Download Infectious disease

Chapter 12
Mechanisms of Infectious
Disease
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Terminology Involved in the Study of
Infectious Disease (cont.)
• 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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Terminology Involved in the Study of
Infectious Disease (cont.)
• 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.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Agents of Infectious Disease
• Prions
• Viruses
• Bacteria
• Rickettsiaceae
• Chlamydiaceae
• Fungi
• Parasites
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Microorganisms
• Eukaryotes (Fungi)
– Contain a membrane-bound nucleus
• Prokaryotes (Bacteria)
– The nucleus is not separated.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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)
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
• Which of the following pathogens does not fit the typical
description of an organism?
− A. Viruses
− B. Bacteria
− C. Rickettsiaceae
− D. Chlamydiaceae
− E. Fungi
− F. Parasites
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
• A. Viruses
• Rationale: Viruses have no organized cellular structure.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Classification of Infectious Disease
• Incidence
• Portal of entry
• Source
• Symptoms
• Disease course
• Site of infection
• Virulence factors
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Portals of Entry
• Penetration
• Direct contact
• Ingestion
• Inhalation
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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)
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Disease Course in Infection
• Incubation period
• Prodromal stage
• Acute stage
• Convalescent stage
• Resolution stage
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Factors Influencing the Site of an
Infectious Disease
• Type of pathogen
• Portal of entry
• Competence of the host’s immunologic defense system
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
• Is the following statement true or false?
• The symptoms of an infection are always obvious and
apparent.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
• False
• Rationale: Symptoms may be covert or nonspecific in
presentation.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Types of Antimicrobial Agents
• Antibacterial agents
• Antiviral agents
• Antifungal agents
• Antiparasitic agents
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Drug Resistance
• Bacterial resistance
mechanisms
• Antiviral resistance
mechanisms
– Inactivate antibiotics
– Nuceoloside analogs
– Genetically alter
antibiotic binding sites
– Protease inhibitors
– Bypass antibiotic
activity
– Changes in the
bacterial cell wall
• Need for combination or
alternating therapy with
multiple antiretroviral
agents
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Techniques for Laboratory Diagnosis of an
Infectious Agent
• Culture
• Serology or detection of characteristic antigens
• Genomic sequences or metabolites produced by the
pathogen
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Categories of Virulence Factors
• Toxins
• Adhesion factors
• Evasive factors
• Invasive factors
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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).
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
• Is the following statement true or false?
• Surgical therapy is used in tandem with antibiotic
treatment in some cases of severe infection.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
• True
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Weapons of Bioterrorism (cont.)
• 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
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins