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Principles of Disease and Epidemiology

 

Introduction

 

  1. Disease- causing microorganisms are called pathogens.
  2. Pathogenic microorganisms have special properties that allow them to invade the human body or produce toxins.
  3. When a microorganism overcomes the body’s defenses, a state of disease results.

 

 Pathology, Infection, and Disease

 

  1. Pathology is the scientific study of disease.
  2. Pathology is concerned with the etiology (cause), pathogenesis (development), and effects of disease.
  3. Infection is the invasion and growth of pathogens in the body.
  4. A host is an organism that shelters and supports the growth of pathogens.
  5. Disease is an abnormal state in which part or all of the body is not properly adjusted or is incapable of performing normal functions (loss of homeostasis).

 

Normal Microbiota

 

  1. Animals, including humans, are usually germ-free in utero.
  2. Microorganisms begin colonization in and on the surface of the body soon after birth.
  3. Microorganisms that establish permanent colonies inside or on the body without producing disease make up the normal microbiota.
  4. Transient microbiota are microbes that are present for various periods and then disappear.

 

Relationships Between the Normal Microbiota and the Host

 

  1. The normal microbiota can prevent pathogens from causing an infection this phenomenon is known as microbial antagonism.
  2. Normal microbiota and the host exist in symbiosis (living together)
  3. The three types of symbiosis are commensalisms (one organism benefits and the other is unaffected), mutualism (both organisms benefit), and parasitism (one organism benefits and one is harmed).

 

Opportunistic Microorganisms

 

  1. opportunistic pathogens do not cause disease under normal conditions but cause disease under special conditions.

 

Cooperation Among Microorganisms

 

  1. In some situations, one microorganism makes it possible for another to cause a disease or produce more severe symptoms.

 

The Etiology of Infectious Diseases

 

Koch’s Postulates

 

  1. Koch’s postulates are criteria for establishing that specific microbes cause specific diseases.
  2. Koch’s postulates have the following requirements:
    1. The same pathogen must be present in every case of the disease
    2. The pathogen must be isolated in pure culture
    3. The pathogen isolated from pure culture must cause the same disease in a healthy, susceptible laboratory animal
    4. The pathogen must be reisolated from the inoculated laboratory animal.

 

Exceptions to Kochs Postulates

 

  1. Kochs postulates are modified to establish etiologies of diseases caused by viruses and some bacteria which cannot be grown on artificial media.
  2. Some disease such as tetanus have unequivocal signs and symptoms.
  3. Some diseases such as pneumonia and nephritis maybe caused by a variety of microbes.
  4. Some pathogens such as S. Pyogenes cause several different diseases.
  5. Certain pathogens such as HIV cause disease in humans only.

 

Classifying Infectious Diseases

 

  1. A patient may exhibit symptoms (subjective changes in body functions) and signs (measurable changes), which a physician uses to make a diagnosis.
  2. A specific group of symptoms of signs that always accompanies a specific disease is called a syndrome.
  3. Communicable diseases are transmitted directly or indirectly from one host to another.
  4. A contagious disease is one that is easily spread from one person to another.
  5. Noncommunicable diseases are caused by microorganisms that normally grow outside the human body and are not transmitted from one host to another.

 

The Occurrence of a Disease

 

  1. Disease occurrence is reported by incidence and prevalence.
    1. Incidence – Number of people contracting the disease
    2. Prevalence – Number of cases at a particular time
  2. Diseases are classified by frequency of occurrence:
    1. Sporadic – occurs occasionally (typhoid fever in the U.S.)
    2. Endemic – constantly present in a population (common cold)
    3. Epidemic – many cases in a given area in short period (influenza)
    4. Pandemic – a world wide epidemic (influenza occasionally, AIDS might be considered pandemic)

 

The Severity or Duration of a Disease

 

1.     The scope of a disease can be defined as:

a.     Acute – develops rapidly but lasts a short time (influenza)

b.     Chronic – develops more slowly, reactions to the disease are less severe, likely to be continual or recurrent for long periods (infectious mononucleosis, TB, hepatitis B)

c.     Subacute – intermediate between acute and chronic (sclerosing panencephalitis)

d.     Latent – the causative agent remains inactive for a time but then becomes active to produce symptoms (shingles)

2.     Herd immunity is the presence of immunity to a disease in most of the population.

 

The Extent of Host Involvement

 

  1. A local infection affects a small area of the body; a systemic infection is spread throughout the body via the circulatory system.
  2. A secondary infection can occur after the host is weakened from a primary infection.
  3. An inapparent, or subclinical, infection does not cause any signs of disease in the host.

 

Patterns of Disease

 

Predisposing Factors

 

  1. A predisposing factor is one that makes the body more susceptible to disease or alters the course of a disease.
  2. Examples include gender, climate, age, fatigue, and inadequate nutrition.

 

The Development Of Disease

 

  1. The incubation period is the time interval between the initial infection and the first appearance of signs and symptoms.
    1. Prodromal period - characterized by the appearance of the first mild signs and symptoms.
    2. Period of illness - the disease is at its height, and all disease signs and symptoms are apparent.

                                                    i.     Death may occur during the period of illness.

                                                   ii.     Crisis is the phase of fever characterized by vasodilation and sweating (the body is trying to return to normal temperature, the fever is breaking).

    1. Period of decline - the signs and symptoms subside.
    2. Period of convalescence - the body returns to its prediseased state, and health is restored.

