BHS 329 Chapter 5

infectivity

the ability of a pathogen to establish an infection in a host.

e.g. a highly infectious virus (like the common cold) spreads easily but may not cause severe illnesss

virulence

the relative capacity of a pathogen to cause damage or severe disease

e.g. a highly virulent pathogen (like ebola) causes severe, often fatal, illness but may be less easily transmitted

toxigenecity

the ability of the pathogen to produce harmful toxins that increase host cell and tissue damage

antigenicity

the level to which a pathogen is viewed by the host immune system as foreign

a more antigenic pathogen elicits a more prominent immune response

a less antigenic pathogen can readily elude immune mechanisms and continue to survive in the host

antigenic variability

a process of eluding the human host defenses and is often a result of altering the antigens present within or on the surface of the microorganism

(infectious organisms escaping defenses, responsible for much of the infectious disease burden in the world)

pathogenic defense mechanisms

the ways in which many pathogens have developed ways to avoid destruction by the host

e.g. thick protective capsules to prevent phagocytosis

confection

a phenomenon of hosting two or more pathogens simultaneously; presents a greater challenge to the immune system

superinfection

when an infection arises in addition to one that is already present; often results from compromised host defenses and over proliferation of resident flora (microorganisms that live within one’s body)

________ require the host for metabolism and reproduction

obligate parasites

_______ may live on the host but can also survive independently (e.g. Helminths)

facultative parasites

• single-celled

• most can reproduce outside of host cells

• those that require oxygen for growth are aerobic

• those that do not are anaerobic

bacteria

_________ _________ require oxygen for survival and for multiplication sufficient to cause disease

aerobic bacteria

anaerobic bacteria

• bacteria that do not require oxygen to survive

• thrive in deep tissues of the body where oxygen supply is limited

Why are anaerobic bacteria infections difficult to treat?

antimicrobial drugs often travel within the vascular system to affect local tissues

the cytoplasm of bacteria

cytosol

independent survival

bacteria can survive outside the human host and can infect and reinfect if not destroyed

structural properties critical to pathogenicity of bacteria

• independent survival,

• stimulation of inflammatory response

• bacterial capsule

• endotoxin

• endospores

Bacteria stimulate this response, resulting in destruction of the surrounding host tissue in an effort to rid the body of the invader

inflammatory response

bacterial capsule

• composed of chemicals not recognized as foreign

• slippery and difficult for phagocytes to engulf

a toxic component of the outer membrane of certain gram-negative bacteria that is released only when the bacteria die

complex of phospholipid-polysaccharide molecules that form the structural component of the gram-negative cell wall.

causes inflammatory mediators to be released

leading to a massive inflammatory response

→ septic shock, severe diarrhea, fever, and leukocytosis

endotoxin

endospores

• a thick-walled protective spore that forms inside a bacterial cell and resists harsh conditions

• survive in a latent state that is resistant to environmental extremes and lack of nutrients

• emerge when the environment is more conducive to replication

exotoxins

• many bacteria are capable of producing these, which results in host cell dysfunction or lysis

• potent bacterial-derived proteins released into surrounding tissues that cause local or systemic injury to the host → brain, GI tract, liver or blood

viruses

• obligate intracellular parasites (cannot replicate outside of the host cell)

• bind to specific receptors on the host cell and move into the host cell, converting the host cellular metabolism to nucleic acids and proteins that are encoded and controlled by it

• can directly kill cells or modify certain cellular functions

• can cause cells to proliferate rapidly and randomly, causing tumors to form in the body

virions

particles released by viruses outside of the cell which can enter and infect other nearby cells

Determined by

(1) the size of the virus that is inoculated into the body;
(2) the process of viral replication;
(3) the viral genotype;
(4) host susceptibility

chronic viral infections

latency

• characteristic of chronic viral infections

• a period of dormancy during which the virus is integrating itself into the host cell’s genetic material
  minimal or no loss of functional capabilities of the host cell

