tuberculosis, brucellosis and also for the body’s rejection of foreign material, such as skin grafts. The importance of cell-mediated immunity can be appreciated from the fact that a child born with a defect in humoral antibody production may survive for as long as 6 years without replacement therapy, but a severe defect in cell-mediated immunity will result in death within the fi rst 6 months of life. Combination of the above: In addition to the B and T lymphoid cells which are responsible for recognizing self and nonself, very often, they co-operate with one another and with certain accessory cells such as macrophages and human K (killer) cells, and their joint functions constitute the complex events of immunity. Active immunity takes time to develop. It is superior to passive immunity because: The duration of protection, like that of the natural infection is frequently long-lasting. Sayfa 25 With few exceptions, severe reactions are rare. The protective efficacy of active immunization exceeds that of passive immunization, and in some instances, approaches 100%. Active immunization is less expensive than passive immunization. Vaccines are cheaper to produce than are antisera. 2. Passive Immunity When antibodies produce in one body (human or animal) are transferred to another to induce protection against disease, it is known as passive immunity. In other words, the body does not produce its own antibodies but depends upon ready-made antibodies. Passive immunity may be induced: By administration of an antibody-containing preparation (immunoglobulin or antiserum) By transfer of maternal antibodies across the placenta. Human milk also contains protective antibodies (IgA) By transfer of lymphocytes, to induce passive cellular immunity—this procedure is still experimental. Immunoglobulins a. Normal human Ig Normal human Ig is an antibody-rich fraction (Cohn fraction II). Obtained from a pool of at least 1000 donors. The WHO has laid down definite standards for its preparation. For example, the preparation should contain at least 90 percent intact IgG; it should be as free as possible from IgG aggregates; all IgG subclasses should be present; there Sayfa 26 should be a low IgA concentration; the level of antibody against at least two bacterial species and two viruses should be ascertained. Normal human Ig is used to prevent measles in highly susceptible individuals and to provide temporary protection (up to 12 weeks) against hepatitis A infection for travelers to endemic areas and to control institutional and household outbreaks of hepatitis A infection. b. Specific human Ig The specific (hyper immune) human Ig should contain at least 5 times the antibody potential of the standard preparation per unit volume. These preparations are made from the plasma of patients who have recently recovered from an infection or are obtained from individuals who have been immunized against a specific infection. They therefore have a high antibody content against an individual infection and provide immediate protection, e.g. specific human Igs are used for chickenpox prophylaxis of highly susceptible individuals and for postexposure prophylaxis of hepatitis B, and rabies and for tetanus prophylaxis in the wounded. Antisera or Antitoxins The term antiserum is applied to materials prepared in animals. Originally passive immunization was achieved by the administration of antisera or antitoxins prepared from non-human sources such as horses. Since human immunoglobulin preparations exist only for a small number of diseases, antitoxins prepared from non-human sources (against tetanus, diphtheria, botulism, gas gangrene and snake bite) are still the mainstay of passive immunization. Administration of antisera may occasionally give rise to serum sickness and anaphylactic shock due to abnormal sensitivity of the recipient. Passive immunity differs from active immunity in the following respects: Immunity is rapidly established. Sayfa 27 Immunity produced is only temporary (days to months) till the antibody is eliminated from the body. There is no education of the reticuloendothelial system. Passive immunization is useful for individual who cannot form antibodies or for the normal host who takes time to develop antibodies following active immunization. Conclusion Health, disease, infection and their concepts have evolved over the years after a series of trials and errors. There is bound to be changes in the future and we will each have an opportunity to reject the existing and take advantage of new knowledge of newer diseases, their diagnosis, prevention and treatment. Whatever the new technological developments one saying is sure to stay “Health is Wealth”. Sayfa 28 Sayfa 29