Disease & Epidemiology PDF

Preview

Summary

Microbiology ...

Description

Microbiology: Lecture: Disease and Epidemiology Dr. J. Lopez Introduction: We all depend on our various defense mechanisms to keep us healthy. However, in spite of our defenses, we are still susceptible to pathogens (disease-causing microorganisms). Some bacteria can invade our tissues and resist our defenses by producing capsules or enzymes, while other bacteria release toxins that can seriously affect our health. The learning objectives of this lecture will include the following: 1. Define pathology, etiology, infection, and disease. 2. Define normal and transient microbiota 3. Compare commensalism, mutualism, and parasitism, and be able to give an example of each. 4. List Koch’s postulates. 5. Differentiate a communicable from a noncommunicable disease. 6. Categorize diseases according to frequency of occurrence. 7. Categorize diseases according to severity 8. Define herd immunity. 9. Identify four predisposing factors for disease. 10. Be able to discuss the following; Incubation period, prodromal period, period of illness, period of decline, period of convalescence. 11. Define reservoir of infection 12. Contrast human, animal, and nonliving reservoirs, and be able to give an example of each. 13. Discuss at least four methods of disease transmission. 14. Define nosocomial infections, and explain their importance 15. Define compromised host 16. Define epidemiology 17. Define morbidity and mortality. Notes: ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________

Pathology, Infection, and Disease: (Objective 1) Pathology is the study of disease. It is primarily concerned with the following: 1. The cause or etiology. 2. The pathogenesis, the manner in which a disease develops. 3. The structural and functional changes that occurs because of the disease. Quite often we use the term infection and disease interchangeably. However, these terms differ somewhat in meaning. Infection is the invasion or colonization of the body by pathogenic microorganisms. Disease, on the other hand, is an abnormal state in which part or all of the body is not properly adjusted or incapable of performing its normal function. An infection may exist in the absence of detectable disease. For example, the body may be infected with a virus that causes AIDS, but there may be no symptoms of the disease.

Note: Not all microorganisms are pathogenic. In fact, it may be argued, that the presence of some microorganisms can even benefit the host. Thus, before discussing the role of microorganisms in causing disease, let us first explore the relationship of the microorganisms to the healthy human body. Notes: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

Normal Microbiota: (Objective 2) Animals, including humans, are generally free of microbes in utero. At birth, however, normal and characteristic microbial populations begin to establish themselves. For example, just before a woman gives birth, lactobacilli in her vagina multiply rapidly. The newborn’s first contact with microorganisms is usually with these lactobacilli, and they become the predominant organisms in the newborn’s intestine. More microorganisms are introduced to the newborn’s body from the environment when breathing begins and feeding starts. After birth, E. coli and other bacteria acquired from foods begin to inhabit the large intestine. These microorganisms remain their throughout life and, in response to altered environmental conditions, may increase or decrease in number and contribute to disease. Many other usually harmless microorganisms establish themselves inside other parts of the body and on its surface. These microorganisms that establish permanent residence (colonize) but do not produce disease under normal conditions are members of the body’s normal microbiota or normal flora. Other microorganisms called transient microbiota, may be present for several days, weeks, or months, and then disappear. Notes: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

Relationships Between the Normal Microbiota and the Host: (Objective 3) Once established, the normal microbiota can benefit the host by preventing the overgrowth of harmful microorganisms. This is often referred to as microbial antagonism. Microbial antagonism involves competition among microbes. One consequence of this competition is that the normal microbiota protects the host against colonization by potentially pathogenic microbes in a number of ways, such as the following: 1. Competing for nutrients 2. Producing substances harming to the invading microbes 3. Affecting conditions such as pH However, when the balance between normal microbiota and pathogenic microbes is upset, disease can result. For example, the normal bacterial microbiota of the adult human vagina maintains a local pH of 3.5-4.5. This pH inhibits the overgrowth of the yeast Candida albicans. However, if the normal bacterial population is eliminated by antibiotics, excessive douching, or deodorants, the pH of the vagina reverts to nearly neutral, allowing Candida albicans to flourish, leading to a condition called vaginitis. Another example, involves another bacterium, Clostridium difficile. The normal microbiota of the large intestine effectively inhibits C. difficile. However, if the normal microbiotas are eliminated, for example, by antibiotics, C. difficile can be a problem. As a matter fact, this microbe is responsible for nearly all gastrointestinal infections that follow antibiotic therapy. Notes: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