• low antigencity

• active viral replication can be triggered weeks to years after the initial inoculation
  

e.g. herpes simplex virus, herpes zoster virus

rickettsiae

• obligate intracellular parasites

• gram negative bacteria capable of producing energy

• target human epithelial cells of the blood vessels and capillaries

mycoplasmas

• lack cells walls and survive on the surface of host cells

  • do not enter host cell for replication

chlamydiae

• reproduce through binary fission yet are obligate intracellular parasites

• use the host metabolism to reproduce

• cause epithelial cell necrosis
  

fungi

• relatively large organisms compared to bacteria and viruses

• have a nuclear membrane, cytoplasm, and organelles

• unicellular forms are yeasts

• multicellular forms are molds

yeasts

produce by budding and form an elongated chain called pseudohypahe

mold

• colonies have tubules that branch to form hyphae;

• clusters of hyphae are called mycelium, mycoses

mycoses

infections with fungi

cause disease when the host’s defenses are compromised or when they grow in part of the body that is not natural to them

fungal infections can be these

opportunistic pathogens

protozoa

• unicellular, complex microorganisms characterized by an irregular or fluctuant shape without a cell wall

• many are motile

• some are parasites and capable of living independent of the host

infectious agents

Bacteria
Fungi
Viruses
Rickettsiae
Protozoa (sterilization)

reservoirs

people, equipment, water

how do you break the chain of infection at the reservoir stage?

sanitizing environment, antimicrobial drugs

portal of exit

excretions, secretions, skin, droplets

What are some direct methods used to block or control the portal of exit, preventing infectious agents from escaping a host and spreading to others?

Hand washing, covering secretions, trash disposal

Means of transmission

Direct contact, ingestion, formats, airborne

What are some ways to interrupt means of transmission and stop the spread of disease?

(hand washing, sterilization, universal precautions, airflow control, food handling, isolation)

Portal of entry

Mucous membrane, GI tract, GU tract, respiratory tract, broken skin

What are some ways to interrupt the portal of entry

Cover

susceptible hosts

Immunosuppression, diabetes, surgery, burns, old age

What are some ways to protect a susceptible host?

Reconition of high-risk patients, treatment of underlying diseases

vector transmission

transmission of an infectious agent by an insect, arthropod, or animal (a vehicle harbors the pathogen and carries it to the host)

prodrome

• subclinical illness

• disease present but few symptoms

  • low-grade fever, nausea, weakness, and generalized muscle aches

• described a feeling under the weather

incubation

• this phase extends from exposure to the onset of any signs or symptoms

• the individual has no idea that he or she has been exposed

• during this period, transmission of microorganisms to others is greatest (for chicken pox this period is 7-21 days from exposure to recognize symptoms)
  

convalescence

• this phase extends from waning clinical manifestations to full recovery from the disease

• fatigue is common during this period

septic shock

a process of systemic vasodilation due to severe infection, often with gram-negative bacteria (the endotoxin component)

influenza

• a viral infection of the epithelial cells of the airway; transmitted via respiratory droplets

• impairs cilia, mucus, and antibodies then moves into the cells and replicates

• well adapted to escape host defenses

• gradually changes genetic composition during replication in the human host cell in a process called reassortment

• viral offspring w/ altered antigenic properties result in ongoing host susceptibility to the virus

• tests have a false-positive or false-negative error rate of 20-30%

hepatitis

• damage to the hepatocytes inhibits bile production and can affect fat emulsification and absorption

• blood glucose fluctuations

Kupffer cells

• aged red blood cells are engulfed and destroyed by these phagocytes houses in the liver

• small capillaries perfuse the liver and are lined with these

• they readily engulf harmful substances

  • because of these, bacteria pose less harm to the liver than viruses

• separate hemoglobin into heme and globin

erthropoeisis

the formation of red blood cells

viral hepatitis

• acute or chronic liver disease

• generally caused by fecal-oral contamination leads to acute, blood and body fluid exposure lead to chronic

• chronic is represented by impaired liver function for more than 6 months

• the liver is infiltrated with macrophages and lymphocytes

• risk of developing hepatocellular carcinoma related to persistent cell injury

Clinical manifestations of viral hepatitis

• prodrome (fatigue, anorexia, malaise, HA, low-grade fever; lasts about 2 weeks)

• Icterus (onset of jaundice, dark urine, clay-colored stools 2 weeks after exposure to virus; lasts approximately 2-6 weeks)

• Recovery (resolution of jaundice around 8 weeks after initial exposure; signs/symptoms improve, but liver remains enlarged for an addition 1-4 weeks)

diagnostic criteria for viral hepatitis

• Detection of viral antibodies (anti-HAV, anti-HCV, anti-HDV, anti-HEV)

• HbsAg may be present in acute infection or in those who are chronic asymptomatic carriers

• bilirubin in the urine

• elevated serum bilirubin (>30 mg/dL means severe disease)

• prolonged clotting time

tuberculosis

• caused by an aerobic, rod-shaped bacterium

• primary site of infection is the lungs, but can affect any organ in the body

• upon encountering bacilli, alveolar macrophages ingest but are unable to destroy the bacteria

• bacilli proliferate within the macrophages, and they present the bacilli as antigens to the T lymphocytes (cell-mediated immunity)

• bacilli do not produce toxins; damage to the lung and other body tissues is through hypersensitivity reaction elicited by the bacilli

  • formation of a granuloma as a result of the inflammatory response (goon focus) with a center called a caseous necrosis that is pasty, yellow, and cheese-like

ghon complex

• the Ghon focus and additional granulomas that develop through the lymph channels

• scar tissue and calcification results in these

• small numbers of dormant bacteria can survive within the calcifications for years

cavitations

• area of necrosis that erode surrounding structures of the lungs, including

  • bronchioles, bronchi, and surrounding blood vessels

• secondary TB is marked by these in an aggressive and destructive form

UTI

Based on a urine culture noting greater than 1,000 colonies of a single organism per mL

bacterial meningitis

• Inflammation of the membranes of the brain and spinal cord

• most common bacterial cause: aerobic, gram negative bacterium

• 13 different types

• mode of transmission: respiratory droplets that enter the respiratory tract and attach to epithelial cells

• inflammatory and immune response is waged

• immunoglobulins in the respiratory mucosa can provide protective immunity

viral meningitis

• often referred to as aseptic meningitis

• generally less severe than other types

• often resolves without specific treatment

• fungi can result in meningitis in immunocompromised individuals

Kernig sign

• extension of the upper leg places a stretch on the meninges and causes pain

• patient is placed supine with knees bent and hips flexed and moves leg upward

• diagnostic criteria for meningitis

Brudzinski sign

• A positive sign of meningitis, where there is an involuntary flexion of the arm, hip, knee when the patients neck is passively flexed

• diagnostic criteria for meningitis

What is the treatment for bacterial meningitis

• Antibiotics (IV ASAP)

• Isolation (24–48 hr droplet precautions)

• Rehydration (careful fluids)

• Breathing support

• Reduce ICP (dexamethasone, mannitol)

• Alleviate stimulation

• Intervene for complications

  • Nurture/support the family
    
    AIRBRAIN

Tinea

• group of fungal skin diseases that occur in various locations

  • tine pedis (feet)

  • tinea unguium (nails)

  • tine capitis (scalp); use wood light to diagnose

  • tine cruris (groin)

  • tine corpori’s (ringworm) and versicolor (skin)

• tx is topical or oral anti-fungal agents (2-8 weeks)

• topical therapy ineffective for tx of hair and nails (dermatophyte-ski, hair, and nails)

• dermatophyte attaches to and produced thickening of keratinized cells

malaria

• Caused by infection with plasmodium protozoa transmitted by mosquitos

• plasmodium falciparum is the most lethal species responsible for 95% of deaths in sub-Saharan Africa

• plasmodia enter circulating RBCs and feed on hemoglobin and other proteins within the cells

• Infection typically develops within a month of exposure

• headache, shivering and chills, high fever, excessive sweating, cough fatigue, malaise and joint/muscle aching

• high fatality due to cerebral malaria

fungal meningitis

• type of meningitis that commonly occurs in immunocompromised

• mild sx