Principles of Disease and Epidemiology Notes

Ch. 14 - Principles of Disease and Epidemiology

Pathology

• Pathology is the scientific study of disease, encompassing:
  • Etiology: the cause of disease.
  • Pathogenesis: the manner in which the disease develops.
  • Changes caused by the disease and its final effects on the body.

Infection vs. Disease

• Infection: invasion or colonization of the body by pathogenic microorganisms.
• Disease: occurs when an infection causes a change from the normal state of health.

Normal Microorganisms

• Normal microbiota (flora): microorganisms that colonize the body without causing disease under normal circumstances.
• Transient microbiota: microorganisms that are present temporarily.
• Microbial antagonism: competition by normal microbiota that prevents disease or overgrowth of harmful organisms.
  • Direct competition for nutrients.
  • Production of bacteriocins: proteins that inhibit other bacteria of similar species.

Microbial Antagonism

• Microbial antagonism is competition between microbes.
• Normal microbiota protect the host by:
  • Occupying niches that pathogens might occupy.
  • Producing acids.
  • Producing bacteriocins.
• Probiotics: live microbes applied to or ingested into the body, intended to exert a beneficial effect (e.g., Lactobacillus spp).

Symbiosis

• Symbiosis: the host and microbiota living together.
  • Commensalism: one benefits, and the other isn't harmed.
  • Mutualism: both benefit.
  • Parasitism: the host is harmed.
• Opportunistic pathogens: organisms that do not normally cause disease in a healthy person but will cause disease if the person is weakened (e.g., from previous disease or immunosuppression such as cancer therapy).

Koch's Postulates

• Koch's Postulates are used to prove the cause of an infectious disease and consist of:
  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.
• Modifications to Koch's Postulates are necessary for unculturable diseases. Molecular methods or comparing cultures from different specimens for intracellular parasites can be used.
  • Example: same organisms from human into mouse, then observed as same again in diseased mouse
• Etiology - Koch's postulates:
  • The disease is reproduced in the second animal; microorganisms are isolated from this animal.
  • Pathogenic microorganisms are grown in pure culture.
  • Identical microorganisms are identified.

Classifying Infectious Diseases

• Symptoms: subjective changes in body function (e.g., pain, malaise - not observable).
• Signs: objective changes that can be observed and measured (e.g., lesions, fever, etc.).
• Syndrome: a group of symptoms and signs that may always accompany a particular disease.
• Communicable disease: disease that spreads from one host to another either directly or indirectly (e.g., genital herpes, influenza).
• Contagious disease: disease that spreads easily from person to person (e.g., Chickenpox).
• Noncommunicable disease: not spread from host to host (e.g., opportunistic infections caused by normal flora, or disease such as tetanus introduced from outside the body).

Occurrence of Disease

• Incidence: the fraction of a population that contracts the disease during a particular time period.
• Prevalence: fraction of a population having the disease at a specified time.
• Frequency:
  • Sporadic: occurs only occasionally.
  • Endemic: constantly present.
  • Epidemic: many people in a given area acquire a certain disease in a relatively short time period.
  • Pandemic: worldwide epidemic.

Classifying Diseases (cont.)

• Severity or Duration:
  • Acute: develops rapidly but lasts only a short time (flu).
  • Chronic: develops more slowly and often is less severe but can be continual or recurrent.
  • Latent: inactive for a time.
• Emerging Infectious Diseases: new or changing diseases showing an increased incidence in the recent past or a potential to increase in the near future. (Table 14.6, p. 419).

Classifying Diseases (cont.)

• Extent of Host Involvement - how much of the body is affected.
  • Local infection: limited to a relatively small portion of the body.
  • Systemic (generalized) infection: spread throughout the body.
  • presence of bacteria in the blood $=$ bacteremia, and if they multiply in the blood, septicemia.
  • toxemia is presence of toxins in the blood (tetanus), and viremia is viruses in the blood.
• Primary infection: an acute infection that causes the initial illness.
• Secondary infection: an opportunistic pathogen taking advantage of the weakened defenses due to the primary infection.

Spread of Infection

• Spread of Infection:
  • Reservoirs: source of infection, can be living or non-living.
    • Humans: transmit from person to person. Called carriers, some of which may not exhibit symptoms.
    • Animals: diseases transmitted to humans from animals are called zoonoses. Can occur through contact with the animal, with animal waste, ingestion of animal products, or by insect vectors.
    • Non-living: soil and water. (ex. fungi in soil, botulism and tetanus; water - cholera)

Spread of Infection (cont.)

• Transmission of Disease:
  • Contact transmission: spread of disease by direct or indirect contact, or by droplet transmission, also knows as person to person transmission. No intermediate. Ex. kissing, touching.
  • Indirect contact: spread to susceptible host through a non-living object, called a fomite.
  • Droplet transmission: mucus droplets spread a short distance (less than one meter) by talking, sneezing, coughing, etc.
  • Vehicle transmission: transmission of disease agents by a medium such as food, water or air (more than 1 meter)
  • Vector transmission: animals which carry pathogens from one host to another Ex

Nosocomial Infections

• Nosocomial Infections: does not show any evidence of being present or incubating at the time of admission to a hospital. (5-15% of all hospital patients acquire nosocomial infections)
• Factors:
  1. microorganisms in the hospital
  2. weakened state of the host
  3. chain of transmission in the hospital
     1. used to be mainly Gram-positive, but now mostly Gram- negatives
     2. impaired due to primary infection, drug or radiation therapy, burns or surgery
     3. can transfer patient to patient, staff to patient
• Control: preventative procedures, isolation, use of disposable or carefully sterilize equipment before reusing, monitoring procedures to trace causes.

Patterns of Disease

• Predisposing factors: situations or factors in the host making them susceptible to certain diseases
• Development of Disease:
  • Incubation period: time between infection and appearance of symptoms or signs.
  • Prodromal period: in some diseases, short period of mild symptoms.
  • Period of Illness: most acute. Exhibits overt signs and symptoms. If not successfully overcome, the patient dies during this period.
  • Period of Decline: signs and symptoms diminish, susceptible to secondary infections.
  • Period of Convalescence: regains strength and returns to prediseased state.

Epidemiology

• Epidemiology - study of when and where diseases occur and how they are transmitted in populations.
  • Descriptive epidemiology - collection of data that describe the occurrence of the disease under study.
  • Analytical epidemiology - analyzes a particular disease to determine its probable cause. (risk factors, etc.)
  • Experimental Epidemiology - makes hypothesis about a particular disease and experiments to test this hypothesis with a group of people.

Epidemiology (continued)

• Case Reporting - requires reporting of certain disease cases to the state and national level. Used to keep track of the spread of diseases.
• Centers for Disease Control and Prevention (CDC) - central source of epidemiological information in the U.S.
• Publishes Morbidity and Mortality Weekly Report.
  • Morbidity - incidence of specific notifiable diseases
  • Mortality - number of deaths from these diseases.