The relationship between the normal microbiota and the host is called symbiosis (living together). In the symbiotic relationship called commensalism, one of the organisms is benefited and the other is unaffected. Many of the microorganisms that make up our normal microbiota are commensals; these include the corynebacteria that inhabit the surface of the eye and certain mycobacteria that inhabit the ear and external genitals. These bacteria live on secretions and sloughed off cells. These bacteria bring no apparent benefit or harm to the host. Mutualism is a type of symbiosis that benefits both organisms. For example, the large intestine contains bacteria, such as E. coli, that synthesize vitamin K and some b vitamins. These vitamins are absorbed into the bloodstream and distributed for use by body cells. In exchange, the large intestine provides nutrients for the bacteria so that they can survive. Parasitism is another type of symbiosis where one organism is benefited at the expense of the other. Many disease-causing bacteria are parasites. Notes: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

Koch’s Postualtes: (Objective 4) Koch was a German physician who played a major role in establishing that microorganisms cause specific diseases. For example Koch showed that a specific infectious disease (anthrax) is caused by a specific microorganism (B. anthracis) that can be isolated and cultured on artificial media. He later showed that Mycobacterium tuberculosis is the causative agent of tuberculosis. Koch’s research provides a framework for the study of the etiology of any infectious disease. Today, we refer to Koch’s experimental requirements as Koch’s postulates. They are summarized as follows: 1. The same pathogen must be present in every case of the disease. 2. The pathogen must be isolated from the diseased host and grown in pure culture.

3. The pathogen from the pure culture must cause the disease when it is inoculated into a healthy, susceptible laboratory animal. 4. The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.

Note: Although Koch’s postulates are useful in determining the causative agent of most bacterial diseases, there are some exceptions. For example, the bacterium Treponema pallidum is known to cause syphilis, but virulent strains have never been cultured on artificial media. The causative agent of leprosy, Mycobacterium leprae, has also never been grown on artificial media. Moreover, many rickettsial and viral pathogens cannot be cultured on artificial media because they only multiply within cells. Notes: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

Classifying Infectious Diseases: (Objective 5) Every disease that affects the body alters body structures and functions in particular ways. For example, the patient may experience certain symptoms, or chances in body function, such as pain and malaise (a vague feeling of body discomfort). These subjective changes are not apparent to an observer. The patient can also exhibit signs, which are objective changes the physician can observe and measure (fever, swelling, etc). At times, a specific group of symptoms or signs may accompany a particular disease; such a group is called a syndrome. The diagnosis of a disease is made on the bases of a clinical history, evaluation of the signs and symptoms, together with the results of laboratory tests. Diseases are often classified in terms of how they behave within a host and within a given population. Any disease that spreads from one host to another, either directly or indirectly is said to be a communicable disease. Chickenpox, measles, genital herpes, typhoid fever, and tuberculosis are examples. Chickenpox and measles are also examples of contagious diseases. Contagious diseases are those that are easily spread from one person to another. A noncommunicable disease is not spread from one host to another. These diseases are caused by microorganisms that normally inhabit the body and only occasionally produce disease, or by microorganisms that reside outside of the body and produce disease only when introduced into the body. For example, Clostridium tetani produces disease only when it is introduced into the body via abrasions or wounds. Notes: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

The occurrence of a Disease: (Objective 6) In order to understand the full scope of a disease, it is important to understand something about its occurrence. The incidence of a disease is the number of people in a population who develop a disease during a particular time period. It may be used as an indicator of the spread of the disease. The prevalence of a disease is the number of people in a population who developed a disease at a specified time, regardless of when it first appeared. Prevalence takes into account both old and new cases. It serves as an indicator of how seriously and how long a disease affects a population. For example, the incidence of AIDS in the United States in 199 was 45,000, while the prevalence in that same year was estimated to be 700,000. Frequency of occurrence is another criterion that is used in the classification of diseases. If a particular disease occurs only occasionally, it is called a sporadic disease. A disease constantly present in a population is called an endemic disease. If many people in a given area acquired a certain disease in a relatively short period, it is called an epidemic disease; influenza is an example of a disease that often achieves epidemic status. An epidemic disease that occurs worldwide is called a pandemic disease. Notes: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ________...