Principles of Disease and Epidemiology: Microbiology Study Notes 14

Principles of Infection and Disease CH 14

Pathology, Etiology, and Pathogenesis

Understanding disease in microbiology involves studying its cause, development, and effects on the host.

• Pathology: The scientific study of disease, focusing on its nature and effects.

• Etiology: The study of the cause of a disease, often identifying the specific pathogen responsible.

• Pathogenesis: The mechanism by which a disease develops in the host.

• Infection: Colonization of the body by pathogens, which may or may not result in disease.

• Disease: An abnormal state in which the body is not functioning normally, often due to infection.

Normal Microbiota and the Host

Symbiosis and Microbial Relationships

Normal microbiota are microorganisms that are present in or on the body for days, weeks, or months without causing disease. Their relationship with the host can be classified as symbiotic.

• Symbiosis: The relationship between normal microbiota and the host, which can be:

  • Commensalism: One organism benefits, the other is unaffected.

  • Mutualism: Both organisms benefit.

  • Parasitism: One organism benefits at the expense of the other.

• Opportunistic Pathogens: Normal microbiota that can cause disease under certain conditions.

Microbial Antagonism and Competition

• Microbial antagonism: Competition between microbes that can prevent colonization by pathogens.

• Niche: The specific habitat that a pathogen might occupy.

Probiotics

• Probiotics: Live microbes applied to or ingested into the body, intended to exert a beneficial effect.

Koch's Postulates

Establishing Causation of Disease

Koch's postulates are a set of criteria used to establish a causative relationship between a microbe and a disease.

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 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.

Exceptions: Some pathogens cannot be cultured in artificial media or cause different symptoms in different hosts.

Classifying Infectious Diseases

Definitions and Transmission

• Symptoms: Subjective changes in body function felt by a patient as a result of disease.

• Signs: Objective changes that can be observed and measured.

• Syndrome: A specific group of signs and symptoms that accompany a disease.

• Communicable disease: A disease that is spread from one host to another.

• Contagious disease: A disease that is easily spread from one host to another.

• Noncommunicable disease: A disease that is not transmitted from one host to another.

Occurrence of a Disease

Frequency and Distribution

• Incidence: Number of people in a population that contract a disease during a specific time.

• Prevalence: Number of people in a population who develop a disease at a specified time, regardless of when it first appeared.

• Sporadic disease: Disease that occurs only occasionally in a population.

• Endemic disease: Disease constantly present in a population.

• Epidemic disease: Disease acquired by many people in a given area in a short time.

• Pandemic disease: Worldwide epidemic.

• Herd immunity: Immunity in most of a population.

Severity or Duration of a Disease

• Acute disease: Symptoms develop rapidly but last a short time.

• Chronic disease: Disease develops slowly.

• Latent disease: Disease with a period of no symptoms when the causative agent is inactive.

Extent of Host Involvement

• Local infection: Pathogens are limited to a small area of the body.

• Systemic (generalized) infection: An infection throughout the body.

• Focal infection: Systemic infection that began as a local infection.

• Sepsis: Toxic inflammatory condition arising from the spread of microbes, especially bacteria or their toxins, from a focus of infection.

• Bacteremia: Bacteria in the blood.

• Septicemia: Growth of bacteria in the blood.

• Toxemia: Toxins in the blood.

• Viremia: Viruses in the blood.

• Primary infection: Acute infection that causes the initial illness.

• Secondary infection: Opportunistic infection after a primary (predisposing) infection.

• Subclinical disease: No noticeable signs or symptoms (inapparent infection).

Reservoirs of Infection

Sources of Pathogens

• Human reservoirs: Carriers may have inapparent infections or latent diseases.

• Animal reservoirs: Zoonoses are diseases transmitted from animals to humans (e.g., rabies, Lyme disease).

• Nonliving reservoirs: Soil, water, and food can harbor pathogens (e.g., botulism, tetanus, cholera, typhoid fever).

Transmission of Disease

• Direct contact: Requires close association between infected and susceptible host.

• Indirect contact: Spread by fomites (inanimate objects).

• Droplet transmission: Transmission via airborne droplets.

Healthcare-Associated Infections (HAIs)

• Acquired while receiving treatment in a healthcare facility.

• Also known as nosocomial infections.

• Result from microorganisms in the hospital environment, compromised host, and chain of transmission.

Emerging Infectious Diseases (EIDs)

• Diseases that are new, increasing in incidence, or showing a potential to increase in the near future.

• Contributing factors include genetic recombination, evolution of new strains, inappropriate use of antibiotics, and changes in weather patterns.

Epidemiology

Study of Disease in Populations

• Descriptive epidemiology: Collection and analysis of data concerning a disease.

• Analytical epidemiology: Comparison of a diseased group and a healthy group to determine probable cause, mode of transmission, and prevention.

• Experimental epidemiology: Involves a hypothesis and controlled experiments.

Centers for Disease Control and Prevention (CDC)

• Collects and analyzes epidemiological information in the United States.

• Publishes the Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Deaths from notifiable diseases.

• Morbidity rate: Number of people affected in relation to the total population in a given time period.

• Mortality rate: Number of deaths from a disease in relation to the population in a given time.

Key Epidemiological Terms Table

Term	Definition
Incidence	Number of new cases of a disease in a given area during a given period of time
Prevalence	Total number of cases of a disease in a given area during a given period of time
Endemic	Disease constantly present in a population
Epidemic	Disease acquired by many people in a given area in a short time
Pandemic	Worldwide epidemic

Example: John Snow and Cholera

• John Snow (1848-1849) mapped the occurrence of cholera cases in London, demonstrating the importance of descriptive epidemiology.

• Ignaz Semmelweis (1846-1848) showed that handwashing decreased the incidence of puerperal sepsis.

• Florence Nightingale (1858) demonstrated that improved sanitation decreased the incidence of epidemic typhus.

Additional info: These notes expand on the original content by providing definitions, examples, and context for key microbiological and epidemiological concepts relevant to the study of infectious diseases.