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Chapter 10: Mechanisms of Infectious Disease

Terminology Involved in the Study of Infectious Disease #1

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 #2

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

Terminology Involved in the Study of Infectious Disease #3

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 (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 are 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.

Rickettsiaceae, Anaplasmataceae, Chlamydiaceae, Coxiella

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

Are 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

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

Penetration

Direct contact

Ingestion

Inhalation

Source of an Infectious Disease

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., symptoms such as 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

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

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

Serology or detection of characteristic antigens

Genomic sequences or metabolites produced by the pathogen

Categories of Virulence Factors

Toxins

Adhesion factors

Evasive factors

Invasive factors

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 #1

Category A Agents

Plague

Tularemia

Smallpox

Hemorrhagic fever viruses

Category B Agents

Agents of food-borne and water-borne diseases

Agents of zoonotic infections

Weapons of Bioterrorism #2

Category B Agents (cont.)

Viral encephalitides

Toxins from castor bean

Category C Agents

Mycobacterium tuberculosis

Nipah virus and hantavirus

Tick-borne and yellow fever viruses

Cryptosporidium parvum