Chapter 10 - Mechanisms of Infectious Disease

Terminology Involved in the Study of Infectious Disease

  • Host — any organism capable of supporting the nutritional and physical growth requirements of another organism

  • Infectious disease — the disease state brought about by the interaction with another organism

  • Colonization — the presence and multiplication of a living organism on or within the host

  • Microflora — bacteria inhabiting exposed surfaces of the body

  • Virulence — the disease-inducing potential

Terminology Involved in the Study of Infectious Disease (continued)

  • Pathogens — microorganisms so virulent that they are rarely found in the absence of disease

  • Saprophytes — free-living organisms obtaining their growth from dead or decaying organic material from the environment

Types of Microbial Relationships

  • Mutualism — an interaction in which the microorganism and the host both derive benefits from the interaction

  • Commensalism — an interaction in which colonizing bacteria acquire nutritional needs and shelter but the host body not affected

  • Parasitic relationship — only the infecting organism benefits from the relationship

  • If the host sustains injury or pathologic damage, the process is called an infectious disease

Agents of Infectious Disease

  • Prions

  • Viruses

  • Bacteria

  • Rickettsiaceae

  • Chlamydiaceae

  • Fungi

  • Parasites

Viruses

  • Smallest pathogens

  • Have no organized cellular structure

  • Consist of a protein coat surrounding a nucleic acid core of DNA or RNA

  • Are incapable of replication outside a living cell

Microorganisms: Eukaryotes vs Prokaryotes

  • Eukaryotes (Fungi) — contain a membrane-bound nucleus

  • Prokaryotes (Bacteria) — the nucleus is not separated

Classification of Bacteria

  • According to the microscopic appearance

  • According to staining of the cell

    • Gram-positive organisms — stained purple by a primary basic dye (usually crystal violet)

    • Gram-negative organisms — not stained by the crystal violet but counterstained red by a second dye (safranin)

Parasites

  • Types — Protozoa, Helminths, Arthropods

  • Method of Infecting — These members of the animal kingdom infect and cause diseases in other animals; These animals then transmit disease to humans

Mixed Infections: Rickettsiaceae, Anaplasmataceae, Chlamydiaceae, Coxiella

  • Organisms that combine the characteristics of viral and bacterial agents to produce disease in humans

  • Obligate intracellular pathogens like the viruses

  • Produce a rigid peptidoglycan cell wall

  • Reproduce asexually by cellular division

  • Contain RNA and DNA similar to the bacteria

Question 1

  • Which of the following pathogens does not fit the typical description of an organism?

    • Viruses, Bacteria, Rickettsiaceae, Chlamydiaceae, Fungi, Parasites

Answer to Question 1

  • A. Viruses

  • Rationale: Viruses have no organized cellular structure

Classification of Infectious Disease Attributes

  • Incidence

  • Portal of entry

  • Source (mode of transmission)

  • Symptoms

  • Disease course

  • Site of infection

  • Virulence factors

Epidemiology (Terminology)

  • Epidemiology — the study of factors, events, and circumstances that influence the transmission of infectious diseases among humans

  • Incidence — the number of new cases of an infectious disease that occur within a defined population

  • Prevalence — the number of active cases at any given time

Incidence of Disease

  • Endemic disease — found in a particular geographic region; the incidence and prevalence are expected and relatively stable

  • Epidemic — abrupt and unexpected increase in the incidence of disease over endemic rates

  • Pandemic — spread of disease beyond continental boundaries

Portals of Entry

  • Direct contact

  • Ingestion

  • Inhalation

Source of an Infectious Disease

  • Endogenous or Exogenous

  • Location

    • Nosocomial — develop in hospitalized patients

    • Community acquired — acquired outside of health care facilities

  • Host — an object or substance from which the infectious agent was acquired; May be endogenous or exogenous

Symptomatology

  • Specific — reflects the site of infection (e.g., diarrhea, rash, convulsions, hemorrhage, pneumonia)

  • Nonspecific — can be shared by a number of diverse infectious diseases (e.g., fever, myalgia, headache)

  • Obvious — predictable patterns (e.g., chickenpox and measles)

  • Covert — may require laboratory testing to detect (e.g., hepatitis or increased white blood cell count)

Disease Course in Infection

  • Incubation period

  • Prodromal stage

  • Acute stage

  • Convalescent stage

  • Resolution stage

  • See Fig. 10.6 (referenced in the source)

Factors Influencing the Site of an Infectious Disease

  • Type of pathogen

  • Portal of entry

  • Competence of the host’s immunologic defense system

Question 2

  • Is the following statement true or false?

