Pathophysiology – Chapter 14: Infection, Infectious Diseases, and Epidemiology
I. Symbiotic Relationships Between Microbes and Their Hosts
A. Symbiosis means “to live together”
B. We have symbiotic relationships with countless microorganisms
C. Types of symbiosis
1. mutualism – both benefit
a. bacteria in our colon
b. termites – protozoa/bacteria digest cellulose (wood)
2. commensalism – one benefits/ other not helped or harmed
- Staphylococcus epidermison skin
3. parasitism – one benefits/other harmed
a. examples
-tapeworm
-tuberculosis bacteria in lung
b. pathogen - any parasite causing disease
c. sometimes fatal
d. sometimes coevolution towards commensalism/ mutualism occurs
D. Normal microbiota in hosts
1. axenic – free of microbes
a. alveoli of lungs
b. central nervous system
c. circulatory system
d. upper urogenital region
e. uterus
2. also termed normal flora and indigenous microbiota
3. organisms that colonize the body’s surfaces without normally causing disease
a. 1 sq. cm of skin – 3 million bacteria
b. colon – 400-1000 types of bacteria live there
4. two types
a. resident microbiota
b. transient microbiota
5. resident microbiota
a. part of normal microbiota
b. most commensal
c. locations: -skin, upper respiratory tract, mucous membrane of digestive tract, distal portion of urethra, vagina
6. transient microbiota
a. remain in body for short period of time
b. found in same area as resident microbiota
c. can’t persist in body
-competition from other microorganisms
-elimination by body’s defense cells
-chemical or physical changes in body
7. acquisition of normal microbiota
a. fetal development in womb - free of microorganisms
b. microbiota begin to develop during birthing process
c. most of resident microbiota established during 1st months of life
E. How Normal microbiota become opportunistic pathogens
1. opportunistic pathogens
-normal microbiota that cause disease under certain circumstances
2. conditions that provide opportunities for pathogens
a. introduction of normal microbiota into unusual site in body
b. immune suppression
c. changes in normal microbiota
-microbial competition (antagonism)
-take lots of antibiotics
-all bacteria killed
-bacteria in vagina keep yeast in check
-bacteria dead – yeast infection results
II. Reservoirs of Infectious Diseases of Humans
A. Most pathogens can’t survive for long outside of host
B. Reservoirs of infection
-sites where pathogens are maintained as a source of infection
C. 3 types of reservoirs
1. animals
2. humans
3. nonliving reservoirs
D. Animal reservoirs
1. zoonoses
-diseases naturally spread from animal host to humans
2. acquire zoonoses through various routes
a. direct contact with animal or its waste
b. eating animals
c. bloodsucking arthropods
3. humans are usually dead-end host to zoonotic pathogens
4. examples: yellow fever, anthrax, Bubonic Plague, rabies
E. Human Carriers
1. infected individuals - asymptomatic but infective to others
2. some individuals eventually develop illness while others never get sick
3. healthy carriers may have defensive systems that protect them
4. examples: TB, syphilis, AIDS
F. Nonliving carriers
1. soil, water, and food
-presence of microorganisms often due to contamination by feces or urine
2. examples: botulism, tetanus
III. The movement of microbes into hosts: infection
A. Exposure to Microbes: Contamination and Infection
1. contamination
-presence of microbes in or on body
2. infection
-when organism evades body’s external defenses, multiplies, and becomes established in body
B. Portals of entry
1. sites through which pathogens enter body
2. four major pathways
a. skin
b. mucous membranes
c. placenta
d. parenteral route
3. skin
a. outer layer of skin – dead – acts as barrier to pathogens
b. pathogens enter through opening or cut
c. others burrow into or digest outer layers of skin
4. mucous membrane
a. lines body cavities that are open to environment
b. provide moist, warm environment – good for pathogens
c. respiratory tract most common site of entry – nose, mouth, eyes
d. gastrointestinal tract – must survive pH of stomach – acidic
5. placenta
a. typically forms effective barrier to pathogens
b. pathogens cross placenta and infect fetus
-can cause spontaneous abortion, birth defects, premature birth
6. parenteral route
a. not true portal of entry
b. means by which portal of entry can be circumvented
c. pathogens deposited directly into tissues beneath skin or mucous membranes
-nails, needles, thorns, etc.