 

The Spread of Infection

 

Reservoirs of Infection

 

  1. A continual source of infection is called a reservoir of infection.
  2. Human reservoirs of infection:
    1. People who have a disease.
    2. People who are carriers of pathogenic microorganisms.
  3. Zoonoses are diseases that affect wild and domestic animals and can be transmitted to humans (Table 14.2).
  4. Nonliving reservoirs include soil (fungi, C. botulinum, C.; tetani), water (V. cholerae, S. typhi, protozoans, algae) and improperly prepared or stored foods (trichinosis and salmonellosis).

 

The Transmission Of Disease

 

  1. Direct contact involves close physical contact between the source of the disease and a susceptible host (sexual contact falls into this category, n’est-ce pas?).
  2. Indirect contact involves transmission by fomites (inanimate objects).
  3. Droplet transmission is transmission via saliva or mucus in coughing or sneezing.
  4. Common vehicle transmission is transmission by a medium such as water, food or air.
  5. Airborne transmission refers to pathogens carried on water droplets or dust for a distance greater than 1 meter.
  6. Mechanical transmission is the passive transport of a pathogen on a vector ‘s feet or other body parts.
  7. Biological transmission involves reproduction of the pathogen in the vector and transmission in saliva or feces.
    1.  Arthropod vectors carry pathogens from one host to another by both mechanical and biological transmission.

 

Portals of Exit

 

  1. Just as pathogens have preferred portals of entry they also have definite portals of exit.
    1. The respiratory tract via coughing or sneezing.
    2. The gastrointestinal tract via saliva or feces.
    3. The urogenital tract via secretion from the vagina or penis.
    4. Wounds
    5. Arthropods
    6. Syringes

 

Nosocomial (Hospital-Acquired) Infections

 

  1. A nosocomial infection is any infection that is acquired during the course of stay in a hospital, nursing home, or other health care facility.
  2. About 5-15% or all hospitalized patients acquire nosocomial infections.
  3. Three factors contribute to nosocomial infections:
    1. Microorganisms in the hospital.
    2. A compromised host.
    3. The chain of transmission

 

Microorganisms In The Hospital

 

  1. Certain normal microbiota are often responsible for nosocomial infection when they are introduced into the body through such medical procedures as surgery and catheterization.
  2. Opportunistic, drug-resistant gram-negative bacteria are the most frequent causes of nosocomial infections.

 

The Compromised Host

 

  1. Patients with burns, surgical wounds, and suppressed immune systems are the most susceptible to nosocomial infections.

 

The Chain Of Transmission

 

  1. Nosocomial infections are transmitted by direct contact between staff members and patients and between patients.
  2. Fomites such as catheters, syringes, and respiratory devices can transmit nosocomial infections.

 

The Control Of Nosocomial Infections

 

  1. Aseptic techniques can prevent nosocomial infections.
  2. Hospital infection control staff members are responsible for overseeing the proper cleaning, storage, and handling of equipment and supplies.

 

Emerging Infections Diseases

 

  1. New diseases and diseases with increasing incidences are called emerging infectious diseases (EIDs).
  2. EIDs can result from the use of antibiotics and pesticides, climatic changes, travel, the lack of vaccination, and insufficient case reporting.
  3. The CDC, NIH, and WHO are responsible for surveillance and responses to emerging infectious diseases.

 

Epidemiology

 

  1. The science of epidemiology is the study of the transmission, incidence, and frequency of disease.
  2. Modern epidemiology betgan in the mid-1800’s with the works of Snow, Semmelweis, and Nightingale.
  3. Descriptive epidemiolgy - data about infected people are collected and analyzed.
    1. Relevant information includes information about:

                                                    i.     Affected individuals.

                                                   ii.     The place the disease occurred.

                                                 iii.     The period in which the disease occurred.

    1. Retrospective studies – done after the episode has ended, backtracks to the cause and source of the disease.
    2. Prospective studies – a group of people who are free of a particular disease are chosen and their subsequent disease experiences recorded for a given period of time.
  1.  Analytical epidemiology – a group of infected people is compared with an uninfected group.
    1. Cross-sectional – all data is collected at one time

                                                    i.     Provides information about prevalence

    1. Case-control method (retrospective) – a group of people who have the disease is compared with another group of people who are free of the disease to determine factors that might have preceded the disease.

                                                    i.     Matched by age, sex, socioeconomic status, location, etc.

                                                   ii.     Statistics compared to determine which factors might be responsible (genetic, environmental, nutritional, etc.).

                                                 iii.     Can’t determine incidence.

                                                 iv.     Can calculate odds ratio – if someone has the disease they are X times more likely to have the predisposing factor.

    1. Cohort method (prospective)– compare a population that has had contact with the agent causing the disease and another group that has not.

                                                    i.     Incidence can be calculated.

                                                   ii.     Relative risk can be calculated – if someone has the predisposing factor (or exposure) they are X times more likely to contract the disease.

  1. Experimental epidemiology – controlled experiments are designed to test hypotheses are performed.
    1. Clinical Drug Trials

                                                    i.     Blind studies

                                                   ii.     Double blind studies

  1. Case reporting provides data on incidence and prevalence to local, state, and national health officials.
    1. Establishes the chain of transmission.
  2. The Centers for Disease Control and Prevention (CDC) is the main source of epidemiologic information in the United States.
  3. The CDC publishes the Morbidity and Mortality Weekly Report to provide information on morbidity (incidence) and deaths (mortality).
    1. Mortality rate is the number of people affected by a disease in a given period of time (incidence) in relation to the total population.
    2. Mortality rate is the number of deaths resulting from a disease in a population in a given period of time in relation to the total population.