    • The symptoms of an infection are always obvious and apparent.

Answer to Question 2

  • False

  • Rationale: Symptoms may be covert or nonspecific in presentation

Types of Antimicrobial Agents

  • Antibacterial agents (see Table 10.4)

  • Antiviral agents

  • Antifungal agents

  • Antiparasitic agents

Drug Resistance

  • Bacterial resistance mechanisms

    • Inactivate antibiotics

    • Genetically alter antibiotic binding sites

    • Bypass antibiotic activity

    • Changes in the bacterial cell wall

  • Antiviral resistance mechanisms

    • Nucleoside analogs

    • Protease inhibitors

  • Need for combination or alternating therapy with multiple antiretroviral agents

Intravenous Immunoglobulin and Cytokine Therapy

  • Supplementing or stimulating the host’s immune response so that the spread of a pathogen is limited or reversed

  • Pathogen-specific antibodies given to the patient as an infusion to facilitate neutralization, phagocytosis, and clearance of infectious agents above and beyond the capabilities of the diseased host

Criteria for Diagnosis of an Infectious Disease

  • The recovery of a probable pathogen or evidence of its presence from the infected sites of a diseased host

  • Accurate documentation of clinical signs and symptoms (symptomatology) compatible with an infectious process

Techniques for Laboratory Diagnosis of an Infectious Agent

  • Culture (see Fig. 10.7)

  • Serology or detection of characteristic antigens

  • DNA and RNA sequencing (see Fig. 10.8)

Categories of Virulence Factors

  • Toxins

  • Adhesion factors

  • Evasive factors

  • Invasive factors (see Table 10.3)

Nonpharmacological Intervention

  • Surgical interventions

    • Providing access to an infected site by antimicrobial agents (drainage of an abscess)

    • Cleaning of the site (debridement)

    • Removing infected organs or tissue (e.g., appendectomy)

Question 3

  • Is the following statement true or false?

    • Surgical therapy is used in tandem with antibiotic treatment in some cases of severe infection.

Answer to Question 3

  • True

Antibiotic Mechanisms

  • Interference with a specific step in bacterial cell wall synthesis

  • Inhibition of bacterial protein synthesis

  • Interruption of bacterial nucleic acid synthesis

  • Interference with normal bacterial metabolism

Classification of Antibiotic Action

  • Bactericidal — if it causes irreversible and lethal damage to the bacterial pathogen

  • Bacteriostatic — if its inhibitory effects on bacterial growth are reversed when the agent is eliminated

Classification and Target Site of Antibacterial Agents

  • Penicillins — cell wall

  • Cephalosporins — cell wall

  • Monobactams — cell wall

  • Aminoglycosides — ribosomes

  • Tetracyclines — ribosomes

  • Macrolides — ribosomes

  • Sulfonamides — folic acid synthesis

  • Glycopeptides — ribosomes

  • Quinolones — DNA synthesis

Weapons of Bioterrorism

  • Category A Agents — Plague, Tularemia, Smallpox, Hemorrhagic fever viruses

  • Category B Agents — Agents of food-borne and water-borne diseases; Agents of zoonotic infections

  • Category B Agents (continued) — Viral encephalitides; Toxins from castor bean

  • Category C Agents — Mycobacterium tuberculosis; Nipah virus and hantavirus; Tick-borne and yellow fever viruses; Cryptosporidium parvum

Global Infectious Diseases

  • The challenges associated with maintaining health throughout a global community are becoming increasingly apparent

  • Aided by a global market and the ease of international travel, the past decade has witnessed the importation and emergence of a host of novel infectious diseases

  • There is also the potential threat of the deliberate use of microorganisms as weapons of bioterrorism