C. Role of adhesion in infection
1. process by which microorganisms attach to cells
2. required to successfully establish colonies within host
3. uses adhesion factors
a. specialized structures
b. attachment proteins
4. attachment proteins
a. found on viruses and many bacteria
b. viral or bacterial ligands bind to host cell receptors
-interaction can determine host cell specificity
-example: gonorrhea – only sticks to cells lining urethra/vagina of humans
5. changing/blocking ligand or its receptor can prevent infection
6. inability to make attachment proteins or adhesins renders microorganisms avirulent
7. some bacterial pathogens attach to each other to form biofilm
a. web of cells and polysaccharides
b. dental plaques – bacteria digest carbohydrates on teeth and secrete acid that causes cavities
IV. The Nature of Infectious Disease
A. Infection - invasion of the host by pathogen
B. disease results if invading pathogen alters normal body functions
C. disease is also referred to as morbidity
D. Manifestations of disease: symptoms, signs and syndromes
1. symptoms
a. subjective characteristics of disease FELT only by patient
b. example: fatigue, pain, nausea, headache, chills, etc.
2. signs
a. objective manifestations of disease observed or measured by others
b. example: fever, swelling, rash, redness, vomiting, shivering, etc.
3. syndrome
a. symptoms and signs – characterize a disease or abnormal condition
b. example: AIDS – characterized by malaise, loss of T cells, diarrhea, weight loss, pneumonia,
toxoplasmosis, TB
4. asymptomatic or subclinical
-infections lack symptoms but may still have signs of infection
E. causation of disease: etiology
1. study of the cause of disease
2. germ theory of disease
-disease caused by infections of pathogenic microorganisms
3. Robert Koch
a. developed Koch’s postulates
b. how to prove a particular pathogen causes a particular disease
-suspected agent (bacteria, virus, etc.) must be present in every case of disease
-agent must be isolated and grown in pure culture
-cultured agent must cause the disease when it is inoculated into a healthy susceptible
experimental host
-same agent must be reisolated from the diseased experimental host
4. exceptions to Koch’s postulates
a. some pathogens can’t be cultured in lab - leprosy
b. diseases caused by combination of pathogens and other cofactors
c. pathogens that require human host
5. difficulties in satisfying Koch’s postulates
a. not possible to establish single cause for infection
b. some pathogens ignored
F. Virulence factors of infectious agents
1. pathogenicity
-ability to cause disease
2. virulence
a. degree or measure of pathogenicity
b. virulence factors –traits that help pathogen enter/adhere to host, get nutrients,
escape detection/removal by immune system (enhance ability to infect/cause disease)
-adhesion factors
-biofilms
-extracellular enzymes
-toxins
-antiphagocytic factors
3. extracellular enzymes
a. secreted by pathogen
b. dissolve structural chemicals in body
c. help pathogen maintain infection, invade, and avoid body defenses
d. examples:
-hyaluronidase and collagenase – allows bacteria to move deeper in tissues by creating pathway between cells and breaks down collagen - the body’s chief structural protein
-coagulase – blood clots and bacterial hide in clot
-kinase – dissolve clots releasing bacteria
4. toxins
a. chemicals that harm tissues or trigger host immune responses that cause damage
b. Toxemia refers to toxins in bloodstream that are carried beyond site of infection
c. 2 types
-exotoxins – secreted by pathogen into environment - destroyshost cell or interferes
with host’s metabolism
-cytotoxins, neurotoxins, enterotoxins
-cause gangrene, botulism, tetanus
-antitoxins – secreted by body - neutralize toxins
-can stimulate body’s production of antitoxins with injection of toxoid (toxins made nontoxic)
-endotoxins
-also called Lipid A – part of cell wall of gram negative bacteria
-released from cell wall into environment upon division or digestion/death of pathogen
-can trigger vasodilation, fever, inflammation, shock, blood clotting, diarrhea,
hemorrhaging
5. antiphagocytic factors
a. factors prevent phagocytosis by host’s phagocytic cells
-bacterial capsule
-composed of chemicals not recognized as foreign
-slippery
b. antiphagocytic chemicals
-prevent fusion of lysosome and phagocytic vesicles
-leukocidins directly destroy phagocytic white blood cells
G. The stages of infectious disease
1. disease process occurs following infection
2. many infectious diseases have 5 stages following infection
a. incubation period
-time between infection and 1st symptoms/signs of disease
b. prodromal period
-short period of time – symptoms mild
c. illness
-most severe stage
-signs/symptoms most evident
d. decline
-returning to normal
e. convalescence
-tissues repaired; patient recovers
V. The Movement of Pathogens Out of Hosts: Portals of Exit
A. Pathogens leave host through portals of exit
B. Many portals of exit same as portals of entry
C. Pathogens often leave hosts in secretions and/or excretions of body
1. secretions: earwax, tears, nasal secretions, saliva, sputum, respiratory droplets, vaginal
