Pathogenicity, Spread, and Transmission of Infectious Diseases Flashcards
Disease Triad - three components
- Host
- Environment
- Pathogen
Pathogenicity
The ability of an organism to cause disease
Definitions!
Virulence
Infectivity
Infectious Dose
Virulence: the ability to cause hot cell damage (degree of pathogenicity)
Infectivity: the ability to infect and colonize a host (infectivity is measured as infectious DOSE)
Infectious Dose: the minimum number of pathogens that will initiate an infection
There is an inverse relationship between ______ and ________
Infectious dose & virulence
Virulence
LD 50 vs ID 50
Can be estimated from experimental studies of determining:
50% Lethal Dose (LD 50) - the number of microorganisms needed to kill half of the hosts that are exposed to the pathogen
50% Infectious Dose (ID 50) - the number of microorganisms needed to cause infection in half of the exposed hosts
Virulence of an organism is determined by its ability to produce various ____ factors
Virulence factors
Attenuation
The decrease or loss of virulence
Virulence Factors:
- Adherence
- Evasiveness
- Invasiveness
- Production of Toxins
- Adherence: the attachment of the microorganism to body cells
- Evasiveness: ability to evade host defence mechanisms
- Invasiveness: ability to invade host tissues
- Production of Toxins: ability to produce/release substances that can cause tissue damage
Adhesive Mechanisms
Adhesion is the initial event in the pathogenesis of many infections when they attach to body cells
Attachment = a specific reaction b/w surface receptors & adhesive structures on the surface of microorganisms
Examples of Adhesive Mechanisms
A) Fimbriae/Pili B) Flagella C) Capsule D) Spikes E) Exosomes by Parasites F) Pseudohyphae by Fungi
Invasiveness
The ability of organism to spread in a host tissue after establishing infection
Less invasive organisms = localized lesion
Highly invasive organisms - generalized infection
This is aided by the production of extracellular substances = invasins
-Most invasins are enzymes that act locally to damage host cells and/or have the immediate effect of facilitating the growth & spread of the pathogen
Toxigenicity
The ability of microorganisms to cause disease as determined by the toxin it produces which partly determines its virulence
EXOtoxins
ENDOtoxins
Exotoxins = a potent toxic substance formed and secreted by species of certain bacteria, outside body
Endotoxins = a complex bacterial toxin that is composed of protein, lipid, and LPS which is released only upon lysis of the cell (gram-negative bacteria dies)
Exotoxins
Action is enzymatic and has specific tissue affinity
Named according to what they do (ex. coagulase prevents phagocytosis by forming fibrin barrier around bacteria, fibrinolysin promotes the spread of infection by breaking down the fibrin barrier in tissues)
And according to where they act (cytotoxins)
Examples of Effects of Exotoxins
- Promotion of bacterial survival/spread
- Damage to host cell membrane
- Disruption of of cell metabolism
- Blockage of nerve-impulse transmission (neurotoxins)
Example = Botulinum Toxin (decreases secretion of Ach)
Endotoxins
Endotoxins = lipid portion of LPS
Released when gram-negative bacteria die & their cell walls undergo lysis, thus liberating the endotoxin
Exert their effects by stimulating macrophages to release cytokines in very high concentrations
Biological activities causing:
- Fever
- Muscle proteolysis
- Uncontrolled intravascular coagulation (DIC)
- Shock
Microbial Pathogenicity Damage to Host
Direct vs Indirect Damage
Direct Damage= tissue damage from disease process (toxins, enzymes)
Indirect Damage= tissue reactions from immunopathological response
Ways in which BACTERIA cause tissue damage?
a. Inflammatory reactions d/t extra/intra cellular multiplication
b. Tissue changes/damage due to toxins
- -Endotoxins
- -Exotoxins
Ways in which VIRUSES cause tissue damage?
- Inhibit/alter host cell metabolism
- Cause host cell lysis due to replication cycle
- Cause host cell lysis due to immune action
- Inflammatory reactions
- Damage to immunocytes
- Neoplastic change
Ways in which FUNGI cause tissue damage?
Fungi are extracellular digesters
ENZYMES and metabolic by-products cause/contribute to tissue damage
- -Zymosan is a ligand found on the surface of fungi, like yeast (endotoxin-like effects)
- Dimorphism
Ways in which PARASITES cause tissue damage?
- Mechanical obstruction
- Competition with host for resources
- Proteases: secretion of enzymes
- Immunological reactions
Acute Disease
Subacute Disease
Chronic Disease
Acute= disease in which symptoms and signs develop rapidly and that runs its course quickly (LESS THAN 3 weeks)
Subacute - disease with time course & symptoms between acute & chronic (b/w 3-8 weeks)
Chronic= disease with usually mild symptoms that develop slowly and last a long time (MORE THAN 8 weeks)
Latent Disease
Disease that appears a long time after infection
Local Infection
Systemic Infection
Focal Infection
Local = infection confined to a small region of the body Systemic = widespread infection of many systems Focal = infection that serves as a source of pathogens for infections at other sites in the body
Primary Infection
Secondary Infection
Primary - initial infection within a given patient
Secondary - infection that follow a primary infection; often by opportunistic pathogens
6 Steps in the Chain of Infection
- Infectious Agent
- Reservoir
- Way out of the body
- Method of transmission
- Person at risk
- Way into the body
- Infectious Agent
The germ AKA the micro-organism that can cause infection
Ex. bacteria, viruses, fungi & parasites
Koch’s Postulates (4)
- The suspected causative agent must be absent from all healthy organisms, but present in all diseased organisms
- The causative agent must be isolated from the diseased organism and grown in pure culture
- The cultured agent must cause the same disease when inoculated into a healthy, susceptible organism
- The same causative agent must then be re-isolated from the inoculated, diseased organism
Limitations to Koch’s Postulates (3)
- Pathogens can be isolated from patients who are NOT manifesting symptoms (asymptomatic carriers)
- Some pathogens that cannot be cultivated on existing media
- Exposures to pathogens do not always result in disease in all hosts
- Reservoir
The place where micro-organisms can thrive & reproduce
Nosocomial
Zoonotic Diseases
Nosocomial - acquired by patients while in health care facilities such as hospitals and clinics
Zoonotic Diseases - caused by infections that are shared between animals and people
- Way Out of The Body
Portal of exit, how the micro-organism exits the reservoir
- Methods of Transmission
Transmission = how a microorganism is moved from one site to another
Modes of Disease Transmission
CONTACT
Direct - directly transferred from an infected person to another person
Indirect - microbes transferred through contaminated intermediate objects
VEHICLE
Airborne, Waterborne, Food Borne, Blood Borne
VECTOR
Mechanical (on insect bodies)
Biological (eg. lice, mites, mosquitos, tics)
Horizontal vs. Vertical Transmission
Horizontal = transmission from one individual to another in the same generation
Vertical = transmission from an individual to its offspring
-During delivery, transplancental, in breastmilk
- Person at Risk
Anyone, however, some people are particularly susceptible to infection
Immunocological Status
Factors that affect the body’s natural ability to fight infection:
- Presence of underlying disease
- Immunocompromised status
- Nutritional status
- Age (very young & very old)
- Stress
- Way Into the Body
Portal of entry is how the microorganism gets into the body
- Ingestion
- Open wounds
- Inhaled
- Injection
- Tubes inserted into body cavities
