Lecture 4: Pathogenesis & Diagnosis of Infectious Diseases

Pathogen

A microbe that causes disease

Pathogenicity

The ability of a microorganism to cause disease

Pathogenesis

The development of a disease

Colonization

The microbe's initial invasion into the host

Infection

Multiplication of a pathogen --> leads to disease

Infectious diseases

Diseases that are caused by the pathogen directly invading the body's tissues

Microbial intoxication

A disease that results from the ingestion of a toxin that was produced by a pathogen

How does location of pathogens prevent infection?

Microbes may land in a place they cannot enter the body (such as intact skin)

How do natural antimicrobial factors prevent infection?

Enzymes in tears and saliva can destroy bacterial cell walls

Microbial antagonism

Bacteria in the human microbiome bacteria can inhibit the growth of pathogens

How do phagocytic cells prevent infection?

Phagocytic cells eat pathogens before they can invade tissues

How does overall health prevent infection?

Immunocompromised persons are more susceptible to infections

How can people acquire immunity to pathogens?

Through previous exposure and vaccines

Incubation period

The period between initial infection and the appearance of signs and symptoms

Prodromal period

The period in which vague signs and symptoms appear

Period of illness

The period in which the disease is the most severe (characterized by inflammation and tissue damage)

Convalescent period

Recovery from infection

Signs

Objective changes a clinician can observe and measure

Symptoms

Subjective characteristics of disease felt only by the patient

Localized infection

An infection that occurs in a specific location in the body and has local symptoms

Systemic infection

An infection that spreads throughout the whole body

Primary infection

Initial infection

Secondary infection

An opportunistic infection after a primary infection

Steps of pathogenesis

1. Entry

2. Attachment to tissue

3. Multiplication

4. Invasion/spread of pathogen

5. Evasion of immune system

6. Damage to host tissues

Virulence

The ability of a microbe to produce disease

Virulence factors

Traits of a microbe that allow it to complete pathogenesis

Adhesion factors

Structures that allow the pathogen to attach to the host cell

Adhesins (ligand)

Molecules found on the pathogen's surface that enable it to recognize and bind to host cells

How are pili used to promote infection?

Pili allow bacteria to anchor themselves to body tissues

Invasion factors

Allow pathogens to break into host cells and evade barriers

Exoenzymes

Enzymes secreted by pathogens that break down and inflict damage on tissues

Evasion factors

Allow pathogens to evade the host's immune system

Capsules

A membrane around bacterium that prevent them from being eaten by phagocytosis

Antigenic variation

Bacterium can alter their surface proteins in order to hide from the immune system

Drifts

Minor antigenic variations

Shifts

Major antigenic variations

Endotoxins

A toxin that is part of the cell walls of gram-negative bacteria wall --> released only when bacteria die and their cell walls break down

Septicemia

Infection in the bloodstream

Exotoxins

Toxic substances that pathogens secrete into their environment

Enterotoxins

Exotoxins that affect cells lining the gastrointestinal tract

Neurotoxins

Exotoxins that affect the nervous system

Opisthotonus

Abnormal posture seen in severe tetanus --> the back becomes extremely arched due to muscle spasms

Clinical specimens

Collected from patients and used to diagnose/follow the progression of infectious diseases

What happens when specimens are improperly selected, collected, or transported?

The disease may not be correctly identified

Appropriate specimen

Healthcare providers must ensure that they take a specimen from the appropriate place

When should the specimen be collected?

Before antimicrobial therapy

Where should the specimen be collected?

In a sterile, disposable container

How can the specimen be protected?

Labeling a specimen and placing it in the correct temperature can prevent it from being damaged/contaminated

Bacteremia

The presence of bacteria in the blood

Septicemia vs bacteremia

Bacteremia is the presence of bacteria in the blood, septicemia is the presence and multiplication of bacteria in the blood

Phlebotomy

Incision into a vein (the technical term for drawing blood)

Why is urine a good specimen?

Urine is almost always sterile!

Clean-catch collection

The external urethra is cleaned with an antiseptic wipe --> removing the resident microbiota

Midstream collection

Urine sample collected in the middle of a flow of urine

Cerebrospinal fluid

The fluid in and around the brain and spinal cord

Spinal tap

A procedure in which a needle is inserted between L3 and L4 in order to withdraw CSF

STAT

Immediately

Sputum

Mucous expelled from the lungs by coughing

When should a sputum specimen be collected?

First thing in the morning, as mucous builds up over night

What are throat swabs collected for?

They are used to test for acute pharyngitis

Pharyngitis

Inflammation of the pharynx (sore throat)

How are specimens collected from shallow wounds?

Using swabs

How are specimens collected from deep wounds?

Using needles

Why is it important to process fecal specimens quickly?

A decrease in temperature could kill some pathogens

Clinical health

Deals with personal healthcare issues and treatment (at the individual level)

Public health

Deals with healthcare issues and treatment on a community/societal level

Epidemiology

The study of the incidence, spread, and control of diseases that affect large numbers of people

Distribution of disease

HOW a disease spreads in a population (who, when, where)

Determinants of disease

Factors that influence the likelihood of disease

Descriptive epidemiology

Epidemiology concerned with the investigation of the distribution of disease

Analytic epidemiology

Epidemiology concerned with the investigation of the determinants of disease

Sir Austin Bradford Hill

British epidemiologist who came up with a set of criteria to determine whether exposure causes a specific disease

Strength of association

How strong is the relationship? strong? moderate? weak?

Consistency

Have other studies consistently found a similar association?

Specificity

Does one exposure lead to one disease?

Temporality

Does the exposure come before the disease?

Biological gradient

Does the disease severity increase with increasing levels of exposure?

Plausibility

Does the association make sense in the light of current biological knowledge?

Coherence

Do new findings align with known facts about the disease?

Experimentation

When someone is no longer exposed, does the disease resolve?

Analogy

Has a similar relationship been observed between similar exposures and similar diseases?

Risk ratio

Incidence

The number of new cases of a disease

Prevalence

The percentage of people within a population who have a specific disease

Mortality

Death rate

Endemic

A disease that has a steady incidence rate in a population over time

Epidemic

A regional outbreak of a disease

Pandemic

An epidemic that is geographically widespread

Epidemiologic triad

Host, agent, environment

Chain of infection

1. Infectious agent

2. Reservoir

3. Portal of exit

4. Mode of transmission

5. Portal of entry

6. Susceptible host

Reservoir

The habitat in which the agent lives, grows, and multiplies

Living reservoirs

Humans, animals, insects

Non-living reservoirs

Soil, water, food

Carriers

People who are colonized with a particular pathogen (which is not currently causing an infection) but who transmit it to others

Zoonotic disease

A disease that can be passed between animals and humans

Vectors

Organisms that carry pathogens from person to person

Fomites

Inanimate objects that transfer pathogens from person to person

Direct contact

Touching or coming in contact with a infected person's blood or saliva

Indirect contact

Exposure/transmission of a disease through droplets, vectors, food/water, or fomites