Secretions, semen, milk, blood (insect bites, needles, wounds)
2. excretions: feces, urine
VI. Modes of Infectious Disease Transmission
A. transmission is from a reservoir or portal of exit to another host’s portal of entry
B. 3 groups of transmission
1. contact transmission
a. direct
-person to person contact
-examples: herpes, warts, gonorrhea, syphilis, rabies, ringworm
b. indirect
-fomites: inanimate objects such as toothbrush, clothing, medical equipment, toys, $
-examples: hepatitis B, AIDS
c. droplet
-pathogens travel less than 1 meter such as speaking, coughing, sneezing
-examples: cold, flu
2. vehicle transmission
a. airborne
-droplets travel more than 1 meter
-examples: staph, strep, Hantavirus, measles, TB
b. waterborne or foodborne
-fecal-oral – drink/eat water or food contaminated by raw sewage
-examples for waterborne: giardiasis, cholera, amebic dysentery
-examples for foodborne: hepatitis A, S. aureus, E. coli
3. vector transmission
a. biological
-animals are host and vector
-examples: malaria, viral encephalitis, Lyme Disease, Bubonic Plague
b. mechanical
-animals are not a host – just a vector
-examples: E. coli, salmonella
4. bodily fluid transmission
-blood, saliva, urine, semen
-examples: hepatitis, herpes, AIDS
VII. Classification of Infectious Diseases
A. diseases can be classified several ways
1. the body system they affect
2. their longevity and severity
3. how they are spread to their host
4. the effects they have on populations (not individuals)
B. terms used to classify infectious disease
1. acute disease
a. develops rapidly – lasts short time
b. example: common cold
2. chronic disease
a. develops slowly; is continual or recurring; less severe
b. examples: mono, hepatitis C, TB, leprosy, TB
3. subacute disease
a. between acute and chronic
b. example: subacute bacterial endocarditis
4. latent disease
a. pathogen remains inactive for period of time before becoming active
b. examples: leprosy, herpes, AIDS
5. communicable
a. disease comes from another host
b. examples: flu, herpes, TB
6. contagious
a. easily transmitted
b. examples: chickenpox, measles
7. noncommunicable –
a. disease comes from outside of host or normal microbiota
b. examples: acne, cavities, tetanus
VIII. Epidemiology of Infectious disease
-study of where/when diseases occur and how transmitted within population
A. frequency of disease
1. track occurrence of diseases using 2 measures
a. incidence
-# of new cases of a disease in a given area during a given period of time
b. prevalence
-# of total cases (new and existing) of a disease in a given area during a given period of time
2. occurrence also evaluated in terms of frequency and geographic distribution
a. endemic – disease stable and currently present in population/area
b. epidemic – disease is occurring more than usual
c. pandemic – disease on more than 1 continent
d. sporadic – disease occasional
B. epidemiological studies
-conduct research to study dynamics of diseases in population
1. descriptive epidemiology
a. careful recording of data concerning a disease
-record location and time of the cases of disease
-collect patient information
b. try to identify the index case (1st case) of the disease
2. analytical epidemiology
a. seeks to determine the probable cause, mode of transmission, and methods of prevention
b. useful in situations when Koch’s postulates can’t be applied
c. often retrospective
-investigation occurs after an outbreak has occurred
3. experimental epidemiology
a. involves testing a hypothesis concerning the cause of a disease
b. information comes from analytical studies
b. application of Koch’s postulates is experimental epidemiology
C. Hospital Epidemiology: Nosocomial Infections (diseases acquired in healthcare setting)
1. types of nosocomial infections
a. exogenous
-pathogen acquired from health care environment
b. endogenous
-pathogen comes from normal microbiota due to factors within the health care setting
c. iatrogenic
-results from modern medical procedures
-examples: catheters, surgery, other invasive procedures
2. factors influencing nosocomial infections
a. exposure to lots of pathogens
b. weakened immune system
c. transmission of pathogens among patients and healthcare workers
3. control of nosocomial infections
a. precautions designed to reduce factors that result in disease
-masks, surgical asepsis, sterileprocedures, gloves, gowns, caps, isolation of contagious
or susceptible patients, infection control committees
b. hand washing most effective way to reduce nosocomial infections
D. Epidemiology and public health
1. local, state, national, and global agencies share information concerning disease
a. The United States Public Health Service
b. World Health Organization (WHO)
2. public health agencies work to limit disease transmission
a. monitor water and food safety
b. reduce # of disease vectors and reservoirs
c. establish/enforce immunization schedule
d. locate and treat people exposed to contagious pathogens
e. establish isolation and quarantine measures – controls spread of pathogens
3. public health agencies campaign to educate the public on healthful choices to